Subject: Re: The importance of caching versus CPU-speed on a PC-based workstation
From: Jim Rosinski (rosinski@bearmtn-e0.cgd.ucar.edu)
Date: Mon Jan 25 1999 - 10:16:44 MST
On Sun, 24 Jan 1999, Frank Peacock wrote:
> I am intending to run CCM3 on a PC-based machine. I am not sure of the
> relative importance of cache size to processor speed for improved
> forecasting time.
>
> The present considered machines are either:
>
> a 450MHz Xion with 512Kb cache OR
>
> a 400MHz Xion with 1 Mb cache.
>
> Is there a simple answer to this dilemma or has anyone tested something
> along these lines?
I don't have a complete answer to either question. But isn't the 400MHz
vs. 450MHz processor cost differential around $250, and the 512Kb vs. 1 Mb
cache cost differential around $4,000? Unfortunately I cannot promise that
paying the extra $$ for the cache will be worth it. Main memory bandwidth
has proven to be the primary limiting factor affecting performance for most
architectures on which CCM3 runs.
> Since our memory setup will be totally in-core does this permit the cache
> to be utilized and is the spectral data ordering scheme for non-vectored
> computers possible, with no added programming, on a cached PC-machine?
The answer to both questions is yes. The CCM3 code as run on non-vector
machines is "cache-friendly" in that most of the inner loops (including
spectral dynamics) are stride 1. However, the code is "cache-unfriendly" in
that most inner loop lengths are long (128@T42) and contain lots of work,
while less than abundant opportunity exists for cache reuse.
I am very curious to hear the outcome of what you decide to buy, and how well
the code performs. I can provide timing benchmarks from other machines if
and when the time comes.
Regards,
Jim Rosinski
CCM Core Group
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