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SubjectRe: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry
On Sun, May 3, 2015 at 10:30 AM, Rik van Riel <riel@redhat.com> wrote:
> On 05/03/2015 09:23 AM, Mike Galbraith wrote:
>
>> Below are v4.1-rc1-172-g6c3c1eb3c35e + patches measurements.
>>
>> 100M * stat() on isolated cpu
>>
>> NO_HZ_FULL off inactive housekeeper nohz_full
>> real 0m14.266s 0m14.367s 0m20.427s 0m27.921s
>> user 0m1.756s 0m1.553s 0m1.976s 0m10.447s
>> sys 0m12.508s 0m12.769s 0m18.400s 0m17.464s
>> (real) 1.000 1.007 1.431 1.957
>> 1.000 1.000
>>
>> real 0m20.423s 0m27.930s +rik 1,2
>> user 0m2.072s 0m10.450s
>> sys 0m18.304s 0m17.471s
>> vs off 1.431 1.957
>> vs prev 1.000 1.000
>>
>> real 0m20.256s 0m27.803s +paolo 1,2 (2 missing prototypes)
>> user 0m1.884s 0m10.551s
>> sys 0m18.353s 0m17.242s
>> vs off 1.419 1.948
>> vs prev .991 .995
>>
>> real 0m19.122s 0m26.946s +rik 3
>> user 0m1.896s 0m10.292s
>> sys 0m17.198s 0m16.644s
>> vs off 1.340 1.888
>> vs prev .944 .969
>
> I'm convinced.
>
> Time to try the remote sampling of CPU use statistics, and
> lighten up the RCU overhead of context tracking.
>

I don't understand the remote sampling proposal. Isn't the whole
point of full nohz to avoid periodically interrupting a busy CPU? If
what you have in mind is sending IPIs, then that's just as bad, right?

If, on the other hand, you're just going to remotely sample the
in-memory context, that sounds good.

--Andy


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