[lttng-dev] Tracing peak size of the running queue

[Michel Dagenais]

> I thinkg it might be useful if I could get some representation 
> (graphical or otherwise) about the number of RUNNable tasks at any
> given time to get an idea of how much contention is ongoing on the CPU.
> 
> So I guess something like the following information:
> - peak measurement of the running queue length
> - max/mean task latency (i.e. time from the task enters the running 
> queue to the time it actually gets the CPU)
> - cpu distribution among tasks
> when measured over a given time window, might help a lot understand 
> these problematic scenarios.

Yes, these would be nice in the GUI views. This is on our TODO list. The current statistics are precomputed in the background but do not provide quick access to intermediate values for intervals. For small intervals this is not a problem but for large intervals rescanning every event would be costly. We now have a nice scheme that we prototyped to keep intermediate results and quickly compute values for any interval. We need to integrate that stuff.

Note that lttng-top offers that type of information and views, for lttng 2.0 and with newer kernels.

> I think the above information could be so useful that I am surprised 
> lttng does not provide it out of the box. After all, ALL scheduling 
> events are traced so it should trivially be a matter of displaying
> this data.

Correct, the challenge is to do it efficiently for large intervals. You may want to contribute a view with a trivial implementation and we would insert our optimized algorithm later.