path: root/doc/TODO.detail/mmap

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From: ncm@zembu.com (Nathan Myers)
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On Sun, Feb 25, 2001 at 11:28:46PM -0500, Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > It allows no backing store on disk.  

I.e. it allows you to map memory without an associated inode; the memory
may still be swapped.  Of course, there is no problem with mapping an 
inode too, so that unrelated processes can join in.  Solarix has a flag
to pin the shared pages in RAM so they can't be swapped out.

> > It is the BSD solution to SysV
> > share memory.  Here are all the BSDi flags:
> 
> >      MAP_ANON    Map anonymous memory not associated with any specific
> >                  file.  The file descriptor used for creating MAP_ANON
> >                  must be -1.  The offset parameter is ignored.
> 
> Hmm.  Now that I read down to the "nonstandard extensions" part of the
> HPUX man page for mmap(), I find
> 
>      If MAP_ANONYMOUS is set in flags:
> 
>           o    A new memory region is created and initialized to all zeros.
>                This memory region can be shared only with descendants of
>                the current process.

This is supported on Linux and BSD, but not on Solarix 7.  It's not 
necessary; you can just map /dev/zero on SysV systems that don't 
have MAP_ANON.

> While I've said before that I don't think it's really necessary for
> processes that aren't children of the postmaster to access the shared
> memory, I'm not sure that I want to go over to a mechanism that makes it
> *impossible* for that to be done.  Especially not if the only motivation
> is to avoid having to configure the kernel's shared memory settings.

There are enormous advantages to avoiding the need to configure kernel 
settings.  It makes PG a better citizen.  PG is much easier to drop in 
and use if you don't need attention from the IT department.

But I don't know of any reason to avoid mapping an actual inode,
so using mmap doesn't necessarily mean giving up sharing among
unrelated processes.

> Besides, what makes you think there's not a limit on the size of shmem
> allocatable via mmap()?

I've never seen any mmap limit documented.  Since mmap() is how 
everybody implements shared libraries, such a limit would be equivalent 
to a limit on how much/many shared libraries are used.  mmap() with 
MAP_ANONYMOUS (or its SysV /dev/zero equivalent) is a common, modern 
way to get raw storage for malloc(), so such a limit would be a limit
on malloc() too.

The mmap architecture comes to us from the Mach microkernel memory
manager, backported into BSD and then copied widely.  Since it was
the fundamental mechanism for all memory operations in Mach, arbitrary
limits would make no sense.  That it worked so well is the reason it 
was copied everywhere else, so adding arbitrary limits while copying 
it would be silly.  I don't think we'll see any systems like that.

Nathan Myers
ncm@zembu.com

From pgsql-hackers-owner+M6138@postgresql.org Mon Mar 19 07:57:59 2001
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Date: Mon, 19 Mar 2001 04:55:01 -0800
From: Alfred Perlstein <bright@wintelcom.net>
To: Rod Taylor <rod.taylor@inquent.com>
Cc: Hackers List <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Fw: [vorbis-dev] ogg123: shared memory by mmap()
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WOOT WOOT! DANGER WILL ROBINSON!

> ----- Original Message -----
> From: "Christian Weisgerber" <naddy@mips.inka.de>
> Newsgroups: list.vorbis.dev
> To: <vorbis-dev@xiph.org>
> Sent: Saturday, March 17, 2001 12:01 PM
> Subject: [vorbis-dev] ogg123: shared memory by mmap()
> 
> 
> > The patch below adds:
> >
> > - acinclude.m4:  A new macro A_FUNC_SMMAP to check that sharing
> pages
> >   through mmap() works.  This is taken from Joerg Schilling's star.
> > - configure.in:  A_FUNC_SMMAP
> > - ogg123/buffer.c:  If we have a working mmap(), use it to create
> >   a region of shared memory instead of using System V IPC.
> >
> > Works on BSD.  Should also work on SVR4 and offspring (Solaris),
> > and Linux.

This is a really bad idea performance wise.  Solaris has a special
code path for SYSV shared memory that doesn't require tons of swap
tracking structures per-page/per-process.  FreeBSD also has this
optimization (it's off by default, but should work since FreeBSD
4.2 via the sysctl kern.ipc.shm_use_phys=1)

Both OS's use a trick of making the pages non-pageable, this allows
signifigant savings in kernel space required for each attached
process, as well as the use of large pages which reduce the amount
of TLB faults your processes will incurr.

Anyhow, if you could make this a runtime option it wouldn't be so
evil, but as a compile time option, it's a really bad idea for
Solaris and FreeBSD.

