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From: Jean-Christophe Hugly (jice_at_[hidden])
Date: 2006-02-09 22:10:01


On Thu, 2006-02-09 at 16:37 -0700, Brightwell, Ronald wrote:

> I apologize if it seems like I'm picking on you.
No offense taken.

> I'm hypersensitive to
> people trying to make judgements based on micro-benchmark performance.
> I've been trying to make an argument that two-node ping-pong latency
> comparisons really only have meaning in the context of a whole system.

It's very clear to me that micro-benchmarks do not tell you very much
about real application behaviour; that's not the question. They are
nevertheless relevant to me because, right or wrong, people who buy
stuff look at them. And I work for a commercial outfit.

I may sound silly saying that, but they might be right to look at it,
they just need to look at the rest too. A micro-benchmarks tells you how
much you have of a given currency, that you can trade for another. It
tells you something about the implementation; how efficient the code is,
how well the hardware is utilized, etc. Not in every respect, but some.

It also tells you how far you can emphasize a given feature at the
expense of all others, if it happens that at some point in time it is
what you most need.

By making the argument that a particular characteristic is irrelevant,
you are essentially making a hard coded tradeoff, rather than letting
the users do it.

Back to the specific issue of latency vs. scale. Okay for CG and FT, the
cross-over may be <32, but that's not for all the cases and the
difference visible at 32 is pretty small. So, it is application
dependent, no question about it, but small-msg rdma is beneficial below
a given (application-dependent) cluster size.

-- 
Jean-Christophe Hugly <jice_at_[hidden]>
PANTA