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Date: Tue, 20 Dec 2011 10:48:26 -0500
From: Vince Weaver <vweaver1@eecs.utk.edu>
To: Ingo Molnar <mingo@elte.hu>
CC: Avi Kivity <avi@redhat.com>, Robert Richter <robert.richter@amd.com>,
        Benjamin Block <bebl@mageta.org>,
        Hans Rosenfeld <hans.rosenfeld@amd.com>, <hpa@zytor.com>,
        <tglx@linutronix.de>, <suresh.b.siddha@intel.com>,
        <eranian@google.com>, <brgerst@gmail.com>, <Andreas.Herrmann3@amd.com>,
        <x86@kernel.org>, <linux-kernel@vger.kernel.org>,
        Benjamin Block <benjamin.block@amd.com>
Subject: Re: [RFC 4/5] x86, perf: implements lwp-perf-integration (rc1)
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On Tue, 20 Dec 2011, Ingo Molnar wrote:
> Granted, LWP was mis-designed to quite a degree, those AMD chip 
> engineers should have talked to people who understand how modern 
> PMU abstractions are added to the OS kernel properly. 

You do realize that LWP was probably in design 5+ years ago, at a time 
when most Linux kernel developers wanted nothing to do with perf counters, 
and thus anyone they did contact for help would have been from the 
since-rejected perfctr or perfmon2 camp.

Also, I'm sure Linux isn't the only Operating System that they had in mind 
when designing this functionality.


Running LWP through the kernel is a foolish idea.  Does anyone have any 
numbers on what that would do to overhead?

perf_events creates huge overhead when doing self monitoring.  For simple 
self-monintoring counter reads it is an *order of magnitude* worse than
doing the same thing with perfctr.
  (see numbers here if you don't believe me:
    http://web.eecs.utk.edu/~vweaver1/projects/perf-events/benchmarks/rdtsc_overhead/ )

Vince