--
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]

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From pgsql-hackers-owner+M6255@postgresql.org Tue Mar 20 18:46:33 2001
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From: Alfred Perlstein <bright@wintelcom.net>
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Subject: Re: [HACKERS] Fw: [vorbis-dev] ogg123: shared memory by mmap()
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* Bruce Momjian <pgman@candle.pha.pa.us> [010320 14:10] wrote:
> > > > The patch below adds:
> > > >
> > > > - acinclude.m4:  A new macro A_FUNC_SMMAP to check that sharing
> > > pages
> > > >   through mmap() works.  This is taken from Joerg Schilling's star.
> > > > - configure.in:  A_FUNC_SMMAP
> > > > - ogg123/buffer.c:  If we have a working mmap(), use it to create
> > > >   a region of shared memory instead of using System V IPC.
> > > >
> > > > Works on BSD.  Should also work on SVR4 and offspring (Solaris),
> > > > and Linux.
> > 
> > This is a really bad idea performance wise.  Solaris has a special
> > code path for SYSV shared memory that doesn't require tons of swap
> > tracking structures per-page/per-process.  FreeBSD also has this
> > optimization (it's off by default, but should work since FreeBSD
> > 4.2 via the sysctl kern.ipc.shm_use_phys=1)
> 
> > 
> > Both OS's use a trick of making the pages non-pageable, this allows
> > signifigant savings in kernel space required for each attached
> > process, as well as the use of large pages which reduce the amount
> > of TLB faults your processes will incurr.
> 
> That is interesting.  BSDi has SysV shared memory as non-pagable, and I
> always thought of that as a bug.  Seems you are saying that having it
> pagable has a significant performance penalty.  Interesting.

Yes, having it pageable is actually sort of bad.

It doesn't allow you to do several important optimizations.

-- 
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]


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From pgsql-general-owner+M14300@postgresql.org Mon Aug 27 13:07:32 2001
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From: Martijn van Oosterhout <kleptog@svana.org>
To: Andrew Snow <andrew@modulus.org>
cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] raw partition
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On Tue, Aug 28, 2001 at 12:02:08AM +1000, Andrew Snow wrote:
> 
> What I think would be better would be moving postgresql to a system of
> using memory-mapped I/O.  instead of the shared buffer cache, files
> would be directly memory-mapped and the OS would do the caching.  I
> can't see this happening though because of platform dependancy, but I
> think its worth another look soon because many unix platforms support
> mmap().  I think it would improve the performance of disk-intensive
> tasks noticeably.

Well, this has other problems. Consider tables that are larger than your
system memory. You'd have to continuously map and unmap different sections.
That can have odd side effects (witness mozilla on linux having 15,000
mapped areas or so...)

You would still however get the advantage that you wouldn't have to copy the
data from the disk buffers to user space, you simply get the disk buffer
mapped into your address space.

I think that for commonly used tables that are under 100K in size (most of
the system tables), this is quite a workable idea. If you don't mind keeping
them mapped the whole time.

-- 
Martijn van Oosterhout <kleptog@svana.org>
http://svana.org/kleptog/
> It would be nice if someone came up with a certification system that
> actually separated those who can barely regurgitate what they crammed over
> the last few weeks from those who command secret ninja networking powers.

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From pgsql-general-owner+M14319@postgresql.org Mon Aug 27 16:57:10 2001
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To: Martijn van Oosterhout <kleptog@svana.org>
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Martijn van Oosterhout <kleptog@svana.org> writes:
> You would still however get the advantage that you wouldn't have to copy the
> data from the disk buffers to user space, you simply get the disk buffer
> mapped into your address space.

AFAICS this would be the *only* advantage.  While it's not negligible,
it's quite unclear that it's worth the bookkeeping and portability
headaches of managing lots of mmap'd areas, either.

Before I take this idea seriously at all, I'd want to see a design that
addresses a couple of critical issues:

1. Postgres' shared buffers are *shared*, potentially across many
processes.  How will you deal with buffers for files that have been
mmap'd by only some of the processes?  (Maybe this means that the
whole concept of shared buffers goes away, and each process does its
own buffer management based on its own mmaps.  Not sure.  That would be
a pretty radical restructuring though, and would completely invalidate
our present approach to page-level locking.)

2. How do you deal with extending a file?  My system's mmap man page
says
     If the size of the mapped file changes after the call to mmap(), the
     effect of references to portions of the mapped region that correspond
     to added or removed portions of the file is unspecified.
This suggests that the only portable way to cope is to issue a separate
mmap for every disk page.  Will typical Unix systems perform well with
umpteen thousand small mmap requests?

3. How do you persuade the other backends to drop their mmaps of a table
you are deleting?

There are probably other gotchas, but without an understanding of how
to address these, I doubt it's worth looking further ...

			regards, tom lane

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From pgsql-hackers-owner+M13750=candle.pha.pa.us=pgman@postgresql.org Mon Oct  1 05:59:15 2001
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From: Janardhana Reddy <jana-reddy@mediaring.com.sg>
To: Bruce Momjian <pgman@candle.pha.pa.us>, Tom Lane <tgl@sss.pgh.pa.us>
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Subject: Re: [HACKERS] PERFORMANCE IMPROVEMENT by mapping  WAL FILES
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     I have just  completed the functional testing  the WAL using mmap  , it is

 working  fine,  I  have tested  by commenting out the  "CreateCheckPoint "
functionality so that
   when i kill the postgres and restart it will redo all the records from the
WAL log file  which
  is updated  using mmap.
     Just i need  to  clean code and to do some stress testing.
 By the end of this week i should able to  complete  the stress test  and
generate the patch file .
    As Tom Lane mentioned  i see the  problem in portability  to all platforms,

      what i propose is to use mmap for only WAL  for some platforms like
  linux,freebsd etc . For  other platforms we can use the existing method by
slightly modifying the
 write()  routine to write only the modified part of the page.

