Christoph Best wrote:
>
> Hi everybody,
>
> I am having a problem benchmarking the L2 cache performance on some
> Alpha 21264 systems from our clusters and wondering if anybody else
> has seen this. We use a benchmark that models the kernel of our main
> application (computational physics/lattice gauge theory). When running
> in L1 cache or beyond L2 cache, it gives perfectly consistent readings
> with deviations of 1% or less. But in L2 cache, the numbers from
> different runs may be off by as much as 20%, for which I cannot find a
> good explanation. If I plot performance vs. memory footprint, there is
> a clear shoulder from the L1 cache (64 KB), but then a kind of
> logarithmic behavior (double the memory use loses 30 MFlops).
>
> The benchmark consists of a completely deterministic set of
> floating-point operations, and I use a version that accesses memory
> completely consecutively. The systems are Compaq DS10 (466 MHz single
> proc.), ES40 (666 MHz 4-proc.), and API UP2000 (666 MHz dbl. proc.)
> under Linux. I did not see this effect under Tru64 on a XP1000 (666
> MHz single proc.).
>
> The question is: Is there anything either in Linux or the 21264 that
> could account for such behavior? Could the cache be polluted by other
> processes that effectively? (The machines were basically idle during
> benchmarks).
>
> In particular, it seems that code running just inside the L2 cache (4
> MB on the UP2000 and ES40) is not performing much better than code in
> main memory, which would be a pity. We expect cache performance to be
> a major determinant of total performance for our application: in L1
> cache, the performance is about 600 MFlops, outside L2 cache it drops
> to about 200 MFlops. Inside L2 it varies between 300 and 450 MFlops.
Just out of curiosity, were you using the Compaq compilers or GNU
compilers for your benchmarking under linux? Frome what I understand
there is a significant difference in performance between the two...
John
--
John Burton, Ph.D.
Senior Associate GATS, Inc.
[EMAIL PROTECTED] 11864 Canon Blvd - Suite 101
[EMAIL PROTECTED] (personal) Newport News, VA 23606
(757) 873-5920 (voice) (757) 873-5924 (fax)
