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SubjectRe: [PATCH] x86: Run checksumming in parallel accross multiple alu's

* H. Peter Anvin <hpa@zytor.com> wrote:

> On 10/17/2013 01:41 AM, Ingo Molnar wrote:
> >
> > To correctly simulate the workload you'd have to:
> >
> > - allocate a buffer larger than your L2 cache.
> >
> > - to measure the effects of the prefetches you'd also have to randomize
> > the individual buffer positions. See how 'perf bench numa' implements a
> > random walk via --data_rand_walk, in tools/perf/bench/numa.c.
> > Otherwise the CPU might learn your simplistic stream direction and the
> > L2 cache might hw-prefetch your data, interfering with any explicit
> > prefetches the code does. In many real-life usecases packet buffers are
> > scattered.
> >
> > Also, it would be nice to see standard deviation noise numbers when two
> > averages are close to each other, to be able to tell whether differences
> > are statistically significant or not.
>
>
> Seriously, though, how much does it matter? All the above seems likely
> to do is to drown the signal by adding noise.

I think it matters a lot and I don't think it 'adds' noise - it measures
something else (cache cold behavior - which is the common case for
first-time csum_partial() use for network packets), which was not measured
before, and that that is by its nature has different noise patterns.

I've done many cache-cold measurements myself and had no trouble achieving
statistically significant results and high precision.

> If the parallel (threaded) checksumming is faster, which theory says it
> should and microbenchmarking confirms, how important are the
> macrobenchmarks?

Microbenchmarks can be totally blind to things like the ideal prefetch
window size. (or whether a prefetch should be done at all: some CPUs will
throw away prefetches if enough regular fetches arrive.)

Also, 'naive' single-threaded algorithms can occasionally be better in the
cache-cold case because a linear, predictable stream of memory accesses
might saturate the memory bus better than a somewhat random looking,
interleaved web of accesses that might not harmonize with buffer depths.

I _think_ if correctly tuned then the parallel algorithm should be better
in the cache cold case, I just don't know with what parameters (and the
algorithm has at least one free parameter: the prefetch window size), and
I don't know how significant the effect is.

Also, more fundamentally, I absolutely detest doing no measurements or
measuring the wrong thing - IMHO there are too many 'blind' optimization
commits in the kernel with little to no observational data attached.

Thanks,

Ingo


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