Regards
jana

>
>
> OK, I have talked to Tom Lane about this on the phone and we have a few
> ideas.
>
> Historically, we have avoided mmap() because of portability problems,
> and because using mmap() to write to large tables could consume lots of
> address space with little benefit.  However, I perhaps can see WAL as
> being a good use of mmap.
>
> First, there is the issue of using mmap().  For OS's that have the
> mmap() MAP_SHARED flag, different backends could mmap the same file and
> each see the changes.  However, keep in mind we still have to fsync()
> WAL, so we need to use msync().
>
> So, looking at the benefits of using mmap(), we have overhead of
> different backends having to mmap something that now sits quite easily
> in shared memory.  Now, I can see mmap reducing the copy from user to
> kernel, but there are other ways to fix that.  We could modify the
> write() routines to write() 8k on first WAL page write and later write
> only the modified part of the page to the kernel buffers.  The old
> kernel buffer is probably still around so it is unlikely to require a
> read from the file system to read in the rest of the page.  This reduces
> the write from 8k to something probably less than 4k which is better
> than we can do with mmap.
>
> I will add a TODO item to this effect.
>
> As far as reducing the write to disk from 8k to 4k, if we have to
> fsync/msync, we have to wait for the disk to spin to the proper location
> and at that point writing 4k or 8k doesn't seem like much of a win.
>
> In summary, I think it would be nice to reduce the 8k transfer from user
> to kernel on secondary page writes to only the modified part of the
> page.  I am uncertain if mmap() or anything else will help the physical
> write to the disk.
>
> --
>   Bruce Momjian                        |  http://candle.pha.pa.us
>   pgman@candle.pha.pa.us               |  (610) 853-3000
>   +  If your life is a hard drive,     |  830 Blythe Avenue
>   +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

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From pgsql-hackers-owner+M23388@postgresql.org Mon Jun  3 17:54:43 2002
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From: "Igor Kovalenko" <Igor.Kovalenko@motorola.com>
To: "Bruce Momjian" <pgman@candle.pha.pa.us>
cc: "Tom Lane" <tgl@sss.pgh.pa.us>, "mlw" <markw@mohawksoft.com>,
   "Marc G. Fournier" <scrappy@hub.org>, <pgsql-hackers@postgresql.org>
References: <200206030047.g530lZi21901@candle.pha.pa.us>
Subject: Re: [HACKERS] HEADS UP: Win32/OS2/BeOS native ports
Date: Mon, 3 Jun 2002 16:53:51 -0500
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That's what Apache does. Note, on most platforms MAP_ANON is equivalent to
mmmap-ing /dev/zero. Solaris for example does not provide MAP_ANON but using

fd=open(/dev/zero)
mmap(fd, ...)
close(fd)

works just fine.

----- Original Message -----
From: "Bruce Momjian" <pgman@candle.pha.pa.us>
To: "Igor Kovalenko" <Igor.Kovalenko@motorola.com>
Cc: "Tom Lane" <tgl@sss.pgh.pa.us>; "mlw" <markw@mohawksoft.com>; "Marc G.
Fournier" <scrappy@hub.org>; <pgsql-hackers@postgresql.org>
Sent: Sunday, June 02, 2002 7:47 PM
Subject: Re: [HACKERS] HEADS UP: Win32/OS2/BeOS native ports


> Igor Kovalenko wrote:
> > It does not have to be anonymous. POSIX also defines shm_open(same
arguments
> > as open) API which will create named object in whatever location
corresponds
> > to shared memory storage on that platform (object is then grown to
needed
> > size by ftruncate() and the fd is then passed to mmap). The object will
> > exist in name space and can be detected by subsequent calls to
shm_open()
> > with same name. It is not really different from doing open(), but more
> > portable (mmap() on regular files may not be supported).
>
> Actually, I think the best shared memory implemention would be
> MAP_ANON | MAP_SHARED mmap(), which could be called from the postmaster
> and passed to child processes.
>
> While all our platforms have mmap(), many don't have MAP_ANON, but those
> that do could use it.  You need MAP_ANON to prevent the shared memory
> from being written to a disk file.
>
> --
>   Bruce Momjian                        |  http://candle.pha.pa.us
>   pgman@candle.pha.pa.us               |  (610) 853-3000
>   +  If your life is a hard drive,     |  830 Blythe Avenue
>   +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026
>


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From pgsql-hackers-owner+M24146@postgresql.org Tue Jun 25 02:27:29 2002
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Date: Tue, 25 Jun 2002 14:05:45 +0900 (JST)
From: Curt Sampson <cjs@cynic.net>
To: "J. R. Nield" <jrnield@usol.com>
cc: Bruce Momjian <pgman@candle.pha.pa.us>, Tom Lane <tgl@sss.pgh.pa.us>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: [HACKERS] Buffer Management
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I'm splitting off this buffer mangement stuff into a separate thread.

