Questions about when we take counter measurements, reset, etc

Robert O'Callahan robert at ocallahan.org
Fri Oct 3 19:26:46 PDT 2014


On Fri, Oct 3, 2014 at 8:18 PM, Kyle Huey <me at kylehuey.com> wrote:

> I've been looking at reviving
> https://github.com/cgjones/rr/compare/match-up-rbc-irc.  The way we
> cache performance counters in Task and reset those, as well as reset
> the actual counters we read from is posing some problems.  This
> caching/resetting/etc happens at points that don't matter for retired
> conditional branches but do matter for instructions retired.  I don't
> think I understand things enough to try to rearrange them though.
> What's the logic/reasoning behind the points at which we take
> measurements, the points at which we reset, etc?
>

The current approach is roughly that we measure and reset the counter every
time we stop and then restart the task. This isn't very clear though; we
should make it clearer by removing ticks_read() and always measuring and
resetting the tick counter in wait().

BTW why do you want to do this work to switch to the instructions-retired
counter? AFAIK there is no guarantee it's viable.

Rob
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