Re: Proposed systemtap access to perfmon hardware

[Maynard Johnson <maynardj at us dot ibm dot com>]

Stone, Joshua I wrote:

> Maynard Johnson wrote:
>  
>
> > William Cohen wrote:
> >    
> >
> > > The individual start and stop operations would be allowed.
> > >      
> > This is not so good.  Besides the fact that it may be difficult (or
> > impossible) to do, I don't see it being all that useful.  But then,
> > I'm a tool developer, not a performance analyst, so I could be
> > missing the point.
> >    
>
> Enabling start & stop lets you narrow the context that you want to
> measure.  Perfmon can only give you thread level virtualization of the
> counters.  With start & stop I can, for example, start the counters when
> I enter sys_open and stop when I return.  Now if I want I can get a
> microbenchmark of IPC for the sys_open call (and its callees).
>
> But this also opens up possibilities for more obscure "contexts" -
> perhaps I want to start counting when a network packet is received and
> stop when it is delivered to the thread.  Any probepoint you can do
> today can become a start/stop point for the counters.
>  
Yes, I can certainly see this benefit.  It gives you PAPI-level control 
without having to modify source code.  My concern, however, was that if 
you have multiple counters configured, then individual control of them 
presents an extra level of difficulty.  But, as I've been thinking about 
this a bit more, I think this could be done if you can guarantee that 
the operation is not preempted or interrupted.  Then, the PMU can be 
disabled, reloaded with any changes, and then re-enabled.  Then, any 
counters that had been running before the operation -- and that were not 
changed by the operation -- will be reloaded with their previous count 
and continue running from where they left off.

-Maynard

> Josh
>