Re: [PATCH] sysdeps/arm/armv7/multiarch/memcpy_impl.S: Improve performance.

["Carlos O'Donell" <carlos at redhat dot com>]

On 09/02/2013 09:58 AM, Will Newton wrote:
>> If it's not obvious how is someone supposed to review this patch? :-)
> 
> With difficulty. ;-)

Thank you for acknowledging that.

> Joseph has raised some good points about the comments and I'll go back
> through the code and make sure everything is correct in that regard.
> The change was actually made to the copy of the code in cortex-strings
> some time ago but I delayed pushing the patch due to the 2.18 release
> so I have to refresh my memory somewhat.
> 
> Ideally we would have an agreed upon benchmark with which everyone
> could analyse the performance of the code on their systems, however
> that does not seem to exist as far as I can tell.

We have one, it's the glibc microbenchmark, and we want to expand it,
otherwise when ACME comes with their patch for ARM and breaks performance
for targets that Linaro cares about I have no way to reject the patch
objectively :-)

> The key advantage of the cortex-strings framework is that it allows
> graphing the results of benchmarks. Often changes to string function
> performance can only really be analysed graphically as otherwise you
> end up with a huge soup of numbers, some going up, some going down and
> it is very hard to separate the signal from the noise.

I disagree strongly. You *must* come up with a measurable answer and
looking at a graph is never a solution I'm going to accept.

You need to statistically analyze the numbers, assign weights to ranges,
and come up with some kind of number that evaluates the results based
on *some* formula. That is the only way we are going to keep moving
performance forward (against some kind of criteria).

> The glibc benchmarks also have some other weaknesses that should
> really be addressed, hopefully I'll have some time to write patches
> for some of this work.

Thank you very much.

Cheers,
Carlos.