* Dave Hansen <dave.hansen@xxxxxxxxx> 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 -- 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>