Re: [RFC 4/5] x86, perf: implements lwp-perf-integration (rc1)

Vince Weaver
2011-12-20T15:48:52Z

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