Re: profiling connection overhead

[Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>]

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> I guess the word "run" is misleading (I wrote the program in 5
> minutes); it's just zeroing the same chunk twice and measuring the
> times.  The difference is presumably the page fault overhead, which
> implies that faulting is two-thirds of the overhead on MacOS X and
> three-quarters of the overhead on Linux.

Ah, cute solution to the measurement problem.  I replicated the
experiment just as a cross-check:

Fedora 13 on x86_64 (recent Nehalem):
first  run: 346767
second run: 103143

Darwin on x86_64 (not-so-recent Penryn):
first  run: 341289
second run: 64535

HPUX on HPPA:
first  run: 2191136
second run: 1199879

(On the last two machines I had to cut the array size to 256MB to avoid
swapping.)  All builds with "gcc -O2".

> This makes me pretty
> pessimistic about the chances of a meaningful speedup here.

Yeah, this is confirmation that what you are seeing in the original test
is mostly about faulting pages in, not about the zeroing.  I think it
would still be interesting to revisit the micro-optimization of
MemSet(), but it doesn't look like massive restructuring to avoid it
altogether is going to be worthwhile.

			regards, tom lane