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Bruce Momjian
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@ -951,3 +951,479 @@ good and what is not matters, but it is good for another view point).
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From pgsql-hackers-owner+M13607=candle.pha.pa.us=pgman@postgresql.org Wed Sep 26 19:14:59 2001
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To: "D. Hageman" <dhageman@dracken.com>
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cc: mlw <markw@mohawksoft.com>,
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"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
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Subject: Re: [HACKERS] Spinlock performance improvement proposal
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References: <Pine.LNX.4.33.0109261600100.1784-100000@typhon.dracken.com>
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From: Doug McNaught <doug@wireboard.com>
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Date: 26 Sep 2001 18:39:44 -0400
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In-Reply-To: "D. Hageman"'s message of "Wed, 26 Sep 2001 16:14:22 -0500 (CDT)"
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Status: OR
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"D. Hageman" <dhageman@dracken.com> writes:
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> Save for the fact that the kernel can switch between threads faster then
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> it can switch processes considering threads share the same address space,
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> stack, code, etc. If need be sharing the data between threads is much
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> easier then sharing between processes.
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This depends on your system. Solaris has a huge difference between
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thread and process context switch times, whereas Linux has very little
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difference (and in fact a Linux process context switch is about as
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fast as a Solaris thread switch on the same hardware--Solaris is just
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a pig when it comes to process context switching).
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> I can't comment on the "isolate data" line. I am still trying to figure
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> that one out.
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I think his point is one of clarity and maintainability. When a
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task's data is explicitly shared (via shared memory of some sort) it's
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fairly clear when you're accessing shared data and need to worry about
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locking. Whereas when all data is shared by default (as with threads)
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it's very easy to miss places where threads can step on each other.
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-Doug
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--
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In a world of steel-eyed death, and men who are fighting to be warm,
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Come in, she said, I'll give you shelter from the storm. -Dylan
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From pgsql-hackers-owner+M13611=candle.pha.pa.us=pgman@postgresql.org Wed Sep 26 21:05:02 2001
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Wed, 26 Sep 2001 20:46:51 -0400 (EDT)
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To: Ian Lance Taylor <ian@airs.com>
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cc: "D. Hageman" <dhageman@dracken.com>, mlw <markw@mohawksoft.com>,
|
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"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
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Subject: Re: [HACKERS] Spinlock performance improvement proposal
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In-Reply-To: <si8zf1vcau.fsf@daffy.airs.com>
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References: <Pine.LNX.4.33.0109261600100.1784-100000@typhon.dracken.com> <si8zf1vcau.fsf@daffy.airs.com>
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Comments: In-reply-to Ian Lance Taylor <ian@airs.com>
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message dated "26 Sep 2001 15:04:41 -0700"
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Date: Wed, 26 Sep 2001 20:46:51 -0400
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Message-ID: <14704.1001551611@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Precedence: bulk
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Sender: pgsql-hackers-owner@postgresql.org
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Status: OR
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Ian Lance Taylor <ian@airs.com> writes:
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> (Actually, though, Postgres is already vulnerable to erratic behaviour
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> because any backend process can corrupt the shared buffer pool.)
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Not to mention the other parts of shared memory.
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Nonetheless, our experience has been that cross-backend failures due to
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memory clobbers in shared memory are very infrequent --- certainly far
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less often than we see localized-to-a-backend crashes. Probably this is
|
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because the shared memory is (a) small compared to the rest of the
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address space and (b) only accessed by certain specific modules within
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Postgres.
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I'm convinced that switching to a thread model would result in a
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significant degradation in our ability to recover from coredump-type
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failures, even given the (implausible) assumption that we introduce no
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new bugs during the conversion. I'm also *un*convinced that such a
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conversion will yield significant performance benefits, unless we
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introduce additional cross-thread dependencies (and more fragility
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and lock contention) by tactics such as sharing catalog caches across
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threads.
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regards, tom lane
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From pgsql-hackers-owner+M13616=candle.pha.pa.us=pgman@postgresql.org Wed Sep 26 23:10:52 2001
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Wed, 26 Sep 2001 22:58:41 -0400 (EDT)
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Date: Wed, 26 Sep 2001 22:58:41 -0400 (EDT)
|
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From: Alex Pilosov <alex@pilosoft.com>
|
||||
To: "D. Hageman" <dhageman@dracken.com>
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cc: "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
|
||||
Subject: Re: [HACKERS] Spinlock performance improvement proposal
|
||||
In-Reply-To: <Pine.LNX.4.33.0109261733050.2225-100000@typhon.dracken.com>
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Status: OR
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On Wed, 26 Sep 2001, D. Hageman wrote:
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> > > Save for the fact that the kernel can switch between threads faster then
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> > > it can switch processes considering threads share the same address space,
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> > > stack, code, etc. If need be sharing the data between threads is much
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> > > easier then sharing between processes.
