13-15 November 2018
America/Vancouver timezone

Towards improved selection of CPU idle states

15 Nov 2018, 09:50
20m
Junior-Ballroom-AB (Sheraton Vancouver Wall Center)

Junior-Ballroom-AB

Sheraton Vancouver Wall Center

100

Speaker

Rafael Wysocki (Intel Open Source Technology Center)

Description

The venerable menu governor does some thigns that are quite questionable in my view. First, it includes timer wakeups in the pattern detection data and mixes them up with wakeups from other sources which in some cases causes it to expect what essentially would be a timer wakeup in a time frame in which no timer wakeups are possible (becuase it knows the time until the next timer event and that is later than the expected wakeup time). Second, it uses the extra exit latency limit based on the predicted idle duration and depending on the number of tasks waiting on I/O, even though those tasks may run on a different CPU when they are woken up. Moreover, the time ranges used by it for the sleep length correction factors are not correlated to the list of available idle states in any way whatever and different correction factors are used depending on whether or not there are tasks waiting on I/O, which again doesn't imply anything in particular.

A major rework of the menu governor would be required to address these issues and it is likely that the performance of some workloads would suffer from that. That raises the question of whether or not to try to improve the menu governor or to introduce an entirely new one to replace it, or to do both these things simultaneously.

Primary author

Rafael Wysocki (Intel Open Source Technology Center)

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