Re: [PATCH] x86: Align TLB invalidation info

[Nadav Amit]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Wed, Jan 31, 2018 at 09:38:46PM -0800, Nadav Amit wrote:
> 
>> I used ftrace to measure the execution time of flush_tlb_func_remote() on a
>> 2-socket Haswell machine, using a microbenchmark I wrote for some research
>> project.
> 
> However cool ftrace is, it is _really_ bad for such uses. The cost of
> using ftrace is many many time higher than any change you could affect
> by this.
> 
> A microbench and/or perf is what you should use for this.

Donât expect to see a remote NUMA access impact, whose cost are few 10s of
nanoseconds on microbenchmarks. (And indeed I did not.) Each iteration of
#PF - MADV_DONTNEED takes several microseconds, and the impact is lost in
the noise.

You are right in the fact that ftrace introduces overheads, but the variance
is relatively low. If I stretch the struct to 3 lines of cache, I see a 20ns
overhead. Anyhow, I think this line of code got more than its fair share of
attention.

Attachment: signature.asc
Description: Message signed with OpenPGP