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> >
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> > When using a kernel threading model, it's not obvious to me that the
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> > kernel will switch between threads much faster than it will switch
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> > between processes. As far as I can see, the only potential savings is
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> > not reloading the pointers to the page tables. That is not nothing,
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> > but it is also
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<major snippage>
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> > > I can't comment on the "isolate data" line. I am still trying to figure
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> > > that one out.
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> >
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> > Sometimes you need data which is specific to a particular thread.
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>
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> When you need data that is specific to a thread you use a TSD (Thread
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> Specific Data).
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Which Linux does not support with a vengeance, to my knowledge.
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As a matter of fact, quote from Linus on the matter was something like
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"Solution to slow process switching is fast process switching, not another
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kernel abstraction [referring to threads and TSD]". TSDs make
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implementation of thread switching complex, and fork() complex.
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The question about threads boils down to: Is there far more data that is
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shared than unshared? If yes, threads are better, if not, you'll be
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abusing TSD and slowing things down.
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I believe right now, postgresql' model of sharing only things that need to
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be shared is pretty damn good. The only slight problem is overhead of
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forking another backend, but its still _fast_.
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IMHO, threads would not bring large improvement to postgresql.
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Actually, if I remember, there was someone who ported postgresql (I think
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it was 6.5) to be multithreaded with major pain, because the requirement
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was to integrate with CORBA. I believe that person posted some benchmarks
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which were essentially identical to non-threaded postgres...
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|
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-alex
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From pgsql-hackers-owner+M13619=candle.pha.pa.us=pgman@postgresql.org Thu Sep 27 00:32:55 2001
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Thu, 27 Sep 2001 00:19:21 -0400 (EDT)
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To: "D. Hageman" <dhageman@dracken.com>
|
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cc: Alex Pilosov <alex@pilosoft.com>,
|
||||
"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
|
||||
Subject: Re: [HACKERS] Spinlock performance improvement proposal
|
||||
In-Reply-To: <Pine.LNX.4.33.0109262224040.1173-100000@typhon.dracken.com>
|
||||
References: <Pine.LNX.4.33.0109262224040.1173-100000@typhon.dracken.com>
|
||||
Comments: In-reply-to "D. Hageman" <dhageman@dracken.com>
|
||||
message dated "Wed, 26 Sep 2001 22:41:39 -0500"
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Date: Thu, 27 Sep 2001 00:19:20 -0400
|
||||
Message-ID: <15403.1001564360@sss.pgh.pa.us>
|
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From: Tom Lane <tgl@sss.pgh.pa.us>
|
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Precedence: bulk
|
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Sender: pgsql-hackers-owner@postgresql.org
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Status: OR
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"D. Hageman" <dhageman@dracken.com> writes:
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> If you look at Myron Scott's post today you will see that it had other
|
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> advantages going for it (like auto-vacuum!) and disadvantages ... rogue
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> thread corruption (already debated today).
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But note that Myron did a number of things that are (IMHO) orthogonal
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to process-to-thread conversion, such as adding prepared statements,
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a separate thread/process/whateveryoucallit for buffer writing, ditto
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for vacuuming, etc. I think his results cannot be taken as indicative
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of the benefits of threads per se --- these other things could be
|
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implemented in a pure process model too, and we have no data with which
|
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to estimate which change bought how much.
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|
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Threading certainly should reduce the context switch time, but this
|
||||
comes at the price of increased overhead within each context (since
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access to thread-local variables is not free). It's by no means
|
||||
obvious that there's a net win there.
|
||||
|
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regards, tom lane
|
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|
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From pgsql-hackers-owner+M13621=candle.pha.pa.us=pgman@postgresql.org Thu Sep 27 01:59:44 2001
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for <pgsql-hackers@postgresql.org>; Wed, 26 Sep 2001 22:24:29 -0700 (PDT)
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Date: Wed, 26 Sep 2001 22:24:29 -0700 (PDT)
|
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From: Myron Scott <mscott@sacadia.com>
|
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X-Sender: mscott@goldengate.kojoworldwide.com.
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To: pgsql-hackers@postgresql.org
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||||
Subject: Re: [HACKERS] Spinlock performance improvement proposal
|
||||
In-Reply-To: <15403.1001564360@sss.pgh.pa.us>
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Message-ID: <Pine.GSO.4.10.10109262146500.1111-100000@goldengate.kojoworldwide.com.>
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Status: OR
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|
||||
> But note that Myron did a number of things that are (IMHO) orthogonal
|
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|
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yes, I did :)
|
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|
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> to process-to-thread conversion, such as adding prepared statements,
|
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> a separate thread/process/whateveryoucallit for buffer writing, ditto
|
||||
> for vacuuming, etc. I think his results cannot be taken as indicative
|
||||
> of the benefits of threads per se --- these other things could be
|
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> implemented in a pure process model too, and we have no data with which
|
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> to estimate which change bought how much.
|
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>
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If you are comparing just process vs. thread, I really don't think I
|
||||
gained much for performance and ended up with some pretty unmanageable
|
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code.