On 24 Jun 2002, J. R. Nield wrote:

> I'll back off on that. I don't know if we want to use the OS buffer
> manager, but shouldn't we try to have our buffer manager group writes
> together by files, and pro-actively get them out to disk?

The only way the postgres buffer manager can "get [data] out to disk"
is to do an fsync(). For data files (as opposed to log files), this can
only slow down overall system throughput, as this would only disrupt the
OS's write management.

> Right now, it
> looks like all our write requests are delayed as long as possible and
> the order in which they are written is pretty-much random, as is the
> backend that writes the block, so there is no locality of reference even
> when the blocks are adjacent on disk, and the write calls are spread-out
> over all the backends.

It doesn't matter. The OS will introduce locality of reference with its
write algorithms. Take a look at

    http://www.cs.wisc.edu/~solomon/cs537/disksched.html

for an example. Most OSes use the elevator or one-way elevator
algorithm.  So it doesn't matter whether it's one back-end or many
writing, and it doesn't matter in what order they do the write.

> Would it not be the case that things like read-ahead, grouping writes,
> and caching written data are probably best done by PostgreSQL, because
> only our buffer manager can understand when they will be useful or when
> they will thrash the cache?

Operating systems these days are not too bad at guessing guessing what
you're doing. Pretty much every OS I've seen will do read-ahead when
it detects you're doing sequential reads, at least in the forward
direction. And Solaris is even smart enough to mark the pages you've
read as "not needed" so that they quickly get flushed from the cache,
rather than blowing out your entire cache if you go through a large
file.

> Would O_DSYNC|O_RSYNC turn off the cache?

No. I suppose there's nothing to stop it doing so, in some
implementations, but the interface is not designed for direct I/O.

> Since you know a lot about NetBSD internals, I'd be interested in
> hearing about what postgresql looks like to the NetBSD buffer manager.

Well, looks like pretty much any program, or group of programs,
doing a lot of I/O. :-)

> Am I right that strings of successive writes get randomized?

No; as I pointed out, they in fact get de-randomized as much as
possible. The more proceses you have throwing out requests, the better
the throughput will be in fact.

> What do our cache-hit percentages look like? I'm going to do some
> experimenting with this.

Well, that depends on how much memory you have and what your working
set is. :-)

cjs
-- 
Curt Sampson  <cjs@cynic.net>   +81 90 7737 2974   http://www.netbsd.org
    Don't you know, in this new Dark Age, we're all light.  --XTC




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From cjs@cynic.net Tue Jun 25 09:52:23 2002
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Date: Tue, 25 Jun 2002 22:52:14 +0900 (JST)
From: Curt Sampson <cjs@cynic.net>
To: "J. R. Nield" <jrnield@usol.com>
cc: Bruce Momjian <pgman@candle.pha.pa.us>, Tom Lane <tgl@sss.pgh.pa.us>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management
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So, while we're at it, what's the current state of people's thinking
on using mmap rather than shared memory for data file buffers? I
see some pretty powerful advantages to this approach, and I'm not
(yet :-)) convinced that the disadvantages are as bad as people think.
I think I can address most of the concerns in doc/TODO.detail/mmap.

Is this worth pursuing a bit? (I.e., should I spend an hour or two
writing up the advantages and thoughts on how to get around the
problems?) Anybody got objections that aren't in doc/TODO.detail/mmap?

cjs
-- 
Curt Sampson  <cjs@cynic.net>   +81 90 7737 2974   http://www.netbsd.org
    Don't you know, in this new Dark Age, we're all light.  --XTC


From tgl@sss.pgh.pa.us Tue Jun 25 10:09:07 2002
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To: Curt Sampson <cjs@cynic.net>
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   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management 
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Comments: In-reply-to Curt Sampson <cjs@cynic.net>
	message dated "Tue, 25 Jun 2002 22:52:14 +0900"
Date: Tue, 25 Jun 2002 10:09:02 -0400
Message-ID: <7298.1025014142@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: ORr

Curt Sampson <cjs@cynic.net> writes:
> So, while we're at it, what's the current state of people's thinking
> on using mmap rather than shared memory for data file buffers?

There seem to be a couple of different threads in doc/TODO.detail/mmap.

