linux-kernel - Re: [PATCH v1 00/19] Increase resolution of load weights

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Message-ID: <BANLkTiktch2mHDLXZCdCED3XoqmkVRfghA@mail.gmail.com>
Date:	Tue, 3 May 2011 17:58:54 -0700
From:	Nikhil Rao <ncrao@...gle.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>, linux-kernel@...r.kernel.org,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Stephan Barwolf <stephan.baerwolf@...ilmenau.de>
Subject: Re: [PATCH v1 00/19] Increase resolution of load weights

On Sun, May 1, 2011 at 11:14 PM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Nikhil Rao <ncrao@...gle.com> wrote:
>
>> 1. Performance costs
>>
>> Ran 50 iterations of Ingo's pipe-test-100k program (100k pipe ping-pongs).
>> See http://thread.gmane.org/gmane.linux.kernel/1129232/focus=1129389 for more
>> info.
>>
>> 64-bit build.
>>
>>   2.6.39-rc5 (baseline):
>>
>>     Performance counter stats for './pipe-test-100k' (50 runs):
>>
>>        905,034,914 instructions             #      0.345 IPC     ( +-   0.016% )
>>      2,623,924,516 cycles                     ( +-   0.759% )
>>
>>         1.518543478  seconds time elapsed   ( +-   0.513% )
>>
>>   2.6.39-rc5 + patchset:
>>
>>     Performance counter stats for './pipe-test-100k' (50 runs):
>>
>>        905,351,545 instructions             #      0.343 IPC     ( +-   0.018% )
>>      2,638,939,777 cycles                     ( +-   0.761% )
>>
>>         1.509101452  seconds time elapsed   ( +-   0.537% )
>>
>> There is a marginal increase in instruction retired, about 0.034%; and marginal
>> increase in cycles counted, about 0.57%.
>
> Not sure this increase is statistically significant: both effects are within
> noise and look at elapsed time, it actually went down.
>
> Btw., to best measure context-switching costs you should do something like:
>
>  taskset 1 perf stat --repeat 50 ./pipe-test-100k
>
> to pin both tasks to the same CPU. This reduces noise and makes the numbers
> more relevant: SMP costs do not increase due to your patchset.
>
> So it would be nice to re-run the 64-bit tests with the pipe test bound to a
> single CPU.

I re-ran the 64-bit tests with the pipe test bound to a single CPU.
Data attached below.

2.6.39-rc5:

 Performance counter stats for './pipe-test-100k' (100 runs):

       855,571,900 instructions             #      0.869 IPC     ( +-   0.637% )
       984,213,635 cycles                     ( +-   0.254% )

        0.796129773  seconds time elapsed   ( +-   0.152% )

2.6.39-rc5  + patchset:

 Performance counter stats for './pipe-test-100k' (100 runs):

       905,553,828 instructions             #      0.934 IPC     ( +-   0.059% )
       969,792,787 cycles                     ( +-   0.168% )

        0.788676004  seconds time elapsed   ( +-   0.122% )


There is a 5.8% increase in instructions which is statistically
significant and well over the error margins. Cycles dropped by about
1.17% and elapsed time also dropped about ~1%. I'm looking into
profiles for this test to understand why instr has increased.

>
>> 32-bit build.
>>
>>   2.6.39-rc5 (baseline):
>>
>>     Performance counter stats for './pipe-test-100k' (50 runs):
>>
>>      1,025,151,722 instructions             #      0.238 IPC     ( +-   0.018% )
>>      4,303,226,625 cycles                     ( +-   0.524% )
>>
>>         2.133056844  seconds time elapsed   ( +-   0.619% )
>>
>>   2.6.39-rc5 + patchset:
>>
>>     Performance counter stats for './pipe-test-100k' (50 runs):
>>
>>      1,070,610,068 instructions             #      0.239 IPC     ( +-   1.369% )
>>      4,478,912,974 cycles                     ( +-   1.011% )
>>
>>         2.293382242  seconds time elapsed   ( +-   0.144% )
>>
>> On 32-bit kernels, instructions retired increases by about 4.4% with this
>> patchset. CPU cycles also increases by about 4%.
>>
>> There is a marginal increase in instruction retired, about 0.034%; and
>> marginal increase in cycles counted, about 0.57%.
>
> These results look more bothersome, a clear increase in both cycles, elapsed
> time, and instructions retired, well beyond measurement noise.
>
> Given that scheduling costs are roughly 30% of that pipe test-case, the cost
> increase to the scheduler is probably around:
>
>        instructions:   +14.5%
>        cycles:         +13.3%
>
> That is rather significant.
>

I'll take a closer look at the performance of this patchset this week.
I'm a little confused about how you calculated the cost to the
scheduler. How did you come up with 14.5 % and 13.3%? Also, out of
curiosity, what's an acceptable tolerance level for a performance hit
on 32-bit?

-Thanks
Nikhil

> Thanks,
>
>        Ingo
>
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