* Ingo Molnar <mingo@xxxxxxxxxx> wrote: > So what I measured agrees generally with the comment you added in the commit: > > + * Each single flush is about 100 ns, so this caps the maximum overhead at > + * _about_ 3,000 ns. > > Let that sink through: 3,000 nsecs = 3 usecs, that's like eternity! > > 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 > > 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 I also have cache-cold numbers from another (Intel) system: [ 0.176473] x86/bench:########################################################################## [ 0.185656] x86/bench: Running x86 benchmarks: cache- hot / cold cycles [ 1.234448] x86/bench: Cost of: null : 35 / 73 cycles [ ........] [ 27.930451] x86/bench:######## MM instructions: ###################################### [ 28.979251] x86/bench: Cost of: __flush_tlb() fn : 251 / 366 cycles [ 30.028795] x86/bench: Cost of: __flush_tlb_global() fn : 746 / 1795 cycles [ 31.077862] x86/bench: Cost of: __flush_tlb_one() fn : 237 / 883 cycles [ 32.127371] x86/bench: Cost of: __flush_tlb_range() fn : 312 / 1603 cycles [ 35.254202] x86/bench: Cost of: wbinvd() insn : 2491761 / 2491922 cycles Note how the numbers are even worse in the cache-cold case: the algorithmic complexity of __flush_tlb_range() versus __flush_tlb() makes it run slower (because we miss the I$), while the TLB cache-preservation argument is probably weaker, because when we are cache cold then TLB refill latency probably matters less (as it can be overlapped). So __flush_tlb_range() is software trying to beat hardware, and that's almost always a bad idea on x86. 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>