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

[Vince Weaver]

On Wed, 21 Dec 2011, Peter Zijlstra wrote:

> On Fri, 2011-12-16 at 17:36 -0500, Vince Weaver wrote:
> > Is it possible to read multiple counters at once with this 
> > interface, i.e. using FORMAT_GROUP?
> 
> No, that's not something that I had considered, I'll see if I can make
> that happen by making an array inside the control page instead of a
> single version.

well it's not strictly necessary, I was just checking.  The perfctr
code just does a loop through all available counters too.

At some point PAPI should just abandon using FORMAT_GROUP as it seems to 
be more trouble than its worth.  Though if we have fast rdpmc-based 
counter reads it becomes a little less necessary.

> > 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?
> 
> Depending on how you use it, but yes that's how I'd use it, avoids
> having to do ioctl()s

currently when I tried doing a start ioctl then an immediate read, the 
counter values returned were a very high value.  So unless I did two 
rdpmcs and subtracted, the results made no sense.

was I doing something wrong?  I'll have to investigate more.

> > 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.
> 
> You can of course also do the start/stop/reset in userspace by keeping
> an offset to the counter, avoiding the ioctl()s and simply keeping the
> counter running.

is this guaranteed to work in the face of context switches and/or 
multiplexing?

I need to go back and look at the perfctr code and make sure I understand 
the games it does to ensure that the values it reads out are valid.

Thanks,

Vince

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