Re: [PATCH 0/5] perf events finer grained context instrumentation /context exclusion

[Ingo Molnar]

* Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:

>  Performance counter stats for './hackbench 5' (10 runs):
> 
>          1313640764  instructions             #      0,241 IPC     ( +-   1,393% )  (scaled from 100,05%)
>           214737441  branches                   ( +-   0,948% )
> 
>          1293802776  instructions             #      0,245 IPC     ( +-   0,343% )
>           209495435  branches                   ( +-   0,392% )

Indeed it's about 4 times less noise, not bad.

Cycles is fundamentally random.

> So yeah, the results look a bit better. Still not perfects:
> 
> - we are still instrumenting the tiny parts between the true interrupt
>   and irq_enter() (same for irq_exit() and the end). Same for softirqs.
> 
> - random randomnesses...

Random randomness shouldnt occur for something like instructions or branches.

Could you try some 'must not be variable' workload, like:

    taskset 1 ./hackbench 1

If the workload is pinned to a single CPU then it ought to not be variable at 
all. (modulo things like hash chain lengths and slab caching details, but 
those should not cause 0.4% kind of noise IMO)

Btw., we could try to record all branches of an execution (using BTS, of a 
relatively short but static-length run), and see where the variance comes 
from. I doubt the current BTS code is ready for that, but it would be 'the' 
magic trace-from-hell that includes all execution of the task, recorded at the 
hardware level.

	Ingo
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