Introduce support for eager check I/O scheduling

24 files changed, 850 insertions, 178 deletions

diff --git a/erts/doc/src/erl.xml b/erts/doc/src/erl.xml
index 528a2d95aa..967226056e 100644
--- a/erts/doc/src/erl.xml
+++ b/erts/doc/src/erl.xml
@@ -1065,6 +1065,23 @@
 	    <p>For more information, see
 	    <seealso marker="erlang#system_info_cpu_topology">erlang:system_info(cpu_topology)</seealso>.</p>
           </item>
+          <tag><marker id="+secio"><c>+secio true|false</c></marker></tag>
+          <item>
+	    <p>Enable or disable eager check I/O scheduling. The default
+	    is currently <c>false</c>, but will most likely be changed
+	    to <c>true</c> in OTP 18. The behaviour before this flag
+	    was introduced corresponds to <c>+secio false</c>.</p>
+	    <p>The flag effects when schedulers will check for I/O
+	    operations possible to execute, and when such I/O operations
+	    will execute. As the name of the parameter implies,
+	    schedulers will be more eager to check for I/O when
+	    <c>true</c> is passed. This however also implies that
+	    execution of outstanding I/O operation will not be
+	    prioritized to the same extent as when <c>false</c> is
+	    passed.</p>
+	    <p><seealso marker="erlang#system_info_eager_check_io"><c>erlang:system_info(eager_check_io)</c></seealso>
+	    returns the value of this parameter used when starting the VM.</p>
+          </item>
           <tag><marker id="+sfwi"><c>+sfwi Interval</c></marker></tag>
           <item>
             <p>Set scheduler forced wakeup interval. All run queues will
diff --git a/erts/doc/src/erlang.xml b/erts/doc/src/erlang.xml
index e3ef48a6c1..e02e4cbbc9 100644
--- a/erts/doc/src/erlang.xml
+++ b/erts/doc/src/erlang.xml
@@ -5611,6 +5611,7 @@ ok
       <name name="system_info" arity="1" clause_i="52"/>
       <name name="system_info" arity="1" clause_i="53"/>
       <name name="system_info" arity="1" clause_i="54"/>
+      <name name="system_info" arity="1" clause_i="55"/>
       <fsummary>Information about the system</fsummary>
       <desc>
         <p>Returns various information about the current system
@@ -5740,6 +5741,16 @@ ok
 	      The return value will always be <c>false</c> since
 	      the elib_malloc allocator has been removed.</p>
           </item>
+          <tag><marker id="system_info_eager_check_io"><c>eager_check_io</c></marker></tag>
+          <item>
+            <p>
+	      Returns the value of the <c>erl</c>
+	      <seealso marker="erl#+secio">+secio</seealso> command line
+	      flag which is either <c>true</c> or <c>false</c>. See the
+	      documentation of the command line flag for information about
+	      the different values.
+	    </p>
+          </item>
           <tag><c>ets_limit</c></tag>
           <item>
             <p>Returns the maximum number of ETS tables allowed. This limit
diff --git a/erts/emulator/beam/erl_alloc.types b/erts/emulator/beam/erl_alloc.types
index bb5eba80be..0bbd3e2dd5 100644
--- a/erts/emulator/beam/erl_alloc.types
+++ b/erts/emulator/beam/erl_alloc.types
@@ -393,6 +393,7 @@ type	DRV_EV_STATE	LONG_LIVED	SYSTEM		driver_event_state
 type	DRV_EV_D_STATE	FIXED_SIZE	SYSTEM		driver_event_data_state
 type	DRV_SEL_D_STATE	FIXED_SIZE	SYSTEM		driver_select_data_state
 type	FD_LIST		SHORT_LIVED	SYSTEM		fd_list
+type	ACTIVE_FD_ARR	SHORT_LIVED	SYSTEM		active_fd_array
 type	POLLSET		LONG_LIVED	SYSTEM		pollset
 type	POLLSET_UPDREQ	SHORT_LIVED	SYSTEM		pollset_update_req
 type	POLL_FDS	LONG_LIVED	SYSTEM		poll_fds
diff --git a/erts/emulator/beam/erl_bif_info.c b/erts/emulator/beam/erl_bif_info.c
index d7f1e2d971..8f4095f236 100755
--- a/erts/emulator/beam/erl_bif_info.c
+++ b/erts/emulator/beam/erl_bif_info.c
@@ -2643,6 +2643,9 @@ BIF_RETTYPE system_info_1(BIF_ALIST_1)
     else if (ERTS_IS_ATOM_STR("ets_limit",BIF_ARG_1)) {
         BIF_RET(make_small(erts_db_get_max_tabs()));
     }
+    else if (ERTS_IS_ATOM_STR("eager_check_io",BIF_ARG_1)) {
+	BIF_RET(erts_eager_check_io ? am_true : am_false);
+    }
 
     BIF_ERROR(BIF_P, BADARG);
 }
@@ -3232,17 +3235,38 @@ BIF_RETTYPE erts_debug_get_internal_state_1(BIF_ALIST_1)
 	    BIF_RET(make_small((Uint) words));
 	}
 	else if (ERTS_IS_ATOM_STR("check_io_debug", BIF_ARG_1)) {
-	    /* Used by (emulator) */
-	    int res;
+	    /* Used by driver_SUITE (emulator) */
+	    Uint sz, *szp;
+	    Eterm res, *hp, **hpp;
+	    int no_errors;
+	    ErtsCheckIoDebugInfo ciodi = {0};
 #ifdef HAVE_ERTS_CHECK_IO_DEBUG
 	    erts_smp_proc_unlock(BIF_P,ERTS_PROC_LOCK_MAIN);
-	    res = erts_check_io_debug();
+	    no_errors = erts_check_io_debug(&ciodi);
 	    erts_smp_proc_lock(BIF_P,ERTS_PROC_LOCK_MAIN);
 #else
-	    res = 0;
-#endif
-	    ASSERT(res >= 0);
-	    BIF_RET(erts_make_integer((Uint) res, BIF_P));
+	    no_errors = 0;
+#endif
+	    sz = 0;
+	    szp = &sz;
+	    hpp = NULL;
+	    while (1) {
+		res = erts_bld_tuple(hpp, szp, 4,
+				     erts_bld_uint(hpp, szp,
+						   (Uint) no_errors),
+				     erts_bld_uint(hpp, szp,
+						   (Uint) ciodi.no_used_fds),
+				     erts_bld_uint(hpp, szp,
+						   (Uint) ciodi.no_driver_select_structs),
+				     erts_bld_uint(hpp, szp,
+						   (Uint) ciodi.no_driver_event_structs));
+		if (hpp)
+		    break;
+		hp = HAlloc(BIF_P, sz);
+		szp = NULL;
+		hpp = &hp;
+	    }
+	    BIF_RET(res);
 	}
 	else if (ERTS_IS_ATOM_STR("process_info_args", BIF_ARG_1)) {
 	    /* Used by process_SUITE (emulator) */
diff --git a/erts/emulator/beam/erl_init.c b/erts/emulator/beam/erl_init.c
index 8c4fffa75b..c3be23c84a 100644
--- a/erts/emulator/beam/erl_init.c
+++ b/erts/emulator/beam/erl_init.c
@@ -537,6 +537,8 @@ void erts_usage(void)
     erts_fprintf(stderr, "            see the erl(1) documentation for more info.\n");
     erts_fprintf(stderr, "-sct cput   set cpu topology,\n");
     erts_fprintf(stderr, "            see the erl(1) documentation for more info.\n");
+    erts_fprintf(stderr, "-secio bool enable/disable eager check I/O scheduling,\n");
+    erts_fprintf(stderr, "            see the erl(1) documentation for more info.\n");
     erts_fprintf(stderr, "-sws val    set scheduler wakeup strategy, valid values are:\n");
     erts_fprintf(stderr, "            default|legacy.\n");
     erts_fprintf(stderr, "-swct val   set scheduler wake cleanup threshold, valid values are:\n");
@@ -1487,6 +1489,19 @@ erl_start(int argc, char **argv)
 		    erts_usage();
 		}
 	    }
+	    else if (has_prefix("ecio", sub_param)) {
+		arg = get_arg(sub_param+4, argv[i+1], &i);
+		if (sys_strcmp("true", arg) == 0)
+		    erts_eager_check_io = 1;
+		else if (sys_strcmp("false", arg) == 0)
+		    erts_eager_check_io = 0;
+		else {
+		    erts_fprintf(stderr,
+				 "bad schedule eager check I/O value '%s'\n",
+				 arg);
+		    erts_usage();
+		}
+	    }
 	    else if (has_prefix("pp", sub_param)) {
 		arg = get_arg(sub_param+2, argv[i+1], &i);
 		if (sys_strcmp(arg, "true") == 0)
diff --git a/erts/emulator/beam/erl_port_task.c b/erts/emulator/beam/erl_port_task.c
index 547a42beb2..ff4fdc70aa 100644
--- a/erts/emulator/beam/erl_port_task.c
+++ b/erts/emulator/beam/erl_port_task.c
@@ -32,6 +32,7 @@
 #include "global.h"
 #include "erl_port_task.h"
 #include "dist.h"
+#include "erl_check_io.h"
 #include "dtrace-wrapper.h"
 #include <stdarg.h>
 
@@ -543,6 +544,16 @@ reset_handle(ErtsPortTask *ptp)
 }
 
 static ERTS_INLINE void
+reset_executed_io_task_handle(ErtsPortTask *ptp)
+{
+    if (ptp->u.alive.handle) {
+	ASSERT(ptp == handle2task(ptp->u.alive.handle));
+	erts_io_notify_port_task_executed(ptp->u.alive.handle);
+	reset_port_task_handle(ptp->u.alive.handle);
+    }
+}
+
+static ERTS_INLINE void
 set_handle(ErtsPortTask *ptp, ErtsPortTaskHandle *pthp)
 {
     ptp->u.alive.handle = pthp;
@@ -1365,10 +1376,7 @@ erts_port_task_schedule(Eterm id,
     ErtsPortTask *ptp = NULL;
     erts_aint32_t act, add_flags;
 
-    if (pthp && erts_port_task_is_scheduled(pthp)) {
-	ASSERT(0);
-	erts_port_task_abort(pthp);
-    }
+    ERTS_LC_ASSERT(!pthp || !erts_port_task_is_scheduled(pthp));
 
     ASSERT(is_internal_port(id));
 
@@ -1654,8 +1662,6 @@ erts_port_task_execute(ErtsRunQueue *runq, Port **curr_port_pp)
 	    goto aborted_port_task;
 	}
 
