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

[Ingo Molnar]

* H. Peter Anvin <hpa@xxxxxxxxx> 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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