Re: [PATCH 00/27] Latest numa/core release, v16

[Ingo Molnar <mingo@kernel.org>]

* 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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