Messages in this thread | | | Subject | Re: [PATCH 0/6] Per-processor private data areas for i386 | From | Rusty Russell <> | Date | Fri, 29 Sep 2006 10:22:22 +1000 |
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On Wed, 2006-09-27 at 13:28 -0700, Jeremy Fitzhardinge wrote: > Pavel Machek wrote: > > So we have 4% slowdown... > > > > Yes, that would be the worst-case slowdown in the hot-cache case. > Rearranging the layout of the GDT would remove any theoretical > cold-cache slowdown (I haven't measured if there's any impact in practice). > > Rusty has also done more comprehensive benchmarks with his variant of > this patch series, and found no statistically interesting performance > difference. Which is pretty much what I would expect, since it doesn't > increase cache-misses at all.
OK, here are my null-syscall results. This is on a Intel(R) Pentium(R) 4 CPU 3.00GHz (stepping 9), single processor (SMP kernel).
I did three sets of tests: before, with saving/restoring %gs, with using %gs for per-cpu vars and current and smp_processor_id().
Before: swarm5.0:Simple syscall: 0.3734 microseconds swarm5.1:Simple syscall: 0.3734 microseconds swarm5.2:Simple syscall: 0.3734 microseconds swarm5.3:Simple syscall: 0.3734 microseconds
With saving/restoring %gs: (per-cpu was same) swarm5.4:Simple syscall: 0.3801 microseconds swarm5.5:Simple syscall: 0.3801 microseconds swarm5.6:Simple syscall: 0.3804 microseconds swarm5.7:Simple syscall: 0.3801 microseconds
That's a 6.5ns cost for saving and restoring gs, and other lmbench syscall benchmarks reflected similar differences where the noise didn't overwhelm them.
On kernbench, the differences were in the noise.
Strangely, I see a 4% drop on fork+exec when I used gs for per-cpu vars, which I am now investigating (71.0831 usec before, 71.1725 usec with saving, 73.7458 usec with per-cpu!).
Cheers, Rusty. -- Help! Save Australia from the worst of the DMCA: http://linux.org.au/law
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