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|
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The one thing that led to most of the gains was scheduling all the writes
|
||||
to one thread which, as noted by Tom, you could do on the process model.
|
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Besides, Most of the advantage in doing this was taken away with the
|
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addition of WAL in 7.1.
|
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|
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The other real gain that I saw with threading was limiting the number of
|
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open files but
|
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that led me to alter much of the file manager in order to synchronize
|
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access to the files which probably slowed things a bit.
|
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|
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To be honest, I don't think I, personally,
|
||||
would try this again. I went pretty far off
|
||||
the beaten path with this thing. It works well for what I am doing
|
||||
( a limited number of SQL statements run many times over ) but there
|
||||
probably was a better way. I'm thinking now that I should have tried to
|
||||
add a CORBA interface for connections. I would have been able to
|
||||
accomplish my original goals without creating a deadend for myself.
|
||||
|
||||
|
||||
Thanks all for a great project,
|
||||
|
||||
Myron
|
||||
mscott@sacadia.com
|
||||
|
||||
|
||||
---------------------------(end of broadcast)---------------------------
|
||||
TIP 2: you can get off all lists at once with the unregister command
|
||||
(send "unregister YourEmailAddressHere" to majordomo@postgresql.org)
|
||||
|
||||
From pgsql-hackers-owner+M13632=candle.pha.pa.us=pgman@postgresql.org Thu Sep 27 10:21:22 2001
|
||||
Return-path: <pgsql-hackers-owner+M13632=candle.pha.pa.us=pgman@postgresql.org>
|
||||
Received: from server1.pgsql.org (server1.pgsql.org [64.39.15.238] (may be forged))
|
||||
by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id f8RELLo08607
|
||||
for <pgman@candle.pha.pa.us>; Thu, 27 Sep 2001 10:21:21 -0400 (EDT)
|
||||
Received: from postgresql.org (webmail.postgresql.org [216.126.85.28])
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|
||||
for <pgman@candle.pha.pa.us>; Thu, 27 Sep 2001 09:21:26 -0500 (CDT)
|
||||
(envelope-from pgsql-hackers-owner+M13632=candle.pha.pa.us=pgman@postgresql.org)
|
||||
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|
||||
for <pgsql-hackers@postgresql.org>; Thu, 27 Sep 2001 10:04:09 -0400 (EDT)
|
||||
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|
||||
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|
||||
Thu, 27 Sep 2001 10:02:06 -0400
|
||||
Message-ID: <3BB3315D.EC99FF65@mohawksoft.com>
|
||||
Date: Thu, 27 Sep 2001 10:02:05 -0400
|
||||
From: mlw <markw@mohawksoft.com>
|
||||
X-Mailer: Mozilla 4.75 [en] (X11; U; Linux 2.4.2 i686)
|
||||
X-Accept-Language: en
|
||||
MIME-Version: 1.0
|
||||
To: "D. Hageman" <dhageman@dracken.com>
|
||||
cc: Ian Lance Taylor <ian@airs.com>,
|
||||
"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
|
||||
Subject: Re: [HACKERS] Spinlock performance improvement proposal
|
||||
References: <Pine.LNX.4.33.0109261733050.2225-100000@typhon.dracken.com>
|
||||
Content-Type: text/plain; charset=us-ascii
|
||||
Content-Transfer-Encoding: 7bit
|
||||
Precedence: bulk
|
||||
Sender: pgsql-hackers-owner@postgresql.org
|
||||
Status: OR
|
||||
|
||||
"D. Hageman" wrote:
|
||||
|
||||
> On 26 Sep 2001, Ian Lance Taylor wrote:
|
||||
> >
|
||||
> > > Save for the fact that the kernel can switch between threads faster then
|
||||
> > > it can switch processes considering threads share the same address space,
|
||||
> > > stack, code, etc. If need be sharing the data between threads is much
|
||||
> > > easier then sharing between processes.
|
||||
> >
|
||||
> > When using a kernel threading model, it's not obvious to me that the
|
||||
> > kernel will switch between threads much faster than it will switch
|
||||
> > between processes. As far as I can see, the only potential savings is
|
||||
> > not reloading the pointers to the page tables. That is not nothing,
|
||||
> > but it is also not a lot.