-	reset_handle(ptp);
-
 	if (erts_system_monitor_long_schedule != 0) {
 	    start_time = erts_timestamp_millis();
 	}
@@ -1666,6 +1672,7 @@ erts_port_task_execute(ErtsRunQueue *runq, Port **curr_port_pp)
 
 	switch (ptp->type) {
 	case ERTS_PORT_TASK_TIMEOUT:
+	    reset_handle(ptp);
 	    reds = ERTS_PORT_REDS_TIMEOUT;
 	    if (!(state & ERTS_PORT_SFLGS_DEAD)) {
                 DTRACE_DRIVER(driver_timeout, pp);
@@ -1679,6 +1686,7 @@ erts_port_task_execute(ErtsRunQueue *runq, Port **curr_port_pp)
 	    /* NOTE some windows drivers use ->ready_input for input and output */
 	    (*pp->drv_ptr->ready_input)((ErlDrvData) pp->drv_data,
 					ptp->u.alive.td.io.event);
+	    reset_executed_io_task_handle(ptp);
 	    io_tasks_executed++;
 	    break;
 	case ERTS_PORT_TASK_OUTPUT:
@@ -1687,6 +1695,7 @@ erts_port_task_execute(ErtsRunQueue *runq, Port **curr_port_pp)
             DTRACE_DRIVER(driver_ready_output, pp);
 	    (*pp->drv_ptr->ready_output)((ErlDrvData) pp->drv_data,
 					 ptp->u.alive.td.io.event);
+	    reset_executed_io_task_handle(ptp);
 	    io_tasks_executed++;
 	    break;
 	case ERTS_PORT_TASK_EVENT:
@@ -1696,10 +1705,12 @@ erts_port_task_execute(ErtsRunQueue *runq, Port **curr_port_pp)
 	    (*pp->drv_ptr->event)((ErlDrvData) pp->drv_data,
 				  ptp->u.alive.td.io.event,
 				  ptp->u.alive.td.io.event_data);
+	    reset_executed_io_task_handle(ptp);
 	    io_tasks_executed++;
 	    break;
 	case ERTS_PORT_TASK_PROC_SIG: {
 	    ErtsProc2PortSigData *sigdp = &ptp->u.alive.td.psig.data;
+	    reset_handle(ptp);
 	    ASSERT((state & ERTS_PORT_SFLGS_DEAD) == 0);
 	    if (!pp->sched.taskq.bpq)
 		reds = ptp->u.alive.td.psig.callback(pp,
@@ -1717,6 +1728,7 @@ erts_port_task_execute(ErtsRunQueue *runq, Port **curr_port_pp)
 	    break;
 	}
 	case ERTS_PORT_TASK_DIST_CMD:
+	    reset_handle(ptp);
 	    reds = erts_dist_command(pp, CONTEXT_REDS - pp->reds);
 	    break;
 	default:
diff --git a/erts/emulator/beam/erl_port_task.h b/erts/emulator/beam/erl_port_task.h
index 123253a057..e1e06b565e 100644
--- a/erts/emulator/beam/erl_port_task.h
+++ b/erts/emulator/beam/erl_port_task.h
@@ -154,7 +154,7 @@ erts_port_task_handle_init(ErtsPortTaskHandle *pthp)
 ERTS_GLB_INLINE int
 erts_port_task_is_scheduled(ErtsPortTaskHandle *pthp)
 {
-    return ((void *) erts_smp_atomic_read_nob(pthp)) != NULL;
+    return ((void *) erts_smp_atomic_read_acqb(pthp)) != NULL;
 }
 
 ERTS_GLB_INLINE void erts_port_task_pre_init_sched(ErtsPortTaskSched *ptsp,
diff --git a/erts/emulator/beam/erl_process.c b/erts/emulator/beam/erl_process.c
index 1efd070afd..4f3ea92cf5 100644
--- a/erts/emulator/beam/erl_process.c
+++ b/erts/emulator/beam/erl_process.c
@@ -143,6 +143,7 @@ extern BeamInstr beam_apply[];
 extern BeamInstr beam_exit[];
 extern BeamInstr beam_continue_exit[];
 
+int erts_eager_check_io = 0;
 int erts_sched_compact_load;
 Uint erts_no_schedulers;
 
@@ -1977,19 +1978,28 @@ try_set_sys_scheduling(void)
 #endif
 
 static ERTS_INLINE int
-prepare_for_sys_schedule(void)
+prepare_for_sys_schedule(int non_blocking)
 {
+    if (non_blocking && erts_eager_check_io) {
 #ifdef ERTS_SMP
-    while (!erts_port_task_have_outstanding_io_tasks()
-	   && try_set_sys_scheduling()) {
-	if (!erts_port_task_have_outstanding_io_tasks())
-	    return 1;
-	clear_sys_scheduling();
+	return try_set_sys_scheduling();
+#else
+	return 1;
+#endif
     }
-    return 0;
+    else {
+#ifdef ERTS_SMP
+	while (!erts_port_task_have_outstanding_io_tasks()
+	       && try_set_sys_scheduling()) {
+	    if (!erts_port_task_have_outstanding_io_tasks())
+		return 1;
+	    clear_sys_scheduling();
+	}
+	return 0;
 #else
-    return !erts_port_task_have_outstanding_io_tasks();
+	return !erts_port_task_have_outstanding_io_tasks();
 #endif
+    }
 }
 
 #ifdef ERTS_SMP
@@ -2307,7 +2317,7 @@ scheduler_wait(int *fcalls, ErtsSchedulerData *esdp, ErtsRunQueue *rq)
      * be waiting in erl_sys_schedule()
      */
 
-    if (!prepare_for_sys_schedule()) {
+    if (!prepare_for_sys_schedule(0)) {
 
 	sched_waiting(esdp->no, rq);
 
@@ -2459,7 +2469,7 @@ scheduler_wait(int *fcalls, ErtsSchedulerData *esdp, ErtsRunQueue *rq)
 		 * Got to check that we still got I/O tasks; otherwise
 		 * we have to continue checking for I/O...
 		 */
-		if (!prepare_for_sys_schedule()) {
+		if (!prepare_for_sys_schedule(0)) {
 		    spincount *= ERTS_SCHED_TSE_SLEEP_SPINCOUNT_FACT;
 		    goto tse_wait;
 		}
@@ -2481,7 +2491,7 @@ scheduler_wait(int *fcalls, ErtsSchedulerData *esdp, ErtsRunQueue *rq)
 	     * Got to check that we still got I/O tasks; otherwise
 	     * we have to wait in erl_sys_schedule() after all...
 	     */
-	    if (!prepare_for_sys_schedule()) {
+	    if (!prepare_for_sys_schedule(0)) {
 		/*
 		 * Not allowed to wait in erl_sys_schedule;
 		 * do tse wait instead...
@@ -7022,7 +7032,7 @@ Process *schedule(Process *p, int calls)
 
 	    goto check_activities_to_run;
 	}
-	else if (fcalls > input_reductions && prepare_for_sys_schedule()) {
+	else if (fcalls > input_reductions && prepare_for_sys_schedule(!0)) {
 	    /*
 	     * Schedule system-level activities.
 	     */
@@ -7030,8 +7040,6 @@ Process *schedule(Process *p, int calls)
 	    erts_smp_atomic32_set_relb(&function_calls, 0);
 	    fcalls = 0;
 
-	    ASSERT(!erts_port_task_have_outstanding_io_tasks());
-
 #if 0 /* Not needed since we wont wait in sys schedule */
 	    erts_sys_schedule_interrupt(0);
 #endif
@@ -7063,7 +7071,7 @@ Process *schedule(Process *p, int calls)
 	if (RUNQ_READ_LEN(&rq->ports.info.len)) {
 	    int have_outstanding_io;
 	    have_outstanding_io = erts_port_task_execute(rq, &esdp->current_port);
-	    if ((have_outstanding_io && fcalls > 2*input_reductions)
+	    if ((!erts_eager_check_io && have_outstanding_io && fcalls > 2*input_reductions)
 		|| rq->halt_in_progress) {
 		/*
 		 * If we have performed more than 2*INPUT_REDUCTIONS since
diff --git a/erts/emulator/beam/erl_process.h b/erts/emulator/beam/erl_process.h
index 8d136f6e8b..41936c1bb0 100644
--- a/erts/emulator/beam/erl_process.h
+++ b/erts/emulator/beam/erl_process.h
@@ -97,6 +97,7 @@ struct saved_calls {
 };
 
 extern Export exp_send, exp_receive, exp_timeout;
+extern int erts_eager_check_io;
 extern int erts_sched_compact_load;
 extern Uint erts_no_schedulers;
 extern Uint erts_no_run_queues;
diff --git a/erts/emulator/beam/sys.h b/erts/emulator/beam/sys.h
index 31252ed78f..e691d5c55c 100644
--- a/erts/emulator/beam/sys.h
+++ b/erts/emulator/beam/sys.h
@@ -64,8 +64,12 @@
  */
 
 #ifndef ERTS_SYS_FD_TYPE
+#define ERTS_SYS_FD_INVALID ((ErtsSysFdType) -1)
 typedef int ErtsSysFdType;
 #else
+#ifndef ERTS_SYS_FD_INVALID
+# error missing ERTS_SYS_FD_INVALID
+#endif
 typedef ERTS_SYS_FD_TYPE ErtsSysFdType;
 #endif
 
@@ -731,6 +735,14 @@ void init_getenv_state(GETENV_STATE *);
 char * getenv_string(GETENV_STATE *);
 void fini_getenv_state(GETENV_STATE *);
 
+#define HAVE_ERTS_CHECK_IO_DEBUG
+typedef struct {
+    int no_used_fds;
+    int no_driver_select_structs;
+    int no_driver_event_structs;
+} ErtsCheckIoDebugInfo;
+int erts_check_io_debug(ErtsCheckIoDebugInfo *ip);
+
 /* xxxP */
 #define SYS_DEFAULT_FLOAT_DECIMALS 20
 void init_sys_float(void);
diff --git a/erts/emulator/beam/utils.c b/erts/emulator/beam/utils.c
index 0d75bbcc77..426e43d5d8 100644
--- a/erts/emulator/beam/utils.c
+++ b/erts/emulator/beam/utils.c
@@ -47,6 +47,7 @@
 #include "erl_sched_spec_pre_alloc.h"
 #include "beam_bp.h"
 #include "erl_ptab.h"
+#include "erl_check_io.h"
 
 #undef M_TRIM_THRESHOLD
 #undef M_TOP_PAD
diff --git a/erts/emulator/sys/common/erl_check_io.c b/erts/emulator/sys/common/erl_check_io.c
index 7035dc77df..e62f34b679 100644
--- a/erts/emulator/sys/common/erl_check_io.c
+++ b/erts/emulator/sys/common/erl_check_io.c
@@ -51,8 +51,17 @@ typedef char EventStateType;
 #define ERTS_EV_TYPE_STOP_USE ((EventStateType) 3) /* pending stop_select */
 
 typedef char EventStateFlags;
-#define ERTS_EV_FLAG_USED   ((EventStateFlags) 1)   /* ERL_DRV_USE has been turned on */
+#define ERTS_EV_FLAG_USED         ((EventStateFlags) 1)   /* ERL_DRV_USE has been turned on */
+#define ERTS_EV_FLAG_DEFER_IN_EV  ((EventStateFlags) 2)
+#define ERTS_EV_FLAG_DEFER_OUT_EV ((EventStateFlags) 4)
 