One envisions mmap as a one-for-one replacement for our current use of
SysV shared memory, the main selling point being to get out from under
kernels that don't have SysV support or have it configured too small.
This might be worth doing, and I think it'd be relatively easy to do
now that the shared memory support is isolated in one file and there's
provisions for selecting a shmem implementation at configure time.
The only thing you'd really have to think about is how to replace the
current behavior that uses shmem attach counts to discover whether any
old backends are left over from a previous crashed postmaster.  I dunno
if mmap offers any comparable facility.

The other discussion seemed to be considering how to mmap individual
data files right into backends' address space.  I do not believe this
can possibly work, because of loss of control over visibility of data
changes to other backends, timing of write-backs, etc.

But as long as you stay away from interpretation #2 and go with
mmap-as-a-shmget-substitute, it might be worthwhile.

(Hey Marc, can one do mmap in a BSD jail?)

			regards, tom lane

From pgsql-hackers-owner+M24158@postgresql.org Tue Jun 25 10:20:42 2002
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Date: Tue, 25 Jun 2002 23:20:15 +0900 (JST)
From: Curt Sampson <cjs@cynic.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: "J. R. Nield" <jrnield@usol.com>, Bruce Momjian <pgman@candle.pha.pa.us>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management 
In-Reply-To: <7298.1025014142@sss.pgh.pa.us>
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On Tue, 25 Jun 2002, Tom Lane wrote:

> The only thing you'd really have to think about is how to replace the
> current behavior that uses shmem attach counts to discover whether any
> old backends are left over from a previous crashed postmaster.  I dunno
> if mmap offers any comparable facility.

Sure. Just mmap a file, and it will be persistent.

> The other discussion seemed to be considering how to mmap individual
> data files right into backends' address space.  I do not believe this
> can possibly work, because of loss of control over visibility of data
> changes to other backends, timing of write-backs, etc.

I don't understand why there would be any loss of visibility of changes.
If two backends mmap the same block of a file, and it's shared, that's
the same block of physical memory that they're accessing. Changes don't
even need to "propagate," because the memory is truly shared. You'd keep
your locks in the page itself as well, of course.

Can you describe the problem in more detail?

> But as long as you stay away from interpretation #2 and go with
> mmap-as-a-shmget-substitute, it might be worthwhile.

It's #2 that I was really looking at. :-)

cjs
-- 
Curt Sampson  <cjs@cynic.net>   +81 90 7737 2974   http://www.netbsd.org
    Don't you know, in this new Dark Age, we're all light.  --XTC




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From pgsql-hackers-owner+M24159@postgresql.org Tue Jun 25 10:25:21 2002
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From: Bruce Momjian <pgman@candle.pha.pa.us>
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Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <7298.1025014142@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 25 Jun 2002 10:20:29 -0400 (EDT)
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
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Tom Lane wrote:
> Curt Sampson <cjs@cynic.net> writes:
> > So, while we're at it, what's the current state of people's thinking
> > on using mmap rather than shared memory for data file buffers?
> 
> There seem to be a couple of different threads in doc/TODO.detail/mmap.
> 
> One envisions mmap as a one-for-one replacement for our current use of
> SysV shared memory, the main selling point being to get out from under
> kernels that don't have SysV support or have it configured too small.
> This might be worth doing, and I think it'd be relatively easy to do
> now that the shared memory support is isolated in one file and there's
> provisions for selecting a shmem implementation at configure time.
> The only thing you'd really have to think about is how to replace the
> current behavior that uses shmem attach counts to discover whether any
> old backends are left over from a previous crashed postmaster.  I dunno
> if mmap offers any comparable facility.
> 
> The other discussion seemed to be considering how to mmap individual
> data files right into backends' address space.  I do not believe this
> can possibly work, because of loss of control over visibility of data
> changes to other backends, timing of write-backs, etc.

Agreed.  Also, there was in intresting thread that mmap'ing /dev/zero is
the same as anonmap for OS's that don't have anonmap.  That should cover
most of them.  The only downside I can see is that SysV shared memory is
locked into RAM on some/most OS's while mmap anon probably isn't. 
Locking in RAM is good in most cases, bad in others.

This will also work well when we have non-SysV semaphore support, like
Posix semaphores, so we would be able to run with no SysV stuff.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026



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From pgsql-hackers-owner+M24160@postgresql.org Tue Jun 25 10:27:40 2002
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Date: Tue, 25 Jun 2002 15:20:49 +0100
To: Tom Lane <tgl@sss.pgh.pa.us>
Subject: Re: [HACKERS] Buffer Management 
In-Reply-To: <7298.1025014142@sss.pgh.pa.us>
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Tom Lane writes:
 > There seem to be a couple of different threads in
 > doc/TODO.detail/mmap.
 > [ snip ]

A place where mmap could be easily used and would offer a good
performance increase is for COPY FROM.

Lee.



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From cjs@cynic.net Tue Jun 25 10:24:49 2002
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Date: Tue, 25 Jun 2002 23:24:44 +0900 (JST)
From: Curt Sampson <cjs@cynic.net>
To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Tom Lane <tgl@sss.pgh.pa.us>, "J. R. Nield" <jrnield@usol.com>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <200206251420.g5PEKT310222@candle.pha.pa.us>
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On Tue, 25 Jun 2002, Bruce Momjian wrote:

> The only downside I can see is that SysV shared memory is
> locked into RAM on some/most OS's while mmap anon probably isn't.

