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Re: CLOG contention

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: CLOG contention
Date: 2012-01-05 22:34:29
Message-ID: 22126.1325802869@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Thu, Jan 5, 2012 at 2:57 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I would be in favor of that, or perhaps some other formula (eg, maybe
>> the minimum should be less than 8 for when you've got very little shmem).

> I have some results that show that, under the right set of
> circumstances, 8->32 is a win, and I can quantify by how much it wins.
>  I don't have any data at all to quantify the cost of dropping the
> minimum from 8->6, or from 8->4, and therefore I'm reluctant to do it.
>  My guess is that it's a bad idea, anyway.  Even on a system where
> shared_buffers is just 8MB, we have 1024 regular buffers and 8 CLOG
> buffers.  If we reduce the number of CLOG buffers from 8 to 4 (i.e. by
> 50%), we can increase the number of regular buffers from 1024 to 1028
> (i.e. by <0.5%).  Maybe you can find a case where that comes out to a
> win, but you might have to look pretty hard.

I think you're rejecting the concept too easily.  A setup with very
little shmem is only going to be suitable for low-velocity systems that
are not pushing too many transactions through per second, so it's not
likely to need so many CLOG buffers.  And frankly I'm not that concerned
about what the performance is like: I'm more concerned about whether
PG will start up at all without modifying the system shmem limits,
on systems with legacy values for SHMMAX etc.  Shaving a few
single-purpose buffers to make back what we spent on SSI, for example,
seems like a good idea to me.

			regards, tom lane

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