[Beowulf] Woodcrest Memory bandwidth

Joe Landman landman at scalableinformatics.com
Mon Aug 14 17:10:13 PDT 2006

Hi Stu:

Stu Midgley wrote:
> sorry, forgot to reply all... don't you hate gmail's interface sometimes?
> What is the memory latency of the woodcrest machines?  Since memory
> latency really determines your memory bandwidth.

Hmmm...  not for large block sequential accesses.  You can prefetch
these assuming enough intelligence in the code generator (heh), or the
hardware if the memory access pattern is fairly consistent.

Latency really defines the random access local node GUPS, well, its
really more complex than that, but roughly that.

That said, I would like to measure this.  I have an old code which does
this, any pointers on code other people would like me to run?  If its
not too hard (e.g. less than 15 minutes) I might do a few.

> If Intel hasn't made any improvements in latency then the limited
> number of out-standing loads in the x86-64 architecture will limit the
> bandwidth regarless of the MB/s you throw at it.

Hmmm... Ok, you are implying that if your processor can consume the
load/store slots faster than it can launch them, and there are a limited
number of memory operations in flight (2? as I remember, not looking at
my notes now), it is going to be load-store pipeline limited, not
necessarily "bandwidth".  That is, the memory system would be
significantly faster than the CPU can consume.

I haven't looked closely at the Woodcrest arch yet.  Don't know
precisely what they are doing here and how it differs from AMD.  Would
be interesting.  So far I haven't been impressed with code that I
thought I should be really impressed with on this machine.  Oddly the
performance was about what we got out of the Core Duo on this platform.

Joseph Landman, Ph.D
Founder and CEO
Scalable Informatics LLC,
email: landman at scalableinformatics.com
web  : http://www.scalableinformatics.com
phone: +1 734 786 8423
fax  : +1 734 786 8452 or +1 866 888 3112
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