|
||||
>
|
||||
> It is my understanding that avoiding a full context switch of the
|
||||
> processor can be of a significant advantage. This is especially important
|
||||
> on processor architectures that can be kinda slow at doing it (x86). I
|
||||
> will admit that most modern kernels have features that assist software
|
||||
> packages utilizing the forking model (copy on write for instance). It is
|
||||
> also my impression that these do a good job. I am the kind of guy that
|
||||
> looks towards the future (as in a year, year and half or so) and say that
|
||||
> processors will hopefully get faster at context switching and more and
|
||||
> more kernels will implement these algorithms to speed up the forking
|
||||
> model. At the same time, I see more and more processors being shoved into
|
||||
> a single box and it appears that the threads model works better on these
|
||||
> type of systems.
|
||||
|
||||
"context" switching happens all the time on a multitasking system. On the x86
|
||||
processor, a context switch happens when you call into the kernel. You have to go
|
||||
through a call-gate to get to a lower privilege ring. "context" switching is very
|
||||
fast. The operating system dictates how heavy or light a process switch is. Under
|
||||
Linux (and I believe FreeBSD with Linux threads, or version 4.x ) threads and
|
||||
processes are virtually identical. The only difference is that the virtual memory
|
||||
pages are not "copy on write." Process vs thread scheduling is also virtually
|
||||
identical.
|
||||
|
||||
If you look to the future, then you should accept that process switching should
|
||||
become more efficient as the operating systems improve.
|
||||
|
||||
>
|
||||
> > > I can't comment on the "isolate data" line. I am still trying to figure
|
||||
> > > that one out.
|
||||
> >
|
||||
> > Sometimes you need data which is specific to a particular thread.
|
||||
>
|
||||
> When you need data that is specific to a thread you use a TSD (Thread
|
||||
> Specific Data).
|
||||
|
||||
Yes, but Postgres has many global variables. The assumption has always been that
|
||||
it is a stand-alone process with an explicitly shared paradigm, not implicitly.
|
||||
|
||||
>
|
||||
> > Basically, you have to look at every global variable in the Postgres
|
||||
> > backend, and determine whether to share it among all threads or to
|
||||
> > make it thread-specific.
|
||||
>
|
||||
> Yes, if one was to implement threads into PostgreSQL I would think that
|
||||
> some re-writing would be in order of several areas. Like I said before,
|
||||
> give a person a chance to restructure things so future TODO items wouldn't
|
||||
> be so hard to implement. Personally, I like to stay away from global
|
||||
> variables as much as possible. They just get you into trouble.
|
||||
|
||||
In real live software, software which lives from year to year with active
|
||||
development, things do get messy. There are always global variables involved in a
|
||||
program. Efforts, of course, should be made to keep them to a minimum, but the
|
||||
reality is that they always happen.
|
||||
|
||||
Also, the very structure of function calls may need to change when going from a
|
||||
process model to a threaded model. Functions never before reentrant are now be
|
||||
reentrant, think about that. That is a huge undertaking. Every single function
|
||||
may need to be examined for thread safety, with little benefit.
|
||||
|
||||
>
|
||||
> > > That last line is a troll if I every saw it ;-) I will agree that threads
|
||||
> > > isn't for everything and that it has costs just like everything else. Let
|
||||
> > > me stress that last part - like everything else. Certain costs exist in
|
||||
> > > the present model, nothing is - how should we say ... perfect.
|
||||
> >
|
||||
> > When writing in C, threading inevitably loses robustness. Erratic
|
||||
> > behaviour by one thread, perhaps in a user defined function, can
|
||||
> > subtly corrupt the entire system, rather than just that thread. Part
|
||||
> > of defensive programming is building barriers between different parts
|
||||
> > of a system. Process boundaries are a powerful barrier.
|
||||
>
|
||||
> I agree with everything you wrote above except for the first line. My
|
||||
> only comment is that process boundaries are only *truely* a powerful
|
||||
> barrier if the processes are different pieces of code and are not
|
||||
> dependent on each other in crippling ways. Forking the same code with the
|
||||
> bug in it - and only 1 in 5 die - is still 4 copies of buggy code running
|
||||
> on your system ;-)
|
||||
|
||||
This is simply not true. All software has bugs, it is an undeniable fact. Some
|
||||
bugs are more likely to be hit than others. 5 processes , when one process hits a
|
||||
bug, that does not mean the other 4 will hit the same bug. Obscure bugs kill
|
||||
software all the time, the trick is to minimize the impact. Software is not
|
||||
perfect, assuming it can be is a mistake.
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
---------------------------(end of broadcast)---------------------------
|
||||
TIP 5: Have you checked our extensive FAQ?
|
||||
|
||||
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|
||||
|
||||
|
||||
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