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Re: Wierd context-switching issue on Xeon

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pg(at)fastcrypt(dot)com,Robert Creager <Robert_Creager(at)LogicalChaos(dot)org>,Josh Berkus <josh(at)agliodbs(dot)com>, Dirk_Lutzebäck <lutzeb(at)aeccom(dot)com>,ohp(at)pyrenet(dot)fr, Joe Conway <mail(at)joeconway(dot)com>,"scott(dot)marlowe" <scott(dot)marlowe(at)ihs(dot)com>,pgsql-performance(at)postgresql(dot)org, Neil Conway <neilc(at)samurai(dot)com>
Subject: Re: Wierd context-switching issue on Xeon
Date: 2004-05-20 03:02:16
Message-ID: 200405200302.i4K32G721749@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > Did we ever come to a conclusion about excessive SMP context switching
> > under load?
> 
> Yeah: it's bad.
> 
> Oh, you wanted a fix?  That seems harder :-(.  AFAICS we need a redesign
> that causes less load on the BufMgrLock.  However, the traditional
> solution to too-much-contention-for-a-lock is to break up the locked
> data structure into finer-grained units, which means *more* lock
> operations in total.  Normally you expect that the finer-grained lock
> units will mean less contention.  But given that the issue here seems to
> be trading physical ownership of the lock's cache line back and forth,
> I'm afraid that the traditional approach would actually make things
> worse.  The SMP issue seems to be not with whether there is
> instantaneous contention for the locked datastructure, but with the cost
> of making it possible for processor B to acquire a lock recently held by
> processor A.

I see.  I don't even see a TODO in there.  :-(

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

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