* Ingo Molnar <mi...@kernel.org> 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
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