Re: [PATCH 0/3] TLB flush multiple pages per IPI v5

Ingo Molnar
2015-06-08T21:51:03Z

* Dave Hansen <dave.hansen@intel.com> wrote:

> On 06/08/2015 12:52 PM, Ingo Molnar wrote:
> > A CR3 driven TLB flush takes less time than a single INVLPG (!):
> >
> > [ 0.389028] x86/fpu: Cost of: __flush_tlb() fn : 96 cycles
> > [ 0.405885] x86/fpu: Cost of: __flush_tlb_one() fn : 260 cycles
> > [ 0.414302] x86/fpu: Cost of: __flush_tlb_range() fn : 404 cycles
>
> How was that measured, btw? Are these instructions running in a loop?

Yes - see the x86 benchmarking patch in the big FPU submission for an earlier
version.

> Does __flush_tlb_one() include the tracepoint?

No tracing overhead.

> (From the commit I referenced) This was (probably) using a different method than
> you did, but "FULL" below is __flush_tlb() while "1" is __flush_tlb_one(). The
> "cycles" includes some overhead from the tracing:
>
> > FULL: 2.20% 2.20% avg cycles: 2283 cycles/page: xxxx samples: 23960
> > 1: 56.92% 59.12% avg cycles: 1276 cycles/page: 1276 samples: 620895
>
> So it looks like we've got some discrepancy, either from the test methodology or
> the CPU. All of the code and my methodology are in the commit. Could you share
> yours?

Yes, you can reproduce it by applying this patch from the FPU series:

Subject: [PATCH 207/208] x86/fpu: Add FPU performance measurement subsystem

(you were Cc:-ed to it, so it should be in your inbox.)

I've got a more advanced version meanwhile, will post it in the next couple of
days or so.

> > it's true that a full flush has hidden costs not measured above, because it has
> > knock-on effects (because it drops non-global TLB entries), but it's not _that_
> > bad due to:
> >
> > - there almost always being a L1 or L2 cache miss when a TLB miss occurs,
> > which latency can be overlaid
> >
> > - global bit being held for kernel entries
> >
> > - user-space with high memory pressure trashing through TLBs typically
> >
> > ... and especially with caches and Intel's historically phenomenally low TLB
> > refill latency it's difficult to measure the effects of local TLB refills, let
> > alone measure it in any macro benchmark.
>
> All that you're saying there is that you need to consider how TLB misses act in
> _practice_ and not just measure worst-case or theoretical TLB miss cost. I
> completely agree with that.

So I'm saying considerably more than that: I consider it likely that a full TLB
flush is not nearly as costly as assumed, for the three reasons outlined above.

It might even be a performance win in Mel's benchmark - although possibly not
measurable within measurement noise levels.

Thanks,

Ingo