cvs commit: src/sys/i386/i386 pmap.c

Julian Elischer julian at
Tue Nov 9 18:02:19 GMT 2004

Robert Watson wrote:

>This change made a large difference, and eliminates the unexplained costs.
>Here's a revised table as compared to the above:
>	sleep mutex	crit section	spin mutex	new spin mutex
>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)?

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