It is if you mlock() it. :-)

cjs
-- 
Curt Sampson  <cjs@cynic.net>   +81 90 7737 2974   http://www.netbsd.org
    Don't you know, in this new Dark Age, we're all light.  --XTC


From tgl@sss.pgh.pa.us Tue Jun 25 10:29:53 2002
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To: Curt Sampson <cjs@cynic.net>
cc: "J. R. Nield" <jrnield@usol.com>, Bruce Momjian <pgman@candle.pha.pa.us>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management 
In-Reply-To: <Pine.NEB.4.43.0206252318020.670-100000@angelic.cynic.net> 
References: <Pine.NEB.4.43.0206252318020.670-100000@angelic.cynic.net>
Comments: In-reply-to Curt Sampson <cjs@cynic.net>
	message dated "Tue, 25 Jun 2002 23:20:15 +0900"
Date: Tue, 25 Jun 2002 10:29:49 -0400
Message-ID: <7498.1025015389@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: ORr

Curt Sampson <cjs@cynic.net> writes:
> On Tue, 25 Jun 2002, Tom Lane wrote:
>> The other discussion seemed to be considering how to mmap individual
>> data files right into backends' address space.  I do not believe this
>> can possibly work, because of loss of control over visibility of data
>> changes to other backends, timing of write-backs, etc.

> I don't understand why there would be any loss of visibility of changes.
> If two backends mmap the same block of a file, and it's shared, that's
> the same block of physical memory that they're accessing.

Is it?  You have a mighty narrow conception of the range of
implementations that's possible for mmap.

But the main problem is that mmap doesn't let us control when changes to
the memory buffer will get reflected back to disk --- AFAICT, the OS is
free to do the write-back at any instant after you dirty the page, and
that completely breaks the WAL algorithm.  (WAL = write AHEAD log;
the log entry describing a change must hit disk before the data page
change itself does.)

			regards, tom lane

From pgsql-hackers-owner+M24164@postgresql.org Tue Jun 25 10:44:39 2002
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To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management 
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Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
	message dated "Tue, 25 Jun 2002 10:20:29 -0400"
Date: Tue, 25 Jun 2002 10:32:10 -0400
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Bruce Momjian <pgman@candle.pha.pa.us> writes:
> This will also work well when we have non-SysV semaphore support, like
> Posix semaphores, so we would be able to run with no SysV stuff.

You do realize that we can use Posix semaphores today?  The Darwin (OS X)
port uses 'em now.  That's one reason I am more interested in mmap as
a shmget substitute than I used to be.

			regards, tom lane



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From pgsql-hackers-owner+M24167@postgresql.org Tue Jun 25 11:02:20 2002
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200206251455.g5PEtst15464@candle.pha.pa.us>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <7524.1025015530@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 25 Jun 2002 10:55:54 -0400 (EDT)
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
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Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > This will also work well when we have non-SysV semaphore support, like
> > Posix semaphores, so we would be able to run with no SysV stuff.
> 
> You do realize that we can use Posix semaphores today?  The Darwin (OS X)
> port uses 'em now.  That's one reason I am more interested in mmap as

No, I didn't realize we had gotten that far.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026



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From pgsql-hackers-owner+M24168@postgresql.org Tue Jun 25 11:05:13 2002
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	Tue, 25 Jun 2002 10:56:58 -0400 (EDT)
From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200206251456.g5PEuwO15564@candle.pha.pa.us>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <7498.1025015389@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 25 Jun 2002 10:56:58 -0400 (EDT)
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
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Status: OR

Tom Lane wrote:
> Curt Sampson <cjs@cynic.net> writes:
> > On Tue, 25 Jun 2002, Tom Lane wrote:
> >> The other discussion seemed to be considering how to mmap individual
> >> data files right into backends' address space.  I do not believe this
> >> can possibly work, because of loss of control over visibility of data
> >> changes to other backends, timing of write-backs, etc.
> 
> > I don't understand why there would be any loss of visibility of changes.
> > If two backends mmap the same block of a file, and it's shared, that's
> > the same block of physical memory that they're accessing.
> 
> Is it?  You have a mighty narrow conception of the range of
> implementations that's possible for mmap.
> 
> But the main problem is that mmap doesn't let us control when changes to
> the memory buffer will get reflected back to disk --- AFAICT, the OS is
> free to do the write-back at any instant after you dirty the page, and
> that completely breaks the WAL algorithm.  (WAL = write AHEAD log;
> the log entry describing a change must hit disk before the data page
> change itself does.)

