Re: B-Tree support function number 3 (strxfrm() optimization) - Mailing list pgsql-hackers

On Sun, Sep 14, 2014 at 7:37 AM, Heikki Linnakangas
<hlinnakangas@vmware.com> wrote:
> Got to be careful to not let the compiler optimize away microbenchmarks like
> this. At least with my version of gcc, the strcoll calls get optimized away,
> as do the memcmp calls, if you don't use the result for anything. Clang was
> even more aggressive; it ran both comparisons in 0.0 seconds. Apparently it
> optimizes away the loops altogether.

I suppose the fact that I saw results that fit my pre-conceived notion
of what was happening made me lose my initial concern about that.

> Also, there should be a setlocale(LC_ALL, "") call somewhere. Otherwise it
> runs in C locale, and we don't use strcoll() at all for C locale.

Oops. This might be a useful mistake, though -- if the strcoll() using
the C locale is enough to make the memcmp() not free, then that
suggests that strcoll() is the "hiding place" for the useless
memcmp(), where instructions relating to the memcmp() can execute in
parallel to instructions relating to strcoll() that add latency from
memory accesses (for non-C locales). With the C locale, strcoll() is
equivalent to strcmp()/memcmp().

Commenting out the setlocale(LC_ALL, "") in your revised versions
shows something like my original numbers (so I guess my compiler
wasn't smart enough to optimize away the strcoll() + memcmp() cases).
Whereas, there is no noticeable regression/difference between each
case when I run the revised program unmodified. That seems to prove
that strcoll() is a good enough "hiding place".

> Both values vary in range 5.9 - 6.1 s, so it's fair to say that the useless
> memcmp() is free with these parameters.
>
> Is this the worst case scenario?

Other than pushing the differences much much later in the strings
(which you surely thought of already), yes. I think it's worse than
the worst, because we've boiled this down to just the comparison part,
leaving only the strcoll() as a "hiding place", which is evidently
good enough. I thought that it was important that there be an
unpredictable access pattern (characteristic of quicksort), so that
memory latency is added here and there. I'm happy to learn that I was
wrong about that, and that a strcoll() alone hides the would-be
memcmp() latency.

Large strings matter much less anyway, I think. If you have a pair of
strings both longer than CACHE_LINE_SIZE bytes, and the first
CACHE_LINE_SIZE bytes are identical, and the lengths are known to
match, it seems like a very sensible bet to anticipate that they're
fully equal. So in a world where that affects the outcome of this test
program, I think it still changes nothing (if, indeed, it matters at
all, which it appears not to anyway, at least with 256 byte strings).

We should probably do the a fully opportunistic "memcmp() == 0" within
varstr_cmp() itself, so that Windows has the benefit of this too, as
well as callers like compareJsonbScalarValue().

Actually, looking at it closely, I think that there might still be a
microscopic regression, as there might have also been with my variant
of your SQL test case [1] - certainly in the noise, but perhaps
measurable with enough runs. If there is, that seems like an
acceptable price to pay.

When I test this stuff, I'm now very careful about power management
settings on my laptop...there are many ways to be left with egg on
your face with this kind of benchmark.

[1] http://www.postgresql.org/message-id/CAM3SWZQY95Sow00b+zJycrGMR-uF1mz8rYv4_Ou2ENcvsTnxYA@mail.gmail.com
-- 
Peter Geoghegan