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

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Date:	Fri, 16 Dec 2011 17:36:35 -0500
From:	Vince Weaver <vweaver1@...s.utk.edu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	<mingo@...e.hu>, William Cohen <wcohen@...hat.com>,
	Stephane Eranian <eranian@...gle.com>,
	Arun Sharma <asharma@...com>, Vince Weaver <vince@...ter.net>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 0/6] perf: x86 RDPMC and RDTSC support

On Mon, 21 Nov 2011, 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've finally had time to apply and test these patches.

Firstly, the documentation was a bit sparse (though I guess that's OK for 
an RFC patch).  Is it possible to read multiple counters at once with this 
interface, i.e. using FORMAT_GROUP?

Also, am I correct that the counters are set to always counting, so you 
always have to do a rdpmc() before and a rdpmc() after?

I ran some benchmarks, the results can be found here:
   http://web.eecs.utk.edu/~vweaver1/projects/perf-events/benchmarks/rdtsc_overhead/

I used the mmap_read_self() routine from your example as the "read" 
performance that I measured.

If start/stop are truly unnecessary when run with your patch then the 
"total" results shift in your favor.  Otherwise perfmon2 and perfctr still 
win the self-monitoring overhead race by a lot.

Vince

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