lkml.org 
[lkml]   [2011]   [Nov]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [patch 4/7] sched: convert rq->avg_idle to rq->avg_event
From
Date
On Wed, 2011-11-23 at 13:27 +0100, Peter Zijlstra wrote:
> On Wed, 2011-11-23 at 13:09 +0100, Mike Galbraith wrote:
> > On Wed, 2011-11-23 at 12:55 +0100, Peter Zijlstra wrote:
> > > On Tue, 2011-11-22 at 15:22 +0100, Mike Galbraith wrote:
> > > > We update rq->clock only at points of interest to the scheduler.
> > > > Using this distance has the same effect as measuring idle time
> > > > for idle_balance() throttling, and allows other uses as well.
> > >
> > > I'm not sure I follow, suppose we're happily context switching away, how
> > > is the avg distance between context switches related to idle time?
> >
> > Average idle time can't be larger.
>
> True :-)
>
> But it can be _much MUCH_ smaller. So the value is a fair upper limit on
> the idle time, but has no relation to the actual idle duration.

Yup, none.

> Now this value seems to be used in 5 to throttle select_idle_sibling(),
> which is again something unrelated to actual idle duration, but also
> unrelated to the avg update_rq_clock() interval.

Yup. Clock update is a thing we do at sched event-of-interest, so this
seemed like a good spot to create a multi-purpose number.

> In patch 6 we use this value to guestimate if we should enter nohz,
> since its a wild over estimation of the actual idle duration it'll be
> less effective and might not hard much.
>
> Also, patch 6's use of sched_migration_cost to reflect the nohz
> enter/exit cost is somewhat iffy, but that's another issue.

Yup.

> Now I'm not saying this all isn't worth it, just saying my brain is
> having difficulty seeing how it all makes sense :-)

They make sense only in that one cheap number generator bandaids three
owies. It's fugly but effective :)

-Mike



\
 
 \ /
  Last update: 2011-11-23 13:59    [W:0.155 / U:0.392 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site