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SubjectRe: [PATCH] x86: Run checksumming in parallel accross multiple alu's
On Fri, Nov 01, 2013 at 10:13:37AM +0100, Ingo Molnar wrote:
>
> * Neil Horman <nhorman@tuxdriver.com> wrote:
>
> > On Thu, Oct 31, 2013 at 11:22:00AM +0100, Ingo Molnar wrote:
> > >
> > > * Neil Horman <nhorman@tuxdriver.com> wrote:
> > >
> > > > > etc. For such short runtimes make sure the last column displays
> > > > > close to 100%, so that the PMU results become trustable.
> > > > >
> > > > > A nehalem+ PMU will allow 2-4 events to be measured in parallel,
> > > > > plus generics like 'cycles', 'instructions' can be added 'for free'
> > > > > because they get counted in a separate (fixed purpose) PMU register.
> > > > >
> > > > > The last colum tells you what percentage of the runtime that
> > > > > particular event was actually active. 100% (or empty last column)
> > > > > means it was active all the time.
> > > > >
> > > > > Thanks,
> > > > >
> > > > > Ingo
> > > > >
> > > >
> > > > Hmm,
> > > >
> > > > I ran this test:
> > > >
> > > > for i in `seq 0 1 3`
> > > > do
> > > > echo $i > /sys/module/csum_test/parameters/module_test_mode
> > > > taskset -c 0 perf stat --repeat 20 -C 0 -e L1-dcache-load-misses -e L1-dcache-prefetches -e cycles -e instructions -ddd ./test.sh
> > > > done
> > >
> > > You need to remove '-ddd' which is a shortcut for a ton of useful
> > > events, but here you want to use fewer events, to increase the
> > > precision of the measurement.
> > >
> > > Thanks,
> > >
> > > Ingo
> > >
> >
> > Thank you ingo, that fixed it. I'm trying some other variants of
> > the csum algorithm that Doug and I discussed last night, but FWIW,
> > the relative performance of the 4 test cases
> > (base/prefetch/parallel/both) remains unchanged. I'm starting to
> > feel like at this point, theres very little point in doing
> > parallel alu operations (unless we can find a way to break the
> > dependency on the carry flag, which is what I'm tinkering with
> > now).
>
> I would still like to encourage you to pick up the improvements that
> Doug measured (mostly via prefetch tweaking?) - that looked like
> some significant speedups that we don't want to lose!
>
Well, yes, I made a line item of that in my subsequent note below. I'm going to
repost that shortly, and I suggested that we revisit this when the AVX
instruction extensions are available.

> Also, trying to stick the in-kernel implementation into 'perf bench'
> would be a useful first step as well, for this and future efforts.
>
> See what we do in tools/perf/bench/mem-memcpy-x86-64-asm.S to pick
> up the in-kernel assembly memcpy implementations:
>
Yes, I'll look into adding this as well
Regards
Neil




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