+#ifdef DEBUG
+#  define ERTS_ACTIVE_FD_INC 2
+#else
+#  define ERTS_ACTIVE_FD_INC 128
+#endif
+
+#define ERTS_CHECK_IO_POLL_RES_LEN 512
 
 #if defined(ERTS_KERNEL_POLL_VERSION)
 #  define ERTS_CIO_EXPORT(FUNC) FUNC ## _kp
@@ -66,6 +75,7 @@ typedef char EventStateFlags;
   (ERTS_POLL_USE_POLL && !ERTS_POLL_USE_KERNEL_POLL)
 
 #define ERTS_CIO_POLL_CTL	ERTS_POLL_EXPORT(erts_poll_control)
+#define ERTS_CIO_POLL_CTLV	ERTS_POLL_EXPORT(erts_poll_controlv)
 #define ERTS_CIO_POLL_WAIT	ERTS_POLL_EXPORT(erts_poll_wait)
 #ifdef ERTS_POLL_NEED_ASYNC_INTERRUPT_SUPPORT
 #define ERTS_CIO_POLL_AS_INTR 	ERTS_POLL_EXPORT(erts_poll_async_sig_interrupt)
@@ -82,6 +92,13 @@ static struct pollset_info
 {
     ErtsPollSet ps;
     erts_smp_atomic_t in_poll_wait;        /* set while doing poll */
+    struct {
+	int six; /* start index */
+	int eix; /* end index */
+	erts_smp_atomic32_t no;
+	int size;
+	ErtsSysFdType *array;
+    } active_fd;
 #ifdef ERTS_SMP
     struct removed_fd* removed_list;       /* list of deselected fd's*/
     erts_smp_spinlock_t removed_list_lock;
@@ -94,9 +111,11 @@ typedef struct {
     SafeHashBucket hb;
 #endif
     ErtsSysFdType fd;
-    union {
-	ErtsDrvEventDataState *event;     /* ERTS_EV_TYPE_DRV_EV */
+    struct {
 	ErtsDrvSelectDataState *select;   /* ERTS_EV_TYPE_DRV_SEL */
+#if ERTS_CIO_HAVE_DRV_EVENT
+	ErtsDrvEventDataState *event;     /* ERTS_EV_TYPE_DRV_EV */
+#endif
 	erts_driver_t* drv_ptr;           /* ERTS_EV_TYPE_STOP_USE */
     } driver;
     ErtsPollEvents events;
@@ -166,6 +185,10 @@ static ERTS_INLINE ErtsDrvEventState* hash_new_drv_ev_state(ErtsSysFdType fd)
     ErtsDrvEventState tmpl;
     tmpl.fd = fd;
     tmpl.driver.select = NULL;
+#if ERTS_CIO_HAVE_DRV_EVENT
+    tmpl.driver.event = NULL;
+#endif
+    tmpl.driver.drv_ptr = NULL;
     tmpl.events = 0;
     tmpl.remove_cnt = 0;
     tmpl.type = ERTS_EV_TYPE_NONE;
@@ -206,6 +229,65 @@ ERTS_SCHED_PREF_QUICK_ALLOC_IMPL(removed_fd, struct removed_fd, 64, ERTS_ALC_T_F
 #endif
 
 static ERTS_INLINE void
+init_iotask(ErtsIoTask *io_task)
+{
+    erts_port_task_handle_init(&io_task->task);
+    erts_smp_atomic_init_nob(&io_task->executed_time, ~((erts_aint_t) 0));
+}
+
+static ERTS_INLINE int
+is_iotask_active(ErtsIoTask *io_task, erts_aint_t current_cio_time)
+{    
+    if (erts_port_task_is_scheduled(&io_task->task))
+	return 1;
+    if (erts_smp_atomic_read_nob(&io_task->executed_time) == current_cio_time)
+	return 1;
+    return 0;
+}
+
+static ERTS_INLINE ErtsDrvSelectDataState *
+alloc_drv_select_data(void)
+{
+    ErtsDrvSelectDataState *dsp = erts_alloc(ERTS_ALC_T_DRV_SEL_D_STATE,
+					     sizeof(ErtsDrvSelectDataState));
+    dsp->inport = NIL;
+    dsp->outport = NIL;
+    init_iotask(&dsp->iniotask);
+    init_iotask(&dsp->outiotask);
+    return dsp;
+}
+
+static ERTS_INLINE void
+free_drv_select_data(ErtsDrvSelectDataState *dsp)
+{
+    ASSERT(!erts_port_task_is_scheduled(&dsp->iniotask.task));
+    ASSERT(!erts_port_task_is_scheduled(&dsp->outiotask.task));
+    erts_free(ERTS_ALC_T_DRV_SEL_D_STATE, dsp);   
+}
+
+static ERTS_INLINE ErtsDrvEventDataState *
+alloc_drv_event_data(void)
+{
+    ErtsDrvEventDataState *dep = erts_alloc(ERTS_ALC_T_DRV_EV_D_STATE,
+					    sizeof(ErtsDrvEventDataState));
+    dep->port = NIL;
+    dep->data = NULL;
+    dep->removed_events = 0;
+#if ERTS_CIO_DEFER_ACTIVE_EVENTS
+    dep->deferred_events = 0;
+#endif
+    init_iotask(&dep->iotask);
+    return dep;
+}
+
+static ERTS_INLINE void
+free_drv_event_data(ErtsDrvEventDataState *dep)
+{
+    ASSERT(!erts_port_task_is_scheduled(&dep->iotask.task));
+    erts_free(ERTS_ALC_T_DRV_EV_D_STATE, dep);   
+}
+
+static ERTS_INLINE void
 remember_removed(ErtsDrvEventState *state, struct pollset_info* psi)
 {
 #ifdef ERTS_SMP
@@ -285,7 +367,7 @@ forget_removed(struct pollset_info* psi)
 		drv_ptr = state->driver.drv_ptr;
 		ASSERT(drv_ptr);
 		state->type = ERTS_EV_TYPE_NONE;
-		state->flags = 0;
+		state->flags &= ~ERTS_EV_FLAG_USED;
 		state->driver.drv_ptr = NULL;  
 		/* Fall through */
 	    case ERTS_EV_TYPE_NONE:
@@ -342,6 +424,10 @@ grow_drv_ev_state(int min_ix)
 	for (i = erts_smp_atomic_read_nob(&drv_ev_state_len); i < new_len; i++) {
 	    drv_ev_state[i].fd = (ErtsSysFdType) i;
 	    drv_ev_state[i].driver.select = NULL;
+#if ERTS_CIO_HAVE_DRV_EVENT
+	    drv_ev_state[i].driver.event = NULL;
+#endif
+	    drv_ev_state[i].driver.drv_ptr = NULL;
 	    drv_ev_state[i].events = 0;
 	    drv_ev_state[i].remove_cnt = 0;
 	    drv_ev_state[i].type = ERTS_EV_TYPE_NONE;
@@ -362,11 +448,7 @@ grow_drv_ev_state(int min_ix)
 static ERTS_INLINE void
 abort_task(Eterm id, ErtsPortTaskHandle *pthp, EventStateType type)
 {
-    if (is_nil(id)) {
-	ASSERT(type == ERTS_EV_TYPE_NONE
-	       || !erts_port_task_is_scheduled(pthp));
-    }
-    else if (erts_port_task_is_scheduled(pthp)) {
+    if (is_not_nil(id) && erts_port_task_is_scheduled(pthp)) {
 	erts_port_task_abort(pthp);
 	ASSERT(erts_is_port_alive(id));
     }
@@ -381,7 +463,7 @@ abort_tasks(ErtsDrvEventState *state, int mode)
 #if ERTS_CIO_HAVE_DRV_EVENT
 	case ERTS_EV_TYPE_DRV_EV:
 	    abort_task(state->driver.event->port,
-		       &state->driver.event->task,
+		       &state->driver.event->iotask.task,
 		       ERTS_EV_TYPE_DRV_EV);
 	    return;
 #endif
@@ -395,14 +477,14 @@ abort_tasks(ErtsDrvEventState *state, int mode)
     case ERL_DRV_WRITE:
 	ASSERT(state->type == ERTS_EV_TYPE_DRV_SEL);
 	abort_task(state->driver.select->outport,
-		   &state->driver.select->outtask,
+		   &state->driver.select->outiotask.task,
 		   state->type);
 	if (mode == ERL_DRV_WRITE)
 	    break;
     case ERL_DRV_READ:
 	ASSERT(state->type == ERTS_EV_TYPE_DRV_SEL);
 	abort_task(state->driver.select->inport,
-		   &state->driver.select->intask,
+		   &state->driver.select->iniotask.task,
 		   state->type);
 	break;
     default:
@@ -440,16 +522,14 @@ deselect(ErtsDrvEventState *state, int mode)
     if (!(state->events)) {
 	switch (state->type) {
 	case ERTS_EV_TYPE_DRV_SEL:
-	    ASSERT(!erts_port_task_is_scheduled(&state->driver.select->intask));
-	    ASSERT(!erts_port_task_is_scheduled(&state->driver.select->outtask));
-	    erts_free(ERTS_ALC_T_DRV_SEL_D_STATE,
-		      state->driver.select);
+	    state->driver.select->inport = NIL;
+	    state->driver.select->outport = NIL;
 	    break;
 #if ERTS_CIO_HAVE_DRV_EVENT
 	case ERTS_EV_TYPE_DRV_EV:
-	    ASSERT(!erts_port_task_is_scheduled(&state->driver.event->task));
-	    erts_free(ERTS_ALC_T_DRV_EV_D_STATE,
-		      state->driver.event);
+	    state->driver.event->port = NIL;
+	    state->driver.event->data = NULL;
+	    state->driver.event->removed_events = (ErtsPollEvents) 0;
 	    break;
 #endif
 	case ERTS_EV_TYPE_NONE:
@@ -459,20 +539,297 @@ deselect(ErtsDrvEventState *state, int mode)
 	    break;
 	}
 	    
-	state->driver.select = NULL;
 	state->type = ERTS_EV_TYPE_NONE;
-	state->flags = 0;
+	state->flags &= ~ERTS_EV_FLAG_USED;
 	remember_removed(state, &pollset);
     }
 }
 
-
 #ifdef ERTS_SYS_CONTINOUS_FD_NUMBERS
 #  define IS_FD_UNKNOWN(state) ((state)->type == ERTS_EV_TYPE_NONE && (state)->remove_cnt == 0)
 #else
 #  define IS_FD_UNKNOWN(state) ((state) == NULL)
 #endif
 
