Re: Linux: more cores = less concurrency.

Scott Marlowe wrote:
> Have you tried running the memory stream benchmark Greg Smith had
> posted here a while back?  It'll let you know if you're memory is
> bottlenecking.  Right now my 48 core machines are the king of that
> benchmark with something like 70+Gig a second.
>

The big Opterons are still the front-runners here, but not with 70GB/s
anymore.  Earlier versions of stream-scaling didn't use nearly enough
data to avoid L3 cache in the processors interfering with results.  More
recent tests I've gotten in done after I expanded the default test size
for them show the Opterons normally hitting the same ~35GB/s maximum
throughput that the Intel processors get out of similar DDR3/1333 sets.
There are some outliers where >50GB/s still shows up.  I'm not sure if I
really believe them though; attempts to increase the test size now hit a
32-bit limit inside stream.c, and I think that's not really big enough
to avoid L3 cache effects here.

In the table at https://github.com/gregs1104/stream-scaling the 4 X 6172
server is similar to Scott's system.  I believe the results for 8
(37613) and 48 cores (32301) there.  I remain somewhat suspicious that
the higher reuslts of 40 - 51GB/s shown between 16 and 32 cores may be
inflated by caching.  At this point I'll probably need direct access to
one of them to resolve this for sure.  I've made a lot of progress with
other people's servers, but complete trust in those particular results
still isn't there yet.

--
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
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