Can we mmap WAL without problems?  Not sure if there is any gain to it
because we just write it and rarely read from it.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026



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From tgl@sss.pgh.pa.us Tue Jun 25 11:00:20 2002
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To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management 
In-Reply-To: <200206251456.g5PEuwO15564@candle.pha.pa.us> 
References: <200206251456.g5PEuwO15564@candle.pha.pa.us>
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
	message dated "Tue, 25 Jun 2002 10:56:58 -0400"
Date: Tue, 25 Jun 2002 11:00:19 -0400
Message-ID: <7805.1025017219@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: ORr

Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Can we mmap WAL without problems?  Not sure if there is any gain to it
> because we just write it and rarely read from it.

Perhaps, but I don't see any point to it.

			regards, tom lane

From pgsql-hackers-owner+M24171@postgresql.org Tue Jun 25 11:14:23 2002
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200206251502.g5PF25r16113@candle.pha.pa.us>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <7805.1025017219@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 25 Jun 2002 11:02:05 -0400 (EDT)
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
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Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Can we mmap WAL without problems?  Not sure if there is any gain to it
> > because we just write it and rarely read from it.
> 
> Perhaps, but I don't see any point to it.

Agreed.  I have been poking around google looking for an article I read
months ago saying that mmap of files is slighly faster in low memory
usage situations, but much slower in high memory usage situations
because the kernel doesn't know as much about the file access in mmap as
it does with stdio.  I will find it.  :-)

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026



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From pgsql-hackers-owner+M24179@postgresql.org Tue Jun 25 12:13:40 2002
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Date: Tue, 25 Jun 2002 12:12:45 -0400
From: Bradley McLean <brad@bradm.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: Mario Weilguni <mario.weilguni@icomedias.com>,
   Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
   Bruce Momjian <pgman@candle.pha.pa.us>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management
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* Tom Lane (tgl@sss.pgh.pa.us) [020625 11:00]:
> 
> msync can force not-yet-written changes down to disk.  It does not
> prevent the OS from choosing to write changes *before* you invoke msync.
> 
> Our problem is that we want to enforce the write ordering "WAL before
> data file".  To do that, we write and fsync (or DSYNC, or something)
> a WAL entry before we issue the write() against the data file.  We
> don't really care if the kernel delays the data file write beyond that
> point, but we can be certain that the data file write did not occur
> too early.
> 
> msync is designed to ensure exactly the opposite constraint: it can
> guarantee that no changes remain unwritten after time T, but it can't
> guarantee that changes aren't written before time T.

Okay, so instead of looking for constraints from the OS on the data file,
use the constraints on the WAL file.  It would work at the cost of a buffer
copy?  Er, maybe two:

mmap the data file and WAL separately.
Copy the data file page to the WAL mmap area.
Modify the page.
msync() the WAL.
Copy the page to the data file mmap area.
msync() or not the data file.

(This is half baked, just thought I'd see if it stirred further thought).

As another approach, how expensive is re-MMAPing portions of the files
compared to the copies.

-Brad

> 
> 			regards, tom lane
> 
> 
> 
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From cjs@cynic.net Wed Jun 26 00:13:45 2002
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Date: Wed, 26 Jun 2002 13:13:42 +0900 (JST)
From: Curt Sampson <cjs@cynic.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: "J. R. Nield" <jrnield@usol.com>, Bruce Momjian <pgman@candle.pha.pa.us>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management 
In-Reply-To: <7498.1025015389@sss.pgh.pa.us>
Message-ID: <Pine.NEB.4.43.0206261149170.670-100000@angelic.cynic.net>
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Status: OR

On Tue, 25 Jun 2002, Tom Lane wrote:

> Curt Sampson <cjs@cynic.net> writes:
>
> > I don't understand why there would be any loss of visibility of changes.
> > If two backends mmap the same block of a file, and it's shared, that's
> > the same block of physical memory that they're accessing.
>
> Is it?  You have a mighty narrow conception of the range of
> implementations that's possible for mmap.

It's certainly possible to implement something that you call mmap
that is not. But if you are using the posix-defined MAP_SHARED flag,
the behaviour above is what you see. It might be implemented slightly
differently internally, but that's no concern of postgres. And I find
it pretty unlikely that it would be implemented otherwise without good
reason.

Note that your proposal of using mmap to replace sysv shared memory
relies on the behaviour I've described too. As well, if you're replacing
sysv shared memory with an mmap'd file, you may end up doing excessive
disk I/O on systems without the MAP_NOSYNC option. (Without this option,
the update thread/daemon may ensure that every buffer is flushed to the
backing store on disk every 30 seconds or so. You might be able to get
around this by using a small file-backed area for things that need to
persist after a crash, and a larger anonymous area for things that don't
need to persist after a crash.)

> But the main problem is that mmap doesn't let us control when changes to
> the memory buffer will get reflected back to disk --- AFAICT, the OS is
> free to do the write-back at any instant after you dirty the page, and
> that completely breaks the WAL algorithm.  (WAL = write AHEAD log;
> the log entry describing a change must hit disk before the data page
> change itself does.)

