cvs commit: src/sys/i386/i386 pmap.c
julian at elischer.org
Tue Nov 9 19:00:51 GMT 2004
Peter Wemm wrote:
>On Tuesday 09 November 2004 10:21 am, Stephan Uphoff wrote:
>>On Tue, 2004-11-09 at 13:02, Julian Elischer wrote:
>>>Robert Watson wrote:
>>>>This change made a large difference, and eliminates the
>>>>unexplained costs. Here's a revised table as compared to the
>>>>sleep mutex crit section spin mutex new spin mutex
>>>>UP SMP UP SMP UP SMP UP SMP
>>>>PIII 21 81 83 81 112 141 95 141
>>>>P4 39 260 120 119 274 342 132 231
>>>>So it basically cut 140 cycles off the P4 UP spin lock, 15 off the
>>>>PIII UP spin lock, and 110 cycles off the P4 SMP spin lock. The
>>>>PIII SMP spin lock looks the same. Keep in mind that all of
>>>>these measurements have a standard deviation of between 0 and 3
>>>>cycles, most in the 1 range. Also keep in mind that these are
>>>>entirely uncontended measurements.
>>>>Assuming that these changes are correct, and pass whatever tests
>>>>people have in mind, this would be a very strong merge candidate
>>>>for performance reasons. The difference is visible in packet
>>>>send tests from user space as a percentage or two improvement on
>>>>UP on my P4, although it's a litte hard to tell due to the noise.
>>>Can you explain why a spin mutex is more expensive than a sleep
>>>mutex (I assume this is uncontested)?
>>cli() and sti() used for the critical section are expensive.
>... on INTEL cpus! Don't make the mistake of assuming that all x86 cpus
>are as slow as Intel's P4 family on this stuff. Other cpus don't have
>the same massive microcode penalty. My recollection is that athlon
>(and athlon64 cpus in 32 bit mode) take about 8-12 clocks to do a cli
>or sti, compared to 300+ for a P4 cpu. And things like 50-90 clocks
>for an invlpg vs 1200-1600 clocks for a P4.
>Please don't accidently penalize those of us with cpus that were
>designed for good all-round performance. The P4 family was designed
>for games and 3d graphics, not all-round performance.
>(This isn't aimed at anybody in particular.. I just wanted to remind
>people that the P4 code is a particularly pathological case (and the
>writing is on the wall for that core). Other cpus, including intel's
>newer non-P4 cores, dont have the same pathological problems.)
maybe robert can send you his benchmarks to run on your jardwqare..
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