* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Nov 21, 2012 at 7:10 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > Because scalability slowdowns are often non-linear.
>
> Only if you hold locks or have other non-cpu-private activity.
>
> Which the vsyscall code really shouldn't have.
Yeah, the faults accessing any sort of thread shared cache line
was my main thinking - the vsyscall faults are so hidden, and
David's transaction score was so low that I could not exclude
some extremely high page fault rate (which would not get
reported by anything other than a strange blip on the profile).
I was thinking of a hundred thousand vsyscall page faults per
second as a possibility - SPECjbb measures time for every
transaction.
So this was just a "maybe-that-has-an-effect" blind theory of
mine - and David's testing did not confirm it so we know it was
a bad idea.
I basically wanted to see a profile from David that looked as
flat as mine - that would have excluded a handful of unknown
unknowns.
> That said, it might be worth removing the
> "prefetchw(&mm->mmap_sem)" from the VM fault path. Partly
> because software prefetches have never ever worked on any
> reasonable hardware, and partly because it could seriously
> screw up things like the vsyscall stuff.
Yeah, I was wondering about that one too ...
> I think we only turn prefetchw into an actual prefetch
> instruction on 3DNOW hardware. Which is the *old* AMD chips. I
> don't think even the Athlon does that.
>
> Anyway, it might be interesting to see a instruction-level
> annotated profile of do_page_fault() or whatever
Yes.
> > So with CONFIG_NUMA_BALANCING=y we are taking a higher page
> > fault rate, in exchange for a speedup.
>
> The thing is, so is autonuma.
>
> And autonuma doesn't show any of these problems. [...]
AutoNUMA regresses on this workload, at least on my box:
v3.7 AutoNUMA | numa/core-v16 [ vs. v3.7]
----- -------- | ------------- -----------
|
[ SPECjbb transactions/sec ] |
[ higher is better ] |
|
SPECjbb single-1x32 524k 507k | 638k +21.7%
It regresses by 3.3% over mainline. [I have not measured a
THP-disabled number for AutoNUMA.]
Maybe it does not regress on David's box - I have just
re-checked all of David's mails and AFAICS he has not reported
AutoNUMA SPECjbb performance.
> Why are you ignoring that fact?
I'm not :-(
Thanks,
Ingo
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