The call to apr_explode_localtime() in mod_log_config is one of the more

expensive operations in the httpd.  This patch attempts to reduce the
overhead by caching the result for 15 seconds.

Submitted by:	Brian Pane <bpane@pacbell.net>
Reviewed by:	Cliff Woolley, Ryan Bloom, Dean Gaudet, Justin Erenkrantz


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91083 13f79535-47bb-0310-9956-ffa450edef68

1 files changed, 175 insertions, 0 deletions

diff --git a/server/util_time.c b/server/util_time.c
new file mode 100644
index 0000000000..3db076bcea
--- /dev/null
+++ b/server/util_time.c
@@ -0,0 +1,175 @@
+/* ====================================================================
+ * The Apache Software License, Version 1.1
+ *
+ * Copyright (c) 2001 The Apache Software Foundation.  All rights
+ * reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * 3. The end-user documentation included with the redistribution,
+ *    if any, must include the following acknowledgment:
+ *       "This product includes software developed by the
+ *        Apache Software Foundation (http://www.apache.org/)."
+ *    Alternately, this acknowledgment may appear in the software itself,
+ *    if and wherever such third-party acknowledgments normally appear.
+ *
+ * 4. The names "Apache" and "Apache Software Foundation" must
+ *    not be used to endorse or promote products derived from this
+ *    software without prior written permission. For written
+ *    permission, please contact apache@apache.org.
+ *
+ * 5. Products derived from this software may not be called "Apache",
+ *    nor may "Apache" appear in their name, without prior written
+ *    permission of the Apache Software Foundation.
+ *
+ * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
+ * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED.  IN NO EVENT SHALL THE APACHE SOFTWARE FOUNDATION OR
+ * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ *
+ */
+
+#include "util_time.h"
+
+/* Cache for exploded values of recent timestamps
+ */
+
+struct exploded_time_cache_element {
+    apr_int64_t t;
+    apr_exploded_time_t xt;
+    apr_int64_t t_validate; /* please see comments in cached_explode() */
+};
+
+/* the "+ 1" is for the current second: */
+#define TIME_CACHE_SIZE (AP_TIME_RECENT_THRESHOLD + 1)
+
+static struct exploded_time_cache_element exploded_cache_localtime[TIME_CACHE_SIZE];
+static struct exploded_time_cache_element exploded_cache_gmt[TIME_CACHE_SIZE];
+
+
+static apr_status_t cached_explode(apr_exploded_time_t *xt, apr_time_t t,
+                                   struct exploded_time_cache_element *cache,
+                                   int use_gmt)
+{
+    apr_int64_t seconds = t / APR_USEC_PER_SEC;
+    struct exploded_time_cache_element *cache_element =
+        &(cache[seconds % TIME_CACHE_SIZE]);
+    struct exploded_time_cache_element cache_element_snapshot;
+
+    /* The cache is implemented as a ring buffer.  Each second,
+     * it uses a different element in the buffer.  The timestamp
+     * in the element indicates whether the element contains the
+     * exploded time for the current second (vs the time
+     * 'now - AP_TIME_RECENT_THRESHOLD' seconds ago).  If the
+     * cached value is for the current time, we use it.  Otherwise,
+     * we compute the apr_exploded_time_t and store it in this
+     * cache element. Note that the timestamp in the cache
+     * element is updated only after the exploded time.  Thus
+     * if two threads hit this cache element simultaneously
+     * at the start of a new second, they'll both explode the
+     * time and store it.  I.e., the writers will collide, but
+     * they'll be writing the same value.
+     */
+    if (cache_element->t >= seconds) {
+        /* There is an intentional race condition in this design:
+         * in a multithreaded app, one thread might be reading
+         * from this cache_element to resolve a timestamp from
+         * TIME_CACHE_SIZE seconds ago at the same time that
+         * another thread is copying the exploded form of the
+         * current time into the same cache_element.  (I.e., the
+         * first thread might hit this element of the ring buffer
+         * just as the element is being recycled.)  This can
+         * also happen at the start of a new second, if a
+         * reader accesses the cache_element after a writer
+         * has updated cache_element.t but before the writer
+         * has finished updating the whole cache_element.
+         *
+         * Rather than trying to prevent this race condition
+         * with locks, we allow it to happen and then detect
+         * and correct it.  The detection works like this:
+         *   Step 1: Take a "snapshot" of the cache element by
+         *           copying it into a temporary buffer.
+         *   Step 2: Check whether the snapshot contains consistent
+         *           data: the timestamps at the start and end of
+         *           the cache_element should both match the 'seconds'
+         *           value that we computed from the input time.
+         *           If these three don't match, then the snapshot
+         *           shows the cache_element in the middle of an
+         *           update, and its contents are invalid.
+         *   Step 3: If the snapshot is valid, use it.  Otherwise,
+         *           just give up on the cache and explode the
+         *           input time.
+         */
+        memcpy(&cache_element_snapshot, cache_element,
+               sizeof(struct exploded_time_cache_element));
+        if ((seconds != cache_element_snapshot.t) ||
+            (seconds != cache_element_snapshot.t_validate)) {
+            /* Invalid snapshot */
+            if (use_gmt) {
+                return apr_explode_gmt(xt, t);
+            }
+            else {
+                return apr_explode_localtime(xt, t);
+            }
+        }
+        else {
+            /* Valid snapshot */
+            memcpy(xt, &(cache_element_snapshot.xt),
+                   sizeof(apr_exploded_time_t));
+        }
+    }
+    else {
+        apr_status_t r;
+        if (use_gmt) {
+            r = apr_explode_gmt(xt, t);
+        }
+        else {
+            r = apr_explode_localtime(xt, t);
+        }
+        if (!APR_STATUS_IS_SUCCESS(r)) {
+            return r;
+        }
+        cache_element->t = seconds;
+        memcpy(&(cache_element->xt), xt, sizeof(apr_exploded_time_t));
+        cache_element->t_validate = seconds;
+    }
+    xt->tm_usec = t % APR_USEC_PER_SEC;
+    return APR_SUCCESS;
+}
+
+
+AP_DECLARE(apr_status_t) ap_explode_recent_localtime(apr_exploded_time_t * tm,
+                                                     apr_time_t t)
+{
+    return cached_explode(tm, t, exploded_cache_localtime, 0);
+}
+
+AP_DECLARE(apr_status_t) ap_explode_recent_gmt(apr_exploded_time_t * tm,
+                                               apr_time_t t)
+{
+    return cached_explode(tm, t, exploded_cache_gmt, 1);
+}