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Re: Seq scans status update

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>
Cc: Jeff Davis <pgsql(at)j-davis(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: Seq scans status update
Date: 2007-05-30 19:56:38
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-patches
Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
> I just ran a quick test with 4 concurrent scans on a dual-core system, 
> and it looks like we do "leak" buffers from the rings because they're 
> pinned at the time they would be recycled.

Yeah, I noticed the same in some tests here.  I think there's not a lot
we can do about that; we don't have enough visibility into why someone
else has the buffer pinned.

Using a larger ring would help, by making it less probable that any
other sync-scanning backend is so far behind as to still have the oldest
element of our ring pinned.  But if we do that we have the L2-cache-size
effect to worry about.  Is there any actual data backing up that it's
useful to keep the ring fitting in L2, or is that just guesswork?  In
the sync-scan case the idea seems pretty bogus anyway, because the
actual working set will be N backends' rings not just one.

			regards, tom lane

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