+static ERTS_INLINE void
+check_fd_cleanup(ErtsDrvEventState *state,
+#if ERTS_CIO_HAVE_DRV_EVENT
+		 ErtsDrvEventDataState **free_event,
+#endif
+		 ErtsDrvSelectDataState **free_select)
+{
+    erts_aint_t current_cio_time;
+
+    ERTS_SMP_LC_ASSERT(erts_smp_lc_mtx_is_locked(fd_mtx(state->fd)));
+
+    current_cio_time = erts_smp_atomic_read_acqb(&erts_check_io_time);
+    *free_select = NULL;
+    if (state->driver.select
+	&& (state->type != ERTS_EV_TYPE_DRV_SEL)
+	&& !is_iotask_active(&state->driver.select->iniotask, current_cio_time)
+	&& !is_iotask_active(&state->driver.select->outiotask, current_cio_time)) {
+	
+	*free_select = state->driver.select;
+	state->driver.select = NULL;
+    }
+
+#if ERTS_CIO_HAVE_DRV_EVENT
+    *free_event = NULL;
+    if (state->driver.event
+	&& (state->type != ERTS_EV_TYPE_DRV_EV)
+	&& !is_iotask_active(&state->driver.event->iotask,  current_cio_time)) {
+	
+	*free_event = state->driver.event;
+	state->driver.event = NULL;
+    }
+#endif
+
+#ifndef ERTS_SYS_CONTINOUS_FD_NUMBERS
+    if (((state->type != ERTS_EV_TYPE_NONE)
+	 | state->remove_cnt
+#if ERTS_CIO_HAVE_DRV_EVENT
+	 | (state->driver.event != NULL)
+#endif
+	 | (state->driver.select != NULL)) == 0) {
+
+	hash_erase_drv_ev_state(state);
+
+    }
+#endif
+}
+
+static ERTS_INLINE int
+check_cleanup_active_fd(ErtsSysFdType fd,
+#if ERTS_CIO_DEFER_ACTIVE_EVENTS
+			ErtsPollControlEntry *pce,
+			int *pce_ix,
+#endif
+			erts_aint_t current_cio_time)
+{
+    ErtsDrvEventState *state;
+    int active = 0;
+    erts_smp_mtx_t *mtx = fd_mtx(fd);
+    void *free_select = NULL;
+#if ERTS_CIO_HAVE_DRV_EVENT
+    void *free_event = NULL;
+#endif
+#if ERTS_CIO_DEFER_ACTIVE_EVENTS
+    ErtsPollEvents evon = 0, evoff = 0;
+#endif
+
+    erts_smp_mtx_lock(mtx);
+
+#ifdef ERTS_SYS_CONTINOUS_FD_NUMBERS
+    state = &drv_ev_state[(int) fd];
+#else
+    state = hash_get_drv_ev_state(fd); /* may be NULL! */
+    if (state)
+#endif
+    {
+	if (state->driver.select) {
+#if ERTS_CIO_DEFER_ACTIVE_EVENTS
+	    if (is_iotask_active(&state->driver.select->iniotask, current_cio_time)) {
+		active = 1;
+		if ((state->events & ERTS_POLL_EV_IN)
+		    && !(state->flags & ERTS_EV_FLAG_DEFER_IN_EV)) {
+		    evoff |= ERTS_POLL_EV_IN;
+		    state->flags |= ERTS_EV_FLAG_DEFER_IN_EV;
+		}
+	    }
+	    else if (state->flags & ERTS_EV_FLAG_DEFER_IN_EV) {
+		if (state->events & ERTS_POLL_EV_IN)
+		    evon |= ERTS_POLL_EV_IN;
+		state->flags &= ~ERTS_EV_FLAG_DEFER_IN_EV;
+	    }
+	    if (is_iotask_active(&state->driver.select->outiotask, current_cio_time)) {
+		active = 1;
+		if ((state->events & ERTS_POLL_EV_OUT)
+		    && !(state->flags & ERTS_EV_FLAG_DEFER_OUT_EV)) {
+		    evoff |= ERTS_POLL_EV_OUT;
+		    state->flags |= ERTS_EV_FLAG_DEFER_OUT_EV;
+		}
+	    }
+	    else if (state->flags & ERTS_EV_FLAG_DEFER_OUT_EV) {
+		if (state->events & ERTS_POLL_EV_OUT)
+		    evon |= ERTS_POLL_EV_OUT;
+		state->flags &= ~ERTS_EV_FLAG_DEFER_OUT_EV;
+	    }
+	    if (active)
+		(void) 0;
+	    else
+#else
+	    if (is_iotask_active(&state->driver.select->iniotask, current_cio_time)
+		|| is_iotask_active(&state->driver.select->outiotask, current_cio_time))
+		active = 1;
+	    else
+#endif
+		if (state->type != ERTS_EV_TYPE_DRV_SEL) {
+		free_select = state->driver.select;
+		state->driver.select = NULL;
+	    }
+	}
+
+#if ERTS_CIO_HAVE_DRV_EVENT
+	if (state->driver.event) {
+	    if (is_iotask_active(&state->driver.event->iotask, current_cio_time)) {
+#if ERTS_CIO_DEFER_ACTIVE_EVENTS
+		ErtsPollEvents evs = state->events & ~state->driver.event->deferred_events;
+		if (evs) {
+		    evoff |= evs;
+		    state->driver.event->deferred_events |= evs;
+		}
+#endif
+		active = 1;
+	    }
+	    else if (state->type != ERTS_EV_TYPE_DRV_EV) {
+		free_event = state->driver.event;
+		state->driver.event = NULL;
+	    }
+#if ERTS_CIO_DEFER_ACTIVE_EVENTS
+	    else {
+		ErtsPollEvents evs = state->events & state->driver.event->deferred_events;
+		if (evs) {
+		    evon |= evs;
+		    state->driver.event->deferred_events = 0;
+		}
+	    }
+#endif
+
+	}
+#endif
+
+#ifndef ERTS_SYS_CONTINOUS_FD_NUMBERS
+	if (((state->type != ERTS_EV_TYPE_NONE) | state->remove_cnt | active) == 0)
+	    hash_erase_drv_ev_state(state);
+#endif
+
+    }
+
+    erts_smp_mtx_unlock(mtx);
+
+    if (free_select)
+	free_drv_select_data(free_select);
+#if ERTS_CIO_HAVE_DRV_EVENT
+    if (free_event)
+	free_drv_event_data(free_event);
+#endif
+
+#if ERTS_CIO_DEFER_ACTIVE_EVENTS
+    if (evoff) {
+	ErtsPollControlEntry *pcep = &pce[(*pce_ix)++];
+	pcep->fd = fd;
+	pcep->events = evoff;
+	pcep->on = 0;
+    }
+    if (evon) {
+	ErtsPollControlEntry *pcep = &pce[(*pce_ix)++];
+	pcep->fd = fd;
+	pcep->events = evon;
+	pcep->on = 1;
+    }
+#endif
+
+    return active;
+}
+
+static void
+check_cleanup_active_fds(erts_aint_t current_cio_time)
+{
+    int six = pollset.active_fd.six;
+    int eix = pollset.active_fd.eix;
+    erts_aint32_t no = erts_smp_atomic32_read_dirty(&pollset.active_fd.no);
+    int size = pollset.active_fd.size;
+    int ix = six;
+#if ERTS_CIO_DEFER_ACTIVE_EVENTS
+    /* every fd might add two entries */
+    Uint pce_sz = 2*sizeof(ErtsPollControlEntry)*no;
+    ErtsPollControlEntry *pctrl_entries = (pce_sz
+					   ? erts_alloc(ERTS_ALC_T_TMP, pce_sz)
+					   : NULL);
+    int pctrl_ix = 0;
+#endif
+
+    while (ix != eix) {
+	ErtsSysFdType fd = pollset.active_fd.array[ix];
+	int nix = ix + 1;
+	if (nix >= size)
+	    nix = 0;
+	ASSERT(fd != ERTS_SYS_FD_INVALID);
+	if (!check_cleanup_active_fd(fd,
+#if ERTS_CIO_DEFER_ACTIVE_EVENTS
+				     pctrl_entries,
+				     &pctrl_ix,
+#endif
+				     current_cio_time)) {
+	    no--;
+	    if (ix == six) {
+#ifdef DEBUG
+		pollset.active_fd.array[ix] = ERTS_SYS_FD_INVALID;
+#endif
+		six = nix;
+	    }
+	    else {
+		pollset.active_fd.array[ix] = pollset.active_fd.array[six];
+#ifdef DEBUG
+		pollset.active_fd.array[six] = ERTS_SYS_FD_INVALID;
+#endif
+		six++;
+		if (six >= size)
+		    six = 0;
+	    }
+	}
+	ix = nix;
+    }
+
+#if ERTS_CIO_DEFER_ACTIVE_EVENTS
+    ASSERT(pctrl_ix <= pce_sz/sizeof(ErtsPollControlEntry));
+    if (pctrl_ix)
+	ERTS_CIO_POLL_CTLV(pollset.ps, pctrl_entries, pctrl_ix);
+    if (pctrl_entries)
+	erts_free(ERTS_ALC_T_TMP, pctrl_entries);
+#endif
+
+    pollset.active_fd.six = six;
+    pollset.active_fd.eix = eix;
+    erts_smp_atomic32_set_relb(&pollset.active_fd.no, no);
+}
+
+static ERTS_INLINE void
+add_active_fd(ErtsSysFdType fd)
+{
+    int eix = pollset.active_fd.eix;
+    int size = pollset.active_fd.size;
+    
+
+    pollset.active_fd.array[eix] = fd;
+
+    erts_smp_atomic32_set_relb(&pollset.active_fd.no,
+			       (erts_smp_atomic32_read_dirty(&pollset.active_fd.no)
+				+ 1));
+
+    eix++;
+    if (eix >= size)
+	eix = 0;
+    if (pollset.active_fd.six == eix) {
+	pollset.active_fd.six = 0;
+	eix = size;
+	size += ERTS_ACTIVE_FD_INC;
+	pollset.active_fd.array = erts_realloc(ERTS_ALC_T_ACTIVE_FD_ARR,
+					       pollset.active_fd.array,
+					       sizeof(ErtsSysFdType)*size);
+	pollset.active_fd.size = size;
+#ifdef DEBUG
+	{
+	    int i;
+	    for (i = eix + 1; i < size; i++)
+		pollset.active_fd.array[i] = ERTS_SYS_FD_INVALID;
+	}
+#endif
+
+    }
+
+    pollset.active_fd.eix = eix;
+}
 
