There's previously been several posts discussing the performance penalty
one suffers when running multiple LL tests on a multiprocessor system
with a single shared system bus. It would be interesting to see whether this
penalty could be alleviated in a reasonably cost-effective fashion through
use of larger L2 caches.
Here's the idea: if the L2 caches are much smaller than the dataset size,
the system bus will be heavily used by each process, leading to memory
delays. If each processor has an L2 cache large enough to hold the full
dataset (between 4 and 5MB for an LL test of an exponent ~10M), there
will be essentially no memory traffic and no shared-bus penalty. Such
large caches are expensive. But it seems that between these two extremes
(very small and very large caches) there should be some kind of optimal
compromise, where the cache size is just large enough that the resulting
reduction in memory traffic allows each process to access the main memory
at basically the same speed it would if there were no other jobs competing for
bus bandwidth, i.e. where the bus traffic just begins to saturate.
I suspect for LL tests in the ~10M range, this happy medium may be as
'small' as 1-2MB. Are PC systems with L2 caches in this size range
available? If so, how much of a premium does one pay for the extra cache?
Cheers,
-Ernst
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