Messages in this thread | | | Date | Mon, 2 May 2011 08:14:11 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v1 00/19] Increase resolution of load weights |
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* Nikhil Rao <ncrao@google.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.
> 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.
Thanks,
Ingo
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