On Wednesday 15 November 2006 18:46, Eric Dumazet wrote:
> On Wednesday 15 November 2006 18:24, Andi Kleen wrote:
> > On Wednesday 15 November 2006 18:20, Ingo Molnar wrote:
> > > * Andi Kleen <[email protected]> wrote:
> > > > On Wednesday 15 November 2006 12:27, Eric Dumazet wrote:
> > > > > Seeing %gs prefixes used now by i386 port, I recalled seeing
> > > > > strange oprofile results on Opteron machines.
> > > > >
> > > > > I really think %gs prefixes can be expensive in some (most ?)
> > > > > cases, even if the Intel/AMD docs say they are free.
> > > >
> > > > They aren't free, just very cheap.
> > >
> > > Eric's test shows a 5% slowdown. That's far from cheap.
> >
> > I have my doubts about the accuracy of his test results. That is why I
> > asked him to double check.
>
> Fair enough :)
>
> I plan doing *lot* of tests as soon as possible (not possible during
> daytime unfortunately, I miss a dev machine)
>
I did *lot* of reboots of my Dell D610 machine, with some trivial benchmarks
using : pipe/write()/read, umask(), or getppid(), using or not oprofile.
I managed to avoid reloading %gs in sysenter_entry .
(avoiding the two instructions : movl $(__KERNEL_PDA), %edx; movl %edx, %gs
I could not avoid reloading %gs in system_call, I dont know why, but modern
glibc use sysenter so I dont care :)
I confirm I got better results with my patched kernel in all tests I've done.
umask : 12.64 s instead of 12.90 s
getppid : 13.37 s instead of 13.72 s
pipe/read/write : 9.10 s instead of 9.52 s
(I got very different results in umask() bench, patching it not to use xchg(),
since this instruction is expensive on x86 and really change oprofile
results. I will submit a patch for this.
Eric
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