Re: [RFC patch 1/2] sched: dynamically adapt granularity with nr_running

Steven Rostedt
2010-09-13T14:43:13Z

On Mon, 2010-09-13 at 16:16 +0200, Peter Zijlstra wrote:

> > > - if (delta_exec < sysctl_sched_min_granularity)
> > > - return;
> >
> > Well, the reason why this test is here seems to be that we don't want to trigger
> > "resched_task" more often than needed, and here it's defined by the granularity.
>
> Right, but its wrong for the weighted case. Letting a light task run
> that long will make its latency suck.
>
> > I don't quite see with what you are replacing this, other than "let's set the
> > resched flag all the time to save a 32-bit division". I figure out it's more
> > expensive the call the scheduler than to do a 32-bit div.
>
> The more divs we put it, the more expensive it all becomes.
>

What about:

if (delta_exec < sysctl_sched_min_granularity ||
delta_exec < __sched_gran(cfs_rq->nr_running))

This way we avoid the div when delta_exec is less than the
min_granularity, and then we can do the div to perhaps avoid a needless
resched?

-- Steve