Re: [Perfctr-devel] Re: Enabling RDPMC in user space by default

[Stephane Eranian <eranian@hpl.hp.com>]

Nicholas,

On Wed, Nov 30, 2005 at 12:22:42AM -0800, Nicholas Miell wrote:
> On Tue, 2005-11-29 at 23:38 -0800, Stephane Eranian wrote:
> > Nicholas,
> > 
> > On Tue, Nov 29, 2005 at 03:29:26PM -0800, Nicholas Miell wrote:
> > > On Wed, 2005-11-30 at 00:17 +0100, Andi Kleen wrote:
> > > > On Tue, Nov 29, 2005 at 03:02:18PM -0800, Nicholas Miell wrote:
> > > > > On Tue, 2005-11-29 at 23:43 +0100, Andi Kleen wrote:
> > > > > > To give an bad analogy RDTSC usage in the last years is
> > > > > > like explicit spinning wait loops for delays in the earlier
> > > > > > times. They tended to work on some subset of computers,
> > > > > > but were always bad and caused problems and people eventually learned
> > > > > > it was better to use operating system services for this.
> > > > > 
> > > > > And you are now suggesting people should use RDPMC instead of OS
> > > > > services?
> > > > 
> > > > For any kind of timers they should use the OS service 
> > > > (gettimeofday/clock_gettime). The OS will go to extraordinary
> > > > means to make it as fast as possible, but when it's slow
> > > > then because it's not possible to do it faster accurately
> > > > (that's the case right now modulo one possible optimization)
> > > > 
> > > > For cycle counting where they previously used RDTSC they should
> > > > use RDPMC 0 now.
> > > 
> > > Well, if that's all you want them to use RDPMC 0 for, why not just make
> > > PMCs programmable from userspace?
> > > 
> > Simply because write a PERFSEL (i.e. an MSR) register is a privileged operation.
> 
> Well, yeah. Design an API (or just steal from Solaris) and expose that
> to userspace.
> 
And that is exactly what I am doing with the perfmon2 interface that I 
am working on. It does expose the performance counters to users via
a system call. With it you can certainly program a PERFSEL to count
cycles and you can use RDPMC from ring 3 to read the current value.

-- 
-Stephane
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