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

[Ingo Molnar]

* 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
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