 int
 ERTS_CIO_EXPORT(driver_select)(ErlDrvPort ix,
@@ -489,6 +846,10 @@ ERTS_CIO_EXPORT(driver_select)(ErlDrvPort ix,
     ErtsDrvEventState *state;
     int wake_poller;
     int ret;
+#if ERTS_CIO_HAVE_DRV_EVENT
+    ErtsDrvEventDataState *free_event = NULL;
+#endif
+    ErtsDrvSelectDataState *free_select = NULL;
 #ifdef USE_VM_PROBES
     DTRACE_CHARBUF(name, 64);
 #endif
@@ -589,9 +950,9 @@ ERTS_CIO_EXPORT(driver_select)(ErlDrvPort ix,
     if (new_events & (ERTS_POLL_EV_ERR|ERTS_POLL_EV_NVAL)) {
 	if (state->type == ERTS_EV_TYPE_DRV_SEL && !state->events) {
 	    state->type = ERTS_EV_TYPE_NONE;
-	    state->flags = 0;
-	    erts_free(ERTS_ALC_T_DRV_SEL_D_STATE, state->driver.select);
-	    state->driver.select = NULL;
+	    state->flags &= ~ERTS_EV_FLAG_USED;
+	    state->driver.select->inport = NIL;
+	    state->driver.select->outport = NIL;
 	}
 	ret = -1;
 	goto done;
@@ -609,18 +970,10 @@ ERTS_CIO_EXPORT(driver_select)(ErlDrvPort ix,
     state->events = new_events;
     if (ctl_events) {
 	if (on) {
-	    if (state->type == ERTS_EV_TYPE_NONE) {
-		ErtsDrvSelectDataState *dsdsp
-		    = erts_alloc(ERTS_ALC_T_DRV_SEL_D_STATE,
-				 sizeof(ErtsDrvSelectDataState));
-		dsdsp->inport = NIL;
-		dsdsp->outport = NIL;
-		erts_port_task_handle_init(&dsdsp->intask);
-		erts_port_task_handle_init(&dsdsp->outtask);
-		ASSERT(state->driver.select == NULL);
-		state->driver.select = dsdsp;
+	    if (!state->driver.select)
+		state->driver.select = alloc_drv_select_data();
+	    if (state->type == ERTS_EV_TYPE_NONE)
 		state->type = ERTS_EV_TYPE_DRV_SEL;
-	    }
 	    ASSERT(state->type == ERTS_EV_TYPE_DRV_SEL);
 	    if (ctl_events & ERTS_POLL_EV_IN)
 		state->driver.select->inport = id;
@@ -641,17 +994,12 @@ ERTS_CIO_EXPORT(driver_select)(ErlDrvPort ix,
 		    state->driver.select->outport = NIL;
 		}
 		if (new_events == 0) {
-		    ASSERT(!erts_port_task_is_scheduled(&state->driver.select->intask));
-		    ASSERT(!erts_port_task_is_scheduled(&state->driver.select->outtask));
 		    if (old_events != 0) {
 			remember_removed(state, &pollset);
 		    }		    
 		    if ((mode & ERL_DRV_USE) || !(state->flags & ERTS_EV_FLAG_USED)) {
 			state->type = ERTS_EV_TYPE_NONE;
-			state->flags = 0;
-			erts_free(ERTS_ALC_T_DRV_SEL_D_STATE,
-				  state->driver.select);
-			state->driver.select = NULL;
+			state->flags &= ~ERTS_EV_FLAG_USED;
 		    }
 		    /*else keep it, as fd will probably be selected upon again */
 		}
@@ -682,13 +1030,15 @@ ERTS_CIO_EXPORT(driver_select)(ErlDrvPort ix,
    
     ret = 0;
 
-done:;
-#ifndef ERTS_SYS_CONTINOUS_FD_NUMBERS
-    if (state->type == ERTS_EV_TYPE_NONE && state->remove_cnt == 0) {
-	hash_erase_drv_ev_state(state);
-    }
+done:
+
+    check_fd_cleanup(state,
+#if ERTS_CIO_HAVE_DRV_EVENT
+		     &free_event,
 #endif
-done_unknown:    
+		     &free_select);
+
+done_unknown:
     erts_smp_mtx_unlock(fd_mtx(fd));
     if (stop_select_fn) {
 	int was_unmasked = erts_block_fpe();
@@ -696,6 +1046,12 @@ done_unknown:
 	(*stop_select_fn)(e, NULL);
 	erts_unblock_fpe(was_unmasked);
     }
+    if (free_select)
+	free_drv_select_data(free_select);
+#if ERTS_CIO_HAVE_DRV_EVENT
+    if (free_event)
+	free_drv_event_data(free_event);
+#endif
     return ret;
 }
 
@@ -715,6 +1071,10 @@ ERTS_CIO_EXPORT(driver_event)(ErlDrvPort ix,
     ErtsDrvEventState *state;
     int do_wake = 0;
     int ret;
+#if ERTS_CIO_HAVE_DRV_EVENT
+    ErtsDrvEventDataState *free_event;
+#endif
+    ErtsDrvSelectDataState *free_select;
     Port *prt = erts_drvport2port(ix);
 
     if (prt == ERTS_INVALID_ERL_DRV_PORT)
@@ -795,10 +1155,8 @@ ERTS_CIO_EXPORT(driver_event)(ErlDrvPort ix,
 	    state->driver.event->removed_events |= remove_events;
 	}
 	else {
-	    state->driver.event
-		= erts_alloc(ERTS_ALC_T_DRV_EV_D_STATE,
-			     sizeof(ErtsDrvEventDataState));
-	    erts_port_task_handle_init(&state->driver.event->task);
+	    if (!state->driver.event)
+		state->driver.event = alloc_drv_event_data();
 	    state->driver.event->port = id;
 	    state->driver.event->removed_events = (ErtsPollEvents) 0;
 	    state->type = ERTS_EV_TYPE_DRV_EV;
@@ -808,10 +1166,10 @@ ERTS_CIO_EXPORT(driver_event)(ErlDrvPort ix,
     else {
 	if (state->type == ERTS_EV_TYPE_DRV_EV) {
 	    abort_tasks(state, 0);
-	    erts_free(ERTS_ALC_T_DRV_EV_D_STATE,
-		      state->driver.event);
+	    state->driver.event->port = NIL;
+	    state->driver.event->data = NULL;
+	    state->driver.event->removed_events = (ErtsPollEvents) 0;
 	}
-	state->driver.select = NULL;
 	state->type = ERTS_EV_TYPE_NONE;
 	remember_removed(state, &pollset);
     }
@@ -821,12 +1179,22 @@ ERTS_CIO_EXPORT(driver_event)(ErlDrvPort ix,
     ret = 0;
 
 done:
-#ifndef ERTS_SYS_CONTINOUS_FD_NUMBERS
-    if (state->type == ERTS_EV_TYPE_NONE && state->remove_cnt == 0) {
-	hash_erase_drv_ev_state(state);
-    }
+
+    check_fd_cleanup(state,
+#if ERTS_CIO_HAVE_DRV_EVENT
+		     &free_event,
 #endif
+		     &free_select);
+
     erts_smp_mtx_unlock(fd_mtx(fd));
+
+    if (free_select)
+	free_drv_select_data(free_select);
+#if ERTS_CIO_HAVE_DRV_EVENT
+    if (free_event)
+	free_drv_event_data(free_event);
+#endif
+
     return ret;
 #endif
 }
@@ -1019,7 +1387,7 @@ steal_pending_stop_select(erts_dsprintf_buf_t *dsbufp, ErlDrvPort ix,
 	 * In either case stop_select should not be called.
 	 */	    
 	state->type = ERTS_EV_TYPE_NONE;
-	state->flags = 0;
+	state->flags &= ~ERTS_EV_FLAG_USED;
 	if (state->driver.drv_ptr->handle) {
 	    erts_ddll_dereference_driver(state->driver.drv_ptr->handle);
 	}
@@ -1091,38 +1459,103 @@ event_large_fd_error(ErlDrvPort ix, ErtsSysFdType fd, ErlDrvEventData event_data
 #endif
 #endif
 
+static ERTS_INLINE int
+io_task_schedule_allowed(ErtsDrvEventState *state,
+			 ErtsPortTaskType type,
+			 erts_aint_t current_cio_time)
+{
+    ErtsIoTask *io_task;
+
+    switch (type) {
+    case ERTS_PORT_TASK_INPUT:
+	if (!state->driver.select)
+	    return 0;
+#if ERTS_CIO_HAVE_DRV_EVENT
+	if (state->driver.event)
+	    return 0;
+#endif
+	io_task = &state->driver.select->iniotask;
+	break;
+    case ERTS_PORT_TASK_OUTPUT:
+	if (!state->driver.select)
+	    return 0;
+#if ERTS_CIO_HAVE_DRV_EVENT
+	if (state->driver.event)
+	    return 0;
+#endif
+	io_task = &state->driver.select->outiotask;
+	break;
+#if ERTS_CIO_HAVE_DRV_EVENT
+    case ERTS_PORT_TASK_EVENT:
+	if (!state->driver.event)
+	    return 0;
+	if (state->driver.select)
+	    return 0;
+	io_task = &state->driver.event->iotask;
+	break;
+#endif
+    default:
+	ERTS_INTERNAL_ERROR("Invalid I/O-task type");
+	return 0;
+    }
+
+    return !is_iotask_active(io_task, current_cio_time);
+}
+
 static ERTS_INLINE void
-iready(Eterm id, ErtsDrvEventState *state)
+iready(Eterm id, ErtsDrvEventState *state, erts_aint_t current_cio_time)
 {
-    if (erts_port_task_schedule(id,
-				&state->driver.select->intask,
-				ERTS_PORT_TASK_INPUT,
-				(ErlDrvEvent) state->fd) != 0) {
-	stale_drv_select(id, state, ERL_DRV_READ);
+    if (io_task_schedule_allowed(state,
+				 ERTS_PORT_TASK_INPUT,
+				 current_cio_time)) {
+	ErtsIoTask *iotask = &state->driver.select->iniotask;
+	erts_smp_atomic_set_nob(&iotask->executed_time, current_cio_time);
+	if (erts_port_task_schedule(id,
+				    &iotask->task,
+				    ERTS_PORT_TASK_INPUT,
+				    (ErlDrvEvent) state->fd) != 0) {
+	    stale_drv_select(id, state, ERL_DRV_READ);
+	}
+	add_active_fd(state->fd);
     }
 }
 
 static ERTS_INLINE void
-oready(Eterm id, ErtsDrvEventState *state)
+oready(Eterm id, ErtsDrvEventState *state, erts_aint_t current_cio_time)
 {
-    if (erts_port_task_schedule(id,
-				&state->driver.select->outtask,
-				ERTS_PORT_TASK_OUTPUT,
-				(ErlDrvEvent) state->fd) != 0) {
-	stale_drv_select(id, state, ERL_DRV_WRITE);
+    if (io_task_schedule_allowed(state,
+				 ERTS_PORT_TASK_OUTPUT,
+				 current_cio_time)) {
+	ErtsIoTask *iotask = &state->driver.select->outiotask;
+	erts_smp_atomic_set_nob(&iotask->executed_time, current_cio_time);
+	if (erts_port_task_schedule(id,
+				    &iotask->task,
+				    ERTS_PORT_TASK_OUTPUT,
+				    (ErlDrvEvent) state->fd) != 0) {
+	    stale_drv_select(id, state, ERL_DRV_WRITE);
+	}
+	add_active_fd(state->fd);
     }
 }
 
