Re: Large (8M) cache vs. dual-core CPUs - Mailing list pgsql-performance

From Bruce Momjian
Subject Re: Large (8M) cache vs. dual-core CPUs
Date
Msg-id 200604262216.k3QMGkk15952@candle.pha.pa.us
Whole thread Raw
In response to Re: Large (8M) cache vs. dual-core CPUs  ("Jim C. Nasby" <jnasby@pervasive.com>)
Responses Re: Large (8M) cache vs. dual-core CPUs  ("Jim C. Nasby" <jnasby@pervasive.com>)
List 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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