Re: [RFC PATCH 00/11] sched: CFS low-latency features

Mike Galbraith
2010-08-27T08:19:49Z

On Fri, 2010-08-27 at 09:42 +0200, Peter Zijlstra wrote:
> On Thu, 2010-08-26 at 19:49 -0400, Mathieu Desnoyers wrote:
> > AFAIK, I don't think we would end up starving the system in any possible way.
>
> Correct, it does maintain fairness.
>
> > So far I cannot see a situation where selecting the next buddy would _not_ make
> > sense in any kind of input-driven wakeups (interactive, timer, disk, network,
> > etc). But maybe it's just a lack of imagination on my part.
>
> The risk is that you end up with always using next-buddy, and we tried
> that a while back and that didn't work well for some, Mike might
> remember.

I turned it off because it was ripping spread apart badly, and last
buddy did a better job of improving scalability without it.

> Also, when you use timers things like time-outs you really couldn't care
> less if its handled sooner rather than later.
>
> Disk is usually so slow you really don't want to consider it
> interactive, but then sometimes you might,.. its a really hard problem.

(very hard)

> The only clear situation is the direct input, that's a direct link
> between the user and our wakeup chain and the user is always important.

Yeah, directly linked wakeups using next could be a good thing, but the
trouble with using any linkage to the user is that you have to pass it
on to reap benefit.. so when do you disconnect?

-Mike