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Re: LWLock contention: I think I understand the problem

From: Tatsuo Ishii <t-ishii(at)sra(dot)co(dot)jp>
To: tgl(at)sss(dot)pgh(dot)pa(dot)us
Cc: pgman(at)candle(dot)pha(dot)pa(dot)us, pgsql-hackers(at)postgresql(dot)org, jwbaker(at)acm(dot)org
Subject: Re: LWLock contention: I think I understand the problem
Date: 2002-01-03 01:18:25
Message-ID: 20020103101825N.t-ishii@sra.co.jp (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-odbc
> I have thought of a further refinement to the patch I produced
> yesterday.  Assume that there are multiple waiters blocked on (eg)
> BufMgrLock.  After we release the first one, we want the currently
> running process to be able to continue acquiring and releasing the lock
> for as long as its time quantum holds out.  But in the patch as given,
> each acquire/release cycle releases another waiter.  This is probably
> not good.
> 
> Attached is a modification that prevents additional waiters from being
> released until the first releasee has a chance to run and acquire the
> lock.  Would you try this and see if it's better or not in your test
> cases?  It doesn't seem to help on a single CPU, but maybe on multiple
> CPUs it'll make a difference.
> 
> To try to make things simple, I've attached the mod in two forms:
> as a diff from current CVS, and as a diff from the previous patch.

Ok, here is a pgbench (-s 10) result on an AIX 5L box (4 way).

"7.2 with patch" is for the previous patch. "7.2 with patch (revised)"
is for the this patch. I see virtually no improvement. Please note
that xy axis are now in log scale.

Attachment: bench.png
Description: image/png (7.9 KB) (inlined above)

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