Messages in this thread | | | From | Paul Turner <> | Date | Wed, 8 May 2013 05:00:34 -0700 | Subject | Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new forked task |
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On Wed, May 8, 2013 at 4:34 AM, Peter Zijlstra <peterz@infradead.org> wrote: > On Tue, May 07, 2013 at 04:20:55AM -0700, Paul Turner wrote: >> Yes, 1024 was only intended as a starting point. We could also >> arbitrarily pick something larger, the key is that we pick >> _something_. >> >> If we wanted to be more exacting about it we could just give them a >> sched_slice() worth; this would have a few obvious "nice" properties >> (pun intended). > > Oh I see I misunderstood again :/ Its not about the effective load but weight > of the initial effective load wrt adjustment. > > Previous schedulers didn't have this aspect at all, so no experience from me > here. Paul would be the one, since he's ran longest with this stuff. > > That said, I would tend to keep it shorter rather than longer so that it would > adjust quicker to whatever it really wanted to be. > > Morten says the load is unstable specifically on loaded systems.
Here, Morten was (I believe) referring to the stability at task startup.
To be clear: Because we have such a small runnable period denominator at this point a single changed observation (for an equivalently behaving thread) could have a very large effect. e.g. fork/exec -- happen to take a major #pf, observe a "relatively" long initial block.
By associating an initial period (along with our full load_contrib) here, we're making the denominator larger so that these effects are less pronounced; achieving better convergence towards what our load contribution should actually be.
Also: We do this conservatively, by converging down, not up.
> I would think > this is because we'd experience scheduling latency, we're runnable more pushing > things up. But if we're really an idle task at heart we'd not run again for a > long while, pushing things down again.
Exactly, this is why we must be careful to use instaneous weights about wake-up decisions. Interactive and background tasks are largely idle.
While this is exactly how we want them to be perceived from a load-balance perspective it's important to keep in mind that while wake-up placement has a very important role in the overall balance of a system, it is not playing quite the same game as the load-balancer.
> > So on that point Paul's suggestion of maybe starting with __sched_slice() might > make sense because it increases the weight of the initial avg with nr_running. > Not sure really, we'll have to play and see what works best for a number of > workloads.
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