On Thu, Dec 09, 2010 at 06:09:38PM +1100, Nick Piggin wrote:
> I was actually discussing this with Linus a while back, and finally
> got around to testing it out now that I have a modern CPU to measure
> it on! CCing linux-arch because it would be interesting to know
> whether your tuned functions do better than gcc or not (I would
> suspect not).
>
> BTW. patch and numbers are on top of my scaling series, just for
> an idea of what it does, I just want to generate some interesting
> discussion.
>
> If people are interested in running benchmarks, I'll be pushing out
> a new update soon, after some more testing and debugging here.
>
> The standard memcmp function on a Westmere system shows up hot in
> profiles in the `git diff` workload (both parallel and single threaded),
> and it is likely due to the costs associated with trapping into
> microcode, and little opportunity to improve memory access (dentry
> name is not likely to take up more than a cacheline).
>
> So replace it with an open-coded byte comparison. This increases code
> size by 24 bytes in the critical __d_lookup_rcu function, but the
> speedup is huge, averaging 10 runs of each:
>
> git diff st user sys elapsed CPU
> before 1.15 2.57 3.82 97.1
> after 1.14 2.35 3.61 96.8
>
> git diff mt user sys elapsed CPU
> before 1.27 3.85 1.46 349
> after 1.26 3.54 1.43 333
>
> Elapsed time for single threaded git diff at 95.0% confidence:
> -0.21 +/- 0.01
> -5.45% +/- 0.24%
Nice.
[..]
> +static inline int dentry_memcmp(const unsigned char *cs,
> + const unsigned char *ct, size_t count)
> +{
> + while (count) {
> + int ret = (*cs != *ct);
> + if (ret)
> + return ret;
> + cs++;
> + ct++;
> + count--;
> + }
> + return 0;
> +}
we have a memcmp() in lib/string.c. Maybe reuse it from there?
--
Regards/Gruss,
Boris.
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