On Sep 17, 2004, at 3:55 PM, Tom Lane wrote:
Marc Colosimo <[EMAIL PROTECTED]> writes:I'm using 7.4.5 on Mac OS X (G5) and was profiling it to see why it is SO SLOW at committing inserts and deletes into a large database. One of the many slowdowns was from MemSet. I found an old (2002) thread about this and retried the tests (see below). The main point is that the system memset crushes pg's!!
Hmm. I tried to duplicate this on my G4 laptop, and found that they were more or less on a par for small-to-middling block sizes (using "gcc -O2"). Darwin's memset code must have some additional tweaks for use on G5 hardware. Good for Apple --- this is the sort of thing that OS vendors *ought* to be doing. The fact that we can beat the system memset on so many platforms is an indictment of those platforms.
Is it possible to add a define to call the system memset at build time! This probably isn't the case on other systems.
Feel free to hack the definition of MemSet in src/include/c.h. See the comments for it for more context.
Note that for small compile-time-constant block sizes (a case your test
program doesn't test, but it's common in pgsql), gcc with a sufficiently
high optimization setting can unroll the loop into a linear sequence of
words zeroings. I would expect that to beat the system memset up to a
few dozen words, no matter how tense the memset coding is. So you
probably want to think in terms of reducing MEMSET_LOOP_LIMIT rather
than diking out the macro code altogether. Or maybe reduce MemSet to
"memset(...)" but leave MemSetAligned and/or MemSetTest/MemSetLoop
as-is. In any case, reporting results without mentioning the compiler
and optimization level in use isn't going to convince anybody ...
Oops, I used the same setting as in the old hacking message (-O2, gcc 3.3). If I understand what you are saying, then it turns out yes, PG's MemSet is faster for smaller blocksizes (see below, between 32 and 64). I just replaced the whole MemSet with memset and it is not very low when I profile. I could squeeze more out of it if I spent more time trying to understand it (change MEMSET_LOOP_LIMIT to 32 and then add memset after that?). I'm now working one understanding Spin Locks and friends. Putting in a sync call (in s_lock.h) is really a time killer and bad for performance (it takes up 35 cycles).
run on a single processor G5 (1.8Gz, other was on a DP 2Gz G5) pgMemSet: * 4 0.070u 0.000s 0:00.15 46.6% 0+0k 0+0io 0pf+0w * 8 0.090u 0.000s 0:00.16 56.2% 0+0k 0+0io 0pf+0w * 16 0.120u 0.000s 0:00.17 70.5% 0+0k 0+0io 0pf+0w * 32 0.180u 0.000s 0:00.29 62.0% 0+0k 0+0io 0pf+0w * 64 0.450u 0.000s 0:00.92 48.9% 0+0k 0+0io 0pf+0w
memset: * 4 0.170u 0.010s 0:00.44 40.9% 0+0k 0+0io 0pf+0w * 8 0.190u 0.000s 0:00.42 45.2% 0+0k 0+0io 0pf+0w * 16 0.190u 0.010s 0:00.39 51.2% 0+0k 0+0io 0pf+0w * 32 0.200u 0.000s 0:00.39 51.2% 0+0k 0+0io 0pf+0w * 64 0.260u 0.000s 0:00.38 68.4% 0+0k 0+0io 0pf+0w
Marc
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