 #if ERTS_CIO_HAVE_DRV_EVENT
 static ERTS_INLINE void
-eready(Eterm id, ErtsDrvEventState *state, ErlDrvEventData event_data)
+eready(Eterm id, ErtsDrvEventState *state, ErlDrvEventData event_data,
+       erts_aint_t current_cio_time)
 {
-    if (erts_port_task_schedule(id,
-				&state->driver.event->task,
-				ERTS_PORT_TASK_EVENT,
-				(ErlDrvEvent) state->fd,
-				event_data) != 0) {
-	stale_drv_select(id, state, 0);
+    if (io_task_schedule_allowed(state,
+				 ERTS_PORT_TASK_EVENT,
+				 current_cio_time)) {
+	ErtsIoTask *iotask = &state->driver.event->iotask;
+	erts_smp_atomic_set_nob(&iotask->executed_time, current_cio_time);
+	if (erts_port_task_schedule(id,
+				    &iotask->task,
+				    ERTS_PORT_TASK_EVENT,
+				    (ErlDrvEvent) state->fd,
+				    event_data) != 0) {
+	    stale_drv_select(id, state, 0);
+	}
+	add_active_fd(state->fd);
     }
 }
 #endif
@@ -1153,10 +1586,11 @@ ERTS_CIO_EXPORT(erts_check_io_interrupt_timed)(int set,
 void
 ERTS_CIO_EXPORT(erts_check_io)(int do_wait)
 {
-    ErtsPollResFd pollres[256];
+    ErtsPollResFd *pollres;
     int pollres_len;
     SysTimeval wait_time;
     int poll_ret, i;
+    erts_aint_t current_cio_time;
 
  restart:
 
@@ -1173,10 +1607,24 @@ ERTS_CIO_EXPORT(erts_check_io)(int do_wait)
 	wait_time.tv_usec = 0;
     }
 
+    /*
+     * No need for an atomic inc op when incrementing
+     * erts_check_io_time, since only one thread can
+     * check io at a time.
+     */
+    current_cio_time = erts_smp_atomic_read_dirty(&erts_check_io_time);
+    current_cio_time++;
+    erts_smp_atomic_set_relb(&erts_check_io_time, current_cio_time);
+
+    check_cleanup_active_fds(current_cio_time);
+
 #ifdef ERTS_ENABLE_LOCK_CHECK
     erts_lc_check_exact(NULL, 0); /* No locks should be locked */
 #endif
-    pollres_len = sizeof(pollres)/sizeof(ErtsPollResFd);
+
+    pollres_len = erts_smp_atomic32_read_dirty(&pollset.active_fd.no) + ERTS_CHECK_IO_POLL_RES_LEN;
+
+    pollres = erts_alloc(ERTS_ALC_T_TMP, sizeof(ErtsPollResFd)*pollres_len);
 
     erts_smp_atomic_set_nob(&pollset.in_poll_wait, 1);
 
@@ -1196,6 +1644,7 @@ ERTS_CIO_EXPORT(erts_check_io)(int do_wait)
     if (poll_ret != 0) {
 	erts_smp_atomic_set_nob(&pollset.in_poll_wait, 0);
 	forget_removed(&pollset);
+	erts_free(ERTS_ALC_T_TMP, pollres);
 	if (poll_ret == EAGAIN) {
 	    goto restart;
 	}
@@ -1255,15 +1704,15 @@ ERTS_CIO_EXPORT(erts_check_io)(int do_wait)
 		if ((revents & ERTS_POLL_EV_IN)
 		    || (!(revents & ERTS_POLL_EV_OUT)
 			&& state->events & ERTS_POLL_EV_IN)) {
-		    iready(state->driver.select->inport, state);
+		    iready(state->driver.select->inport, state, current_cio_time);
 		}
 		else if (state->events & ERTS_POLL_EV_OUT) {
-		    oready(state->driver.select->outport, state);
+		    oready(state->driver.select->outport, state, current_cio_time);
 		}
 	    }
 	    else if (revents & (ERTS_POLL_EV_IN|ERTS_POLL_EV_OUT)) {
 		if (revents & ERTS_POLL_EV_OUT) {
-		    oready(state->driver.select->outport, state);
+		    oready(state->driver.select->outport, state, current_cio_time);
 		}
 		/* Someone might have deselected input since revents
 		   was read (true also on the non-smp emulator since
@@ -1271,7 +1720,7 @@ ERTS_CIO_EXPORT(erts_check_io)(int do_wait)
 		   revents... */
 		revents &= ~(~state->events & ERTS_POLL_EV_IN);
 		if (revents & ERTS_POLL_EV_IN) {
-		    iready(state->driver.select->inport, state);
+		    iready(state->driver.select->inport, state, current_cio_time);
 		}
 	    }
 	    else if (revents & ERTS_POLL_EV_NVAL) {
@@ -1279,6 +1728,7 @@ ERTS_CIO_EXPORT(erts_check_io)(int do_wait)
 				  state->driver.select->inport,
 				  state->driver.select->outport,
 				  state->events);
+		add_active_fd(state->fd);
 	    }
 	    break;
 	}
@@ -1296,8 +1746,7 @@ ERTS_CIO_EXPORT(erts_check_io)(int do_wait)
 	    if (revents) {
 		event_data->events = state->events;
 		event_data->revents = revents;
-
-		eready(state->driver.event->port, state, event_data);
+		eready(state->driver.event->port, state, event_data, current_cio_time);
 	    }
 	    break;
 	}
@@ -1315,6 +1764,7 @@ ERTS_CIO_EXPORT(erts_check_io)(int do_wait)
 			  (int) state->type);
 	    ASSERT(0);
 	    deselect(state, 0);
+	    add_active_fd(state->fd);
 	    break;
 	}
 	}
@@ -1326,6 +1776,7 @@ ERTS_CIO_EXPORT(erts_check_io)(int do_wait)
     }
 
     erts_smp_atomic_set_nob(&pollset.in_poll_wait, 0);
+    erts_free(ERTS_ALC_T_TMP, pollres);
     forget_removed(&pollset);
 }
 
@@ -1440,10 +1891,27 @@ static void drv_ev_state_free(void *des)
 void
 ERTS_CIO_EXPORT(erts_init_check_io)(void)
 {
+    erts_smp_atomic_init_nob(&erts_check_io_time, 0);
     erts_smp_atomic_init_nob(&pollset.in_poll_wait, 0);
+
     ERTS_CIO_POLL_INIT();
     pollset.ps = ERTS_CIO_NEW_POLLSET();
 
+    pollset.active_fd.six = 0;
+    pollset.active_fd.eix = 0;
+    erts_smp_atomic32_init_nob(&pollset.active_fd.no, 0);
+    pollset.active_fd.size = ERTS_ACTIVE_FD_INC;
+    pollset.active_fd.array = erts_alloc(ERTS_ALC_T_ACTIVE_FD_ARR,
+					 sizeof(ErtsSysFdType)*ERTS_ACTIVE_FD_INC);
+#ifdef DEBUG
+    {
+	int i;
+	for (i = 0; i < ERTS_ACTIVE_FD_INC; i++)
+	    pollset.active_fd.array[i] = ERTS_SYS_FD_INVALID;
+    }
+#endif
+
+
 #ifdef ERTS_SMP
     init_removed_fd_alloc();
     pollset.removed_list = NULL;
@@ -1519,12 +1987,27 @@ Eterm
 ERTS_CIO_EXPORT(erts_check_io_info)(void *proc)
 {
     Process *p = (Process *) proc;
-    Eterm tags[15], values[15], res;
+    Eterm tags[16], values[16], res;
     Uint sz, *szp, *hp, **hpp, memory_size;
     Sint i;
     ErtsPollInfo pi;
-    
-    ERTS_CIO_POLL_INFO(pollset.ps, &pi);
+    erts_aint_t cio_time = erts_smp_atomic_read_acqb(&erts_check_io_time);
+    int active_fds = (int) erts_smp_atomic32_read_acqb(&pollset.active_fd.no);
+
+    while (1) {
+	erts_aint_t post_cio_time;
+	int post_active_fds;
+
+	ERTS_CIO_POLL_INFO(pollset.ps, &pi);
+
+	post_cio_time = erts_smp_atomic_read_mb(&erts_check_io_time);
+	post_active_fds = (int) erts_smp_atomic32_read_acqb(&pollset.active_fd.no);
+	if (cio_time == post_cio_time && active_fds == post_active_fds)
+	    break;
+	cio_time = post_cio_time;
+	active_fds = post_active_fds;
+    }
+
     memory_size = pi.memory_size;
 #ifdef ERTS_SYS_CONTINOUS_FD_NUMBERS
     memory_size += sizeof(ErtsDrvEventState) * erts_smp_atomic_read_nob(&drv_ev_state_len);
@@ -1588,6 +2071,9 @@ ERTS_CIO_EXPORT(erts_check_io_info)(void *proc)
     tags[i] = erts_bld_atom(hpp, szp, "max_fds");
     values[i++] = erts_bld_uint(hpp, szp, (Uint) pi.max_fds);
 
+    tags[i] = erts_bld_atom(hpp, szp, "active_fds");
+    values[i++] = erts_bld_uint(hpp, szp, (Uint) active_fds);
+
 #ifdef ERTS_POLL_COUNT_AVOIDED_WAKEUPS
     tags[i] = erts_bld_atom(hpp, szp, "no_avoided_wakeups");
     values[i++] = erts_bld_uint(hpp, szp, (Uint) pi.no_avoided_wakeups);
@@ -1642,6 +2128,8 @@ print_events(ErtsPollEvents ev)
 typedef struct {
     int used_fds;
     int num_errors;
+    int no_driver_select_structs;
+    int no_driver_event_structs;
 #ifdef ERTS_SYS_CONTINOUS_FD_NUMBERS
     int internal_fds;
     ErtsPollEvents *epep;
@@ -1664,6 +2152,13 @@ static void doit_erts_check_io_debug(void *vstate, void *vcounters)
     struct stat stat_buf;
 #endif
 
+    if (state->driver.select)
+	counters->no_driver_select_structs++;
+#if ERTS_CIO_HAVE_DRV_EVENT
+    if (state->driver.event)
+	counters->no_driver_event_structs++;
+#endif
+
 #ifdef ERTS_SYS_CONTINOUS_FD_NUMBERS
     if (state->events || ep_events) {
 	if (ep_events & ERTS_POLL_EV_NVAL) {
@@ -1802,6 +2297,7 @@ static void doit_erts_check_io_debug(void *vstate, void *vcounters)
 		}
 	    }
 	}
+#if ERTS_CIO_HAVE_DRV_EVENT
 	else if (state->type == ERTS_EV_TYPE_DRV_EV) {
 	    Eterm id;
 	    erts_printf("driver_event ");
@@ -1837,6 +2333,7 @@ static void doit_erts_check_io_debug(void *vstate, void *vcounters)
 		erts_free_port_names(pnp);
 	    }
 	}
+#endif
 #ifdef ERTS_SYS_CONTINOUS_FD_NUMBERS
 	else if (internal) {
 	    erts_printf("internal ");
@@ -1876,7 +2373,7 @@ static void doit_erts_check_io_debug(void *vstate, void *vcounters)
 }
     
 int
-ERTS_CIO_EXPORT(erts_check_io_debug)(void)
+ERTS_CIO_EXPORT(erts_check_io_debug)(ErtsCheckIoDebugInfo *ciodip)
 {
 #ifdef ERTS_SYS_CONTINOUS_FD_NUMBERS
     int fd, len;
@@ -1885,6 +2382,10 @@ ERTS_CIO_EXPORT(erts_check_io_debug)(void)
     ErtsDrvEventState null_des;
 
     null_des.driver.select = NULL;
+#if ERTS_CIO_HAVE_DRV_EVENT
+    null_des.driver.event = NULL;
+#endif
+    null_des.driver.drv_ptr = NULL;
     null_des.events = 0;
     null_des.remove_cnt = 0;
     null_des.type = ERTS_EV_TYPE_NONE;
@@ -1904,6 +2405,8 @@ ERTS_CIO_EXPORT(erts_check_io_debug)(void)
 #endif
     counters.used_fds = 0;
     counters.num_errors = 0;
+    counters.no_driver_select_structs = 0;
+    counters.no_driver_event_structs = 0;
 
