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SubjectRe: [PATCH v1 00/19] Increase resolution of load weights

* 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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