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

From: Ingo Molnar
Date: Thu Jun 10 2010 - 06:17:12 EST

* 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.

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