 #ifdef ERTS_SYS_CONTINOUS_FD_NUMBERS
     len = erts_smp_atomic_read_nob(&drv_ev_state_len);
@@ -1920,8 +2423,16 @@ ERTS_CIO_EXPORT(erts_check_io_debug)(void)
 
     erts_smp_thr_progress_unblock();
 
+    ciodip->no_used_fds = counters.used_fds;
+    ciodip->no_driver_select_structs = counters.no_driver_select_structs;
+    ciodip->no_driver_event_structs = counters.no_driver_event_structs;
+
     erts_printf("\n");
     erts_printf("used fds=%d\n", counters.used_fds);
+    erts_printf("Number of driver_select() structures=%d\n", counters.no_driver_select_structs);
+#if ERTS_CIO_HAVE_DRV_EVENT
+    erts_printf("Number of driver_event() structures=%d\n", counters.no_driver_event_structs);
+#endif
 #ifdef ERTS_SYS_CONTINOUS_FD_NUMBERS
     erts_printf("internal fds=%d\n", counters.internal_fds);
 #endif
@@ -1930,6 +2441,7 @@ ERTS_CIO_EXPORT(erts_check_io_debug)(void)
 #ifdef ERTS_SYS_CONTINOUS_FD_NUMBERS
     erts_free(ERTS_ALC_T_TMP, (void *) counters.epep);
 #endif
+
     return counters.num_errors;
 }
 
diff --git a/erts/emulator/sys/common/erl_check_io.h b/erts/emulator/sys/common/erl_check_io.h
index edab7947ba..d01297d55c 100644
--- a/erts/emulator/sys/common/erl_check_io.h
+++ b/erts/emulator/sys/common/erl_check_io.h
@@ -26,6 +26,7 @@
 #ifndef ERL_CHECK_IO_H__
 #define ERL_CHECK_IO_H__
 
+#include "sys.h"
 #include "erl_sys_driver.h"
 
 #ifdef ERTS_ENABLE_KERNEL_POLL
@@ -52,8 +53,8 @@ void erts_check_io_kp(int);
 void erts_check_io_nkp(int);
 void erts_init_check_io_kp(void);
 void erts_init_check_io_nkp(void);
-int erts_check_io_debug_kp(void);
-int erts_check_io_debug_nkp(void);
+int erts_check_io_debug_kp(ErtsCheckIoDebugInfo *);
+int erts_check_io_debug_nkp(ErtsCheckIoDebugInfo *);
 
 #else /* !ERTS_ENABLE_KERNEL_POLL */
 
@@ -70,6 +71,27 @@ void erts_init_check_io(void);
 
 #endif
 
+extern erts_smp_atomic_t erts_check_io_time;
+
+typedef struct {
+    ErtsPortTaskHandle task;
+    erts_smp_atomic_t executed_time;
+} ErtsIoTask;
+
+ERTS_GLB_INLINE void erts_io_notify_port_task_executed(ErtsPortTaskHandle *pthp);
+
+#if ERTS_GLB_INLINE_INCL_FUNC_DEF
+
+ERTS_GLB_INLINE void
+erts_io_notify_port_task_executed(ErtsPortTaskHandle *pthp)
+{
+    ErtsIoTask *itp = (ErtsIoTask *) (((char *) pthp) - offsetof(ErtsIoTask, task));
+    erts_aint_t ci_time = erts_smp_atomic_read_acqb(&erts_check_io_time);
+    erts_smp_atomic_set_relb(&itp->executed_time, ci_time);
+}
+
+#endif
+
 #endif /*  ERL_CHECK_IO_H__ */
 
 #if !defined(ERL_CHECK_IO_C__) && !defined(ERTS_ALLOC_C__)
@@ -81,6 +103,16 @@ void erts_init_check_io(void);
 #include "erl_poll.h"
 #include "erl_port_task.h"
 
+#ifdef __WIN32__
+/*
+ * Current erts_poll implementation for Windows cannot handle
+ * active events in the set of events polled.
+ */
+#  define ERTS_CIO_DEFER_ACTIVE_EVENTS 1
+#else
+#  define ERTS_CIO_DEFER_ACTIVE_EVENTS 0
+#endif
+
 /*
  * ErtsDrvEventDataState is used by driver_event() which is almost never
  * used. We allocate ErtsDrvEventDataState separate since we dont wan't
@@ -91,13 +123,16 @@ typedef struct {
     Eterm port;
     ErlDrvEventData data;
     ErtsPollEvents removed_events;
-    ErtsPortTaskHandle task;
+#if ERTS_CIO_DEFER_ACTIVE_EVENTS
+    ErtsPollEvents deferred_events;
+#endif
+    ErtsIoTask iotask;
 } ErtsDrvEventDataState;
 
 typedef struct {
     Eterm inport;
     Eterm outport;
-    ErtsPortTaskHandle intask;
-    ErtsPortTaskHandle outtask;
+    ErtsIoTask iniotask;
+    ErtsIoTask outiotask;
 } ErtsDrvSelectDataState;
 #endif /* #ifndef ERL_CHECK_IO_INTERNAL__ */
diff --git a/erts/emulator/sys/common/erl_sys_common_misc.c b/erts/emulator/sys/common/erl_sys_common_misc.c
index 31ad3b82d5..c5e804a62d 100644
--- a/erts/emulator/sys/common/erl_sys_common_misc.c
+++ b/erts/emulator/sys/common/erl_sys_common_misc.c
@@ -44,6 +44,14 @@
 #endif
 #endif
 
+/*
+ * erts_check_io_time is used by the erl_check_io implementation. The
+ * global erts_check_io_time variable is declared here since there
+ * (often) exist two versions of erl_check_io (kernel-poll and
+ * non-kernel-poll), and we dont want two versions of this variable.
+ */
+erts_smp_atomic_t erts_check_io_time;
+
 /* Written once and only once */
 
 static int filename_encoding = ERL_FILENAME_UNKNOWN;
diff --git a/erts/emulator/sys/unix/erl_unix_sys.h b/erts/emulator/sys/unix/erl_unix_sys.h
index 2c47aa06c2..2c86bf5fd1 100644
--- a/erts/emulator/sys/unix/erl_unix_sys.h
+++ b/erts/emulator/sys/unix/erl_unix_sys.h
@@ -130,9 +130,6 @@
 /* File descriptors are numbers anc consecutively allocated on Unix */
 #define  ERTS_SYS_CONTINOUS_FD_NUMBERS
 
-#define HAVE_ERTS_CHECK_IO_DEBUG
-int erts_check_io_debug(void);
-
 #ifndef ERTS_SMP
 #  undef ERTS_POLL_NEED_ASYNC_INTERRUPT_SUPPORT
 #  define ERTS_POLL_NEED_ASYNC_INTERRUPT_SUPPORT
diff --git a/erts/emulator/sys/unix/sys.c b/erts/emulator/sys/unix/sys.c
index 61f9f6a59a..ada62f71f9 100644
--- a/erts/emulator/sys/unix/sys.c
+++ b/erts/emulator/sys/unix/sys.c
@@ -277,7 +277,7 @@ struct {
     void (*check_io)(int);
     Uint (*size)(void);
     Eterm (*info)(void *);
-    int (*check_io_debug)(void);
+    int (*check_io_debug)(ErtsCheckIoDebugInfo *);
 } io_func = {0};
 
 
@@ -299,9 +299,9 @@ Eterm erts_check_io_info(void *p)
 }
 
 int
-erts_check_io_debug(void)
+erts_check_io_debug(ErtsCheckIoDebugInfo *ip)
 {
-    return (*io_func.check_io_debug)();
+    return (*io_func.check_io_debug)(ip);
 }
 
 
diff --git a/erts/emulator/sys/win32/erl_poll.c b/erts/emulator/sys/win32/erl_poll.c
index 7a1d129cd5..972170d465 100644
--- a/erts/emulator/sys/win32/erl_poll.c
+++ b/erts/emulator/sys/win32/erl_poll.c
@@ -1085,7 +1085,7 @@ void erts_poll_controlv(ErtsPollSet ps,
 					 pcev[i].events,
 					 pcev[i].on);
     }
-    ERTS_POLLSET_LOCK(ps);
+    ERTS_POLLSET_UNLOCK(ps);
     HARDTRACEF(("Out erts_poll_controlv"));
 }
 
diff --git a/erts/emulator/sys/win32/erl_win_sys.h b/erts/emulator/sys/win32/erl_win_sys.h
index 0deb097b1a..74b15d3534 100644
--- a/erts/emulator/sys/win32/erl_win_sys.h
+++ b/erts/emulator/sys/win32/erl_win_sys.h
@@ -107,12 +107,10 @@
 /*
  * Our own type of "FD's"
  */
+#define ERTS_SYS_FD_INVALID INVALID_HANDLE_VALUE
 #define ERTS_SYS_FD_TYPE HANDLE
 #define NO_FSTAT_ON_SYS_FD_TYPE 1 /* They are events, not files */
 
-#define HAVE_ERTS_CHECK_IO_DEBUG
-int erts_check_io_debug(void);
-
 /*
  * For erl_time_sup
  */
diff --git a/erts/emulator/test/a_SUITE.erl b/erts/emulator/test/a_SUITE.erl
index 195c9c0a5f..17579be416 100644
--- a/erts/emulator/test/a_SUITE.erl
+++ b/erts/emulator/test/a_SUITE.erl
@@ -97,23 +97,13 @@ display_check_io(ChkIo) ->
     catch erlang:display('--- CHECK IO INFO ---'),
     catch erlang:display(ChkIo),
     catch erts_debug:set_internal_state(available_internal_state, true),
-    NoOfErrorFds = (catch erts_debug:get_internal_state(check_io_debug)),
+    NoOfErrorFds = (catch element(1, erts_debug:get_internal_state(check_io_debug))),
     catch erlang:display({'NoOfErrorFds', NoOfErrorFds}),
     catch erts_debug:set_internal_state(available_internal_state, false),
     catch erlang:display('--- CHECK IO INFO ---'),
     ok.
 
 get_check_io_info() ->
-    ChkIo = erlang:system_info(check_io),
-    case lists:keysearch(pending_updates, 1, ChkIo) of
-	{value, {pending_updates, 0}} ->
-	    display_check_io(ChkIo),
-	    ChkIo;
-	false ->
-	    ChkIo;
-	_ ->
-	    receive after 10 -> ok end,
-	    get_check_io_info()
-    end.
+    z_SUITE:get_check_io_info().
 
 
diff --git a/erts/emulator/test/driver_SUITE.erl b/erts/emulator/test/driver_SUITE.erl
index 7087542899..269e50f3ce 100644
--- a/erts/emulator/test/driver_SUITE.erl
+++ b/erts/emulator/test/driver_SUITE.erl
@@ -31,8 +31,9 @@
 	 end_per_suite/1, init_per_group/2,end_per_group/2,
 	 init_per_testcase/2,
 	 end_per_testcase/2,
+
+	 a_test/1,
 	 outputv_echo/1,
-	
 	 timer_measure/1,
 	 timer_cancel/1,
 	 timer_change/1,
@@ -79,7 +80,8 @@
 	 thr_free_drv/1,
 	 async_blast/1,
 	 thr_msg_blast/1,
-	 consume_timeslice/1]).
+	 consume_timeslice/1,
+	 z_test/1]).
 
