Re: IV.2 - Features impacting all counters

Ingo Molnar
2009-06-22T12:00:20Z

> 2/ Features impacting all counters
>
> On some PMU models, e.g., Itanium, they are certain features which
> have an influence on all counters that are active. For instance,
> there is a way to restrict monitoring to a range of continuous
> code or data addresses using both some PMU registers and the debug
> registers.
>
> Given that the API exposes events (counters) as independent of
> each other, I wonder how range restriction could be implemented.

A solution is to make it a per-counter attribute and fail to
schedule multiple counters at the same time when these constraints
differ.

> Similarly, on Itanium, there are global behaviors. For instance,
> on counter overflow the entire PMU freezes all at once. That seems
> to be contradictory with the design of the API which creates the
> illusion of independence.
>
> What solutions do you propose?

We propose the same solution as last time: live with the small
imprecisions caused by such hardware limitations. We submit that
natural workload noise is probably far bigger than any such effect.
We could certainly list it as a platform limitation.