Skip site navigation (1) Skip section navigation (2)

Re: Wierd context-switching issue on Xeon patch for 7.4.1

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, Josh Berkus <josh(at)agliodbs(dot)com>,Paul Tuckfield <paul(at)tuckfield(dot)com>, Anjan Dave <adave(at)vantage(dot)com>,Neil Conway <neilc(at)samurai(dot)com>, Dirk Lutzeb├Ąck <lutzeb(at)aeccom(dot)com>,pgsql-performance(at)postgresql(dot)org
Subject: Re: Wierd context-switching issue on Xeon patch for 7.4.1
Date: 2004-04-22 04:56:15
Message-ID: 200404220456.i3M4uFv04710@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Tom Lane wrote:
> Dave Cramer <pg(at)fastcrypt(dot)com> writes:
> > I tried increasing the NUM_SPINS to 1000 and it works better.
> 
> Doesn't surprise me.  The value of 100 is about right on the assumption
> that the spinlock instruction per se is not too much more expensive than
> any other instruction.  What I was seeing from oprofile suggested that
> the spinlock instruction cost about 100x more than an ordinary
> instruction :-( ... so maybe 200 or so would be good on a Xeon.
> 
> > This is certainly heading in the right direction ? Although it looks
> > like it is highly dependent on the system you are running on.
> 
> Yeah.  I don't know a reasonable way to tune this number automatically
> for particular systems ... but at the very least we'd need to find a way
> to distinguish uniprocessor from multiprocessor, because on a
> uniprocessor the optimal value is surely 1.

Have you looked at the code pointed to by our TODO item:
	
	* Add code to detect an SMP machine and handle spinlocks accordingly
	  from distributted.net, http://www1.distributed.net/source,
	  in client/common/cpucheck.cpp

For BSDOS it has:

    #if (CLIENT_OS == OS_FREEBSD) || (CLIENT_OS == OS_BSDOS) || \
        (CLIENT_OS == OS_OPENBSD) || (CLIENT_OS == OS_NETBSD)
    { /* comment out if inappropriate for your *bsd - cyp (25/may/1999) */
      int ncpus; size_t len = sizeof(ncpus);
      int mib[2]; mib[0] = CTL_HW; mib[1] = HW_NCPU;
      if (sysctl( &mib[0], 2, &ncpus, &len, NULL, 0 ) == 0)
      //if (sysctlbyname("hw.ncpu", &ncpus, &len, NULL, 0 ) == 0)
        cpucount = ncpus;
    }

and I can confirm that on my computer it works:

	hw.ncpu = 2

-- 
  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

In response to

Responses

pgsql-performance by date

Next:From: Tom LaneDate: 2004-04-22 05:13:47
Subject: Re: Wierd context-switching issue on Xeon patch for 7.4.1
Previous:From: Christopher Kings-LynneDate: 2004-04-22 04:44:07
Subject: Re: Wierd context-switching issue on Xeon patch for 7.4.1

Privacy Policy | About PostgreSQL
Copyright © 1996-2014 The PostgreSQL Global Development Group