Re: [PATCH 6/6] perf: Increase round-robin fairness of flexible events

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Jan 11, 2010 at 11:39:05AM +1100, Paul Mackerras wrote:
> On Mon, Jan 11, 2010 at 12:57:59AM +0100, Frederic Weisbecker wrote:
> 
> > I think the constraint of "either every or none get
> > scheduled in a group" makes a lot of sense for pinned
> > groups.
> > 
> > But I don't see the point in applying this
> > rule inside flexible groups because the nature
> > of flexible events implies these have been created to
> > fight against a limited resource. So if this fight
> > is done only between groups, this is like raising
> > a voluntary starvation.
> > 
> > Or..or..May be I just realize too late that the semantic
> > of a group implies that all events inside must be always
> > counted simultaneously? In which case I agree with you,
> > this patch makes no sense and must be dropped.
> 
> The original idea of the groups was for situations where you want to
> take the difference or ratio of two counts.  For example, if you want
> to measure cache hits but the hardware can only count cache accesses
> and cache misses.  In that situation you want to compute accesses
> minus misses, but if the counters for accesses and for misses are
> independently scheduled, statistical fluctuations can mean there is a
> lot of noise in the result, and it might even be negative.  Putting
> the two counters into one group means that you can meaningfully
> compute the difference or ratio since the two counter values relate to
> the same set of instructions (even if that isn't the whole execution
> of the program).
> 
> The default situation is that each event is in its own group, so the
> starvation you talk about won't arise.  If the user has gone to the
> trouble of putting two events into one group, then they are saying
> that they need the events to be scheduled on and off together, and if
> that leads to starvation, that's unfortunate but we can't do any
> better within the limitations of the hardware.



Agreed. This patch came from my misunderstanding of the purpose of
groups.