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Re: Large (8M) cache vs. dual-core CPUs

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>
Cc: Scott Marlowe <smarlowe(at)g2switchworks(dot)com>, mark(at)mark(dot)mielke(dot)cc, Bill Moran <wmoran(at)collaborativefusion(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Large (8M) cache vs. dual-core CPUs
Date: 2006-04-26 22:16:46
Message-ID: 200604262216.k3QMGkk15952@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Jim C. Nasby wrote:
> On Wed, Apr 26, 2006 at 10:27:18AM -0500, Scott Marlowe wrote:
> > If you haven't actually run a heavy benchmark of postgresql on the two
> > architectures, please don't make your decision based on other
> > benchmarks.  Since you've got both a D920 and an X2 3800, that'd be a
> > great place to start.  Mock up some benchmark with a couple dozen
> > threads hitting the server at once and see if the Intel can keep up.  It
> 
> Or better yet, use dbt* or even pgbench so others can reproduce...

For why Opterons are superior to Intel for PostgreSQL, see:

	http://techreport.com/reviews/2005q2/opteron-x75/index.x?pg=2

Section "MESI-MESI-MOESI Banana-fana...".  Specifically, this part about
the Intel implementation:

	The processor with the Invalid data in its cache (CPU 0, let's say)
	might then wish to modify that chunk of data, but it could not do so
	while the only valid copy of the data is in the cache of the other
	processor (CPU 1). Instead, CPU 0 would have to wait until CPU 1 wrote
	the modified data back to main memory before proceeding.and that takes
	time, bus bandwidth, and memory bandwidth. This is the great drawback of
	MESI.

AMD transfers the dirty cache line directly from cpu to cpu.  I can
imaging that helping our test-and-set shared memory usage quite a bit.

-- 
  Bruce Momjian   http://candle.pha.pa.us
  EnterpriseDB    http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +

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