* 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>