linux-kernel - Re: [RFC][PATCH 0/6] perf: x86 RDPMC and RDTSC support

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Date:	Fri, 2 Dec 2011 14:22:02 -0800
From:	Stephane Eranian <eranian@...gle.com>
To:	Arun Sharma <asharma@...com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, mingo@...e.hu,
	William Cohen <wcohen@...hat.com>,
	Vince Weaver <vince@...ter.net>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 0/6] perf: x86 RDPMC and RDTSC support

Have you tried with inheritance disabled?
I don't remember if perf stat has it enabled buy default.

On Fri, Dec 2, 2011 at 11:26 AM, Arun Sharma <asharma@...com> wrote:
> On 11/21/11 6:51 AM, Peter Zijlstra wrote:
>>
>> These few patches implement x86 RDPMC support and add an extention to the
>> self
>> monitoring data to also allow additional time updates using userspace TSC
>> reads.
>>
>> There's a few loose ends, but it mostly seems to work.
>
>
> I haven't had a chance to test this out yet. But low overhead, always on
> perf counters is something we're very interested in. Thanks for implementing
> it.
>
> However, I suspect the major cost of leaving the perf counters always on
> seems to be in the hit on context switches, rather than the cost of reading
> the perf counters themselves. For eg:
>
>  Baseline:
>
>   (for i in `seq 1 10`; do numactl --cpunodebind 1 ./lat_ctx -P1 -s32k 4;
> done) 2>&1 | tee lmbench1.log
>
>  1 event:
>
>  (for i in `seq 1 10`; do numactl --cpunodebind 1 perf stat -e instructions
> ./lat_ctx -P1 -s32k 4; done) 2>&1 | tee lmbench2.log
>
>  2 events:
>
>  (for i in `seq 1 10`; do numactl --cpunodebind 1 perf stat -e
> cycles,instructions ./lat_ctx -P1 -s32k 4; done) 2>&1 | tee lmbench3.log
>
>  Baseline: 2.2us
>  One event: 6.8us
>  Two events: 7.2us
>
> The cost seems to be at roughly 5us (I measured 2.6.38 and 3.2-rc2). I'll
> dig a bit more on what may be going on here.
>
>  -Arun
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