 -export([bin_prefix/2]).
 
@@ -122,19 +124,19 @@ init_per_testcase(Case, Config) when is_atom(Case), is_list(Config) ->
 	_ -> erts_debug:set_internal_state(available_internal_state, true)
     end,
     erlang:display({init_per_testcase, Case}),
-    ?line 0 = erts_debug:get_internal_state(check_io_debug),
+    ?line 0 = element(1, erts_debug:get_internal_state(check_io_debug)),
     [{watchdog, Dog},{testcase, Case}|Config].
 
 end_per_testcase(Case, Config) ->
     Dog = ?config(watchdog, Config),
     erlang:display({end_per_testcase, Case}),
-    ?line 0 = erts_debug:get_internal_state(check_io_debug),
+    ?line 0 = element(1, erts_debug:get_internal_state(check_io_debug)),
     ?t:timetrap_cancel(Dog).
 
 suite() -> [{ct_hooks,[ts_install_cth]}].
 
-all() -> 
-    [outputv_errors, outputv_echo, queue_echo, {group, timer},
+all() -> %% Keep a_test first and z_test last...
+    [a_test, outputv_errors, outputv_echo, queue_echo, {group, timer},
      driver_unloaded, io_ready_exit, use_fallback_pollset,
      bad_fd_in_pollset, driver_event, fd_change,
      steal_control, otp_6602, driver_system_info_base_ver,
@@ -151,7 +153,8 @@ all() ->
      thr_free_drv,
      async_blast,
      thr_msg_blast,
-     consume_timeslice].
+     consume_timeslice,
+     z_test].
 
 groups() -> 
     [{timer, [],
@@ -917,8 +920,7 @@ steal_control_test(Hndl = {erts_poll_info, Before}) ->
 	  end.
 
 chkio_test_init(Config) when is_list(Config) ->
-    ?line wait_until_no_pending_updates(),
-    ?line ChkIo = erlang:system_info(check_io),
+    ?line ChkIo = get_stable_check_io_info(),
     ?line case catch lists:keysearch(name, 1, ChkIo) of
 	      {value, {name, erts_poll}} ->
 		  ?line ?t:format("Before test: ~p~n", [ChkIo]),
@@ -937,8 +939,7 @@ chkio_test_fini({skipped, _} = Res) ->
 chkio_test_fini({chkio_test_result, Res, Before}) ->
     ?line ok = erl_ddll:unload_driver('chkio_drv'),
     ?line ok = erl_ddll:stop(),
-    ?line wait_until_no_pending_updates(),
-    ?line After = erlang:system_info(check_io),
+    ?line After = get_stable_check_io_info(),
     ?line ?t:format("After test: ~p~n", [After]),
     ?line verify_chkio_state(Before, After),
     ?line Res.
@@ -985,7 +986,7 @@ chkio_test({erts_poll_info, Before},
 		  ?line Fun(),
 		  ?line During = erlang:system_info(check_io),
 		  ?line erlang:display(During),
-		  ?line 0 = erts_debug:get_internal_state(check_io_debug),
+		  ?line 0 = element(1, erts_debug:get_internal_state(check_io_debug)),
 		  ?line ?t:format("During test: ~p~n", [During]),
 		  ?line chk_chkio_port(Port),
 		  ?line case erlang:port_control(Port, ?CHKIO_STOP, "") of
@@ -1034,18 +1035,22 @@ verify_chkio_state(Before, After) ->
 						       After)
 	  end,
     ?line ok.
-    
-    
 
-wait_until_no_pending_updates() ->
-    case lists:keysearch(pending_updates, 1, erlang:system_info(check_io)) of
-	{value, {pending_updates, 0}} ->
-	    ok;
-	false ->
-	    ok;
+get_stable_check_io_info() ->
+    ChkIo = erlang:system_info(check_io),
+    PendUpdNo = case lists:keysearch(pending_updates, 1, ChkIo) of
+		    {value, {pending_updates, PendNo}} ->
+			PendNo;
+		    false ->
+			0
+		end,
+    {value, {active_fds, ActFds}} = lists:keysearch(active_fds, 1, ChkIo),
+    case {PendUpdNo, ActFds} of
+	{0, 0} ->
+	    ChkIo;
 	_ ->
 	    receive after 10 -> ok end,
-	    wait_until_no_pending_updates()
+	    get_stable_check_io_info()
     end.
 
 otp_6602(doc) -> ["Missed port lock when stealing control of fd from a "
@@ -2374,10 +2379,25 @@ count_proc_sched(Ps, PNs) ->
 	    PNs
     end.
     
+a_test(Config) when is_list(Config) ->
+    check_io_debug().
+
+z_test(Config) when is_list(Config) ->
+    check_io_debug().
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 %% 		Utilities
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
+check_io_debug() ->
+    get_stable_check_io_info(),
+    {NoErrorFds, NoUsedFds, NoDrvSelStructs, NoDrvEvStructs}
+	= erts_debug:get_internal_state(check_io_debug),
+    0 = NoErrorFds,
+    NoUsedFds = NoDrvSelStructs,
+    0 = NoDrvEvStructs,
+    ok.
+
 %flush_msgs() ->
 %    receive
 %	M ->
diff --git a/erts/emulator/test/z_SUITE.erl b/erts/emulator/test/z_SUITE.erl
index 4b3075a164..b0c6224dfe 100644
--- a/erts/emulator/test/z_SUITE.erl
+++ b/erts/emulator/test/z_SUITE.erl
@@ -38,7 +38,7 @@
 
 -export([schedulers_alive/1, node_container_refc_check/1,
 	 long_timers/1, pollset_size/1,
-	 check_io_debug/1]).
+	 check_io_debug/1, get_check_io_info/0]).
 
 -define(DEFAULT_TIMEOUT, ?t:minutes(5)).
 
@@ -288,11 +288,14 @@ check_io_debug(Config) when is_list(Config) ->
 	  end.
 
 check_io_debug_test() ->
+    ?line erlang:display(get_check_io_info()),
     ?line erts_debug:set_internal_state(available_internal_state, true),
-    ?line erlang:display(erlang:system_info(check_io)),
-    ?line NoOfErrorFds = erts_debug:get_internal_state(check_io_debug),
+    ?line {NoErrorFds, NoUsedFds, NoDrvSelStructs, NoDrvEvStructs}
+	= erts_debug:get_internal_state(check_io_debug),
     ?line erts_debug:set_internal_state(available_internal_state, false),
-    ?line 0 = NoOfErrorFds,
+    ?line 0 = NoErrorFds,
+    ?line NoUsedFds = NoDrvSelStructs,
+    ?line 0 = NoDrvEvStructs,
     ?line ok.
 
 
@@ -305,7 +308,7 @@ display_check_io(ChkIo) ->
     catch erlang:display('--- CHECK IO INFO ---'),
     catch erlang:display(ChkIo),
     catch erts_debug:set_internal_state(available_internal_state, true),
-    NoOfErrorFds = (catch erts_debug:get_internal_state(check_io_debug)),
+    NoOfErrorFds = (catch element(1, erts_debug:get_internal_state(check_io_debug))),
     catch erlang:display({'NoOfErrorFds', NoOfErrorFds}),
     catch erts_debug:set_internal_state(available_internal_state, false),
     catch erlang:display('--- CHECK IO INFO ---'),
@@ -313,14 +316,19 @@ display_check_io(ChkIo) ->
 
 get_check_io_info() ->
     ChkIo = erlang:system_info(check_io),
-    case lists:keysearch(pending_updates, 1, ChkIo) of
-	{value, {pending_updates, 0}} ->
+    PendUpdNo = case lists:keysearch(pending_updates, 1, ChkIo) of
+		    {value, {pending_updates, PendNo}} ->
+			PendNo;
+		    false ->
+			0
+		end,
+    {value, {active_fds, ActFds}} = lists:keysearch(active_fds, 1, ChkIo),
+    case {PendUpdNo, ActFds} of
+	{0, 0} ->
 	    display_check_io(ChkIo),
 	    ChkIo;
-	false ->
-	    ChkIo;
 	_ ->
-	    receive after 10 -> ok end,
+	    receive after 100 -> ok end,
 	    get_check_io_info()
     end.
 
diff --git a/erts/etc/common/erlexec.c b/erts/etc/common/erlexec.c
index f098e56a2e..9248b3844f 100644
--- a/erts/etc/common/erlexec.c
+++ b/erts/etc/common/erlexec.c
@@ -128,6 +128,7 @@ static char *pluss_val_switches[] = {
     "bwt",
     "cl",
     "ct",
+    "ecio",
     "fwi",
     "tbt",
     "wct",
diff --git a/erts/preloaded/ebin/erlang.beam b/erts/preloaded/ebin/erlang.beam
index 9dd826fa26..82e9de382a 100644
--- a/erts/preloaded/ebin/erlang.beam
+++ b/erts/preloaded/ebin/erlang.beam
diff --git a/erts/preloaded/src/erlang.erl b/erts/preloaded/src/erlang.erl
index 291926578e..58c08fcb34 100644
--- a/erts/preloaded/src/erlang.erl
+++ b/erts/preloaded/src/erlang.erl
@@ -2106,6 +2106,7 @@ tuple_to_list(_Tuple) ->
 	 (dynamic_trace) -> none | dtrace | systemtap;
          (dynamic_trace_probes) -> boolean();
          (elib_malloc) -> false;
+         (eager_check_io) -> boolean();
          (ets_limit) -> pos_integer();
          (fullsweep_after) -> {fullsweep_after, non_neg_integer()};
          (garbage_collection) -> [{atom(), integer()}];