Messages in this thread | | | Date | Wed, 21 Nov 2012 23:46:53 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 00/27] Latest numa/core release, v16 |
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Wed, Nov 21, 2012 at 7:10 AM, Ingo Molnar <mingo@kernel.org> 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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