Hm. Well ,we could try not to write the data to the page until
after we receive notification that our WAL data is committed to
stable storage. However, new the data has to be availble to all of
the backends at the exact time that the commit happens. Perhaps a
shared list of pending writes?

Another option would be to just let it write, but on startup, scan
all of the data blocks in the database for tuples that have a
transaction ID later than the last one we updated to, and remove
them. That could pretty darn expensive on a large database, though.

cjs
-- 
Curt Sampson  <cjs@cynic.net>   +81 90 7737 2974   http://www.netbsd.org
    Don't you know, in this new Dark Age, we're all light.  --XTC


From tgl@sss.pgh.pa.us Wed Jun 26 09:22:05 2002
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To: Curt Sampson <cjs@cynic.net>
cc: "J. R. Nield" <jrnield@usol.com>, Bruce Momjian <pgman@candle.pha.pa.us>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management 
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References: <Pine.NEB.4.43.0206261149170.670-100000@angelic.cynic.net>
Comments: In-reply-to Curt Sampson <cjs@cynic.net>
	message dated "Wed, 26 Jun 2002 13:13:42 +0900"
Date: Wed, 26 Jun 2002 09:21:59 -0400
Message-ID: <1696.1025097719@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: ORr

Curt Sampson <cjs@cynic.net> writes:
> Note that your proposal of using mmap to replace sysv shared memory
> relies on the behaviour I've described too.

True, but I was not envisioning mapping an actual file --- at least
on HPUX, the only way to generate an arbitrary-sized shared memory
region is to use MAP_ANONYMOUS and not have the mmap'd area connected
to any file at all.  It's not farfetched to think that this aspect
of mmap might work differently from mapping pieces of actual files.

In practice of course we'd have to restrict use of any such
implementation to platforms where mmap behaves reasonably ... according
to our definition of "reasonably".

			regards, tom lane

From pgsql-hackers-owner+M24252@postgresql.org Wed Jun 26 16:14:36 2002
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Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <1696.1025097719@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Wed, 26 Jun 2002 13:11:19 -0400 (EDT)
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
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Tom Lane wrote:
> Curt Sampson <cjs@cynic.net> writes:
> > Note that your proposal of using mmap to replace sysv shared memory
> > relies on the behaviour I've described too.
> 
> True, but I was not envisioning mapping an actual file --- at least
> on HPUX, the only way to generate an arbitrary-sized shared memory
> region is to use MAP_ANONYMOUS and not have the mmap'd area connected
> to any file at all.  It's not farfetched to think that this aspect
> of mmap might work differently from mapping pieces of actual files.
> 
> In practice of course we'd have to restrict use of any such
> implementation to platforms where mmap behaves reasonably ... according
> to our definition of "reasonably".

Yes, I am told mapping /dev/zero is the same as the anon map.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026



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From pgsql-hackers-owner+M24292@postgresql.org Wed Jun 26 23:39:10 2002
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From: Curt Sampson <cjs@cynic.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: "J. R. Nield" <jrnield@usol.com>, Bruce Momjian <pgman@candle.pha.pa.us>,
   PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management 
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On Wed, 26 Jun 2002, Tom Lane wrote:

> Curt Sampson <cjs@cynic.net> writes:
> > Note that your proposal of using mmap to replace sysv shared memory
> > relies on the behaviour I've described too.
>
> True, but I was not envisioning mapping an actual file --- at least
> on HPUX, the only way to generate an arbitrary-sized shared memory
> region is to use MAP_ANONYMOUS and not have the mmap'd area connected
> to any file at all.  It's not farfetched to think that this aspect
> of mmap might work differently from mapping pieces of actual files.

I find it somewhat farfetched, for a couple of reasons:

    1. Memory mapped with the MAP_SHARED flag is shared memory,
    anonymous or not. POSIX is pretty explicit about how this works,
    and the "standard" for mmap that predates POSIX is the same.
    Anonymous memory does not behave differently.

    You could just as well say that some systems might exist such
    that one process can write() a block to a file, and then another
    might read() it afterwards but not see the changes. Postgres
    should not try to deal with hypothetical systems that are so
    completely broken.

    2. Mmap is implemented as part of a unified buffer cache system
    on all of today's operating systems that I know of. The memory
    is backed by swap space when anonymous, and by a specified file
    when not anonymous; but the way these two are handled is
    *exactly* the same internally.

    Even on older systems without unified buffer cache, the behaviour
    is the same between anonymous and file-backed mmap'd memory.
    And there would be no point in making it otherwise. Mmap is
    designed to let you share memory; why make a broken implementation
    under certain circumstances?

> In practice of course we'd have to restrict use of any such
> implementation to platforms where mmap behaves reasonably ... according
> to our definition of "reasonably".

Of course. As we do already with regular I/O.

cjs
-- 
Curt Sampson  <cjs@cynic.net>   +81 90 7737 2974   http://www.netbsd.org
    Don't you know, in this new Dark Age, we're all light.  --XTC




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