Messages in this thread |  | | | Date | Tue, 20 Dec 2011 19:27:54 +0100 | | From | Ingo Molnar <> | | Subject | Re: [RFC 4/5] x86, perf: implements lwp-perf-integration (rc1) |
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* Vince Weaver <vweaver1@eecs.utk.edu> wrote:
> 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.
That does not really contradict what i said.
> 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?
At most an LLWPCB instruction is needed.
> 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.
Only if you are comparing apples to oranges: if you compare a full kernel based read of self-profiling counters with an RDPMC instruction.
But as we told you previously, you could use RDPMC under perf as well, last i checked PeterZ posted experimental patches for that. Peter, what's the status of that?
Thanks,
Ingo
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