Re: Nick's scheduler v17

[Nick Piggin]

Andrew Theurer wrote:

> > > >    *imbalance = min(this_load - load_avg, load_avg - max_load)
> > > >
> > > > That way you take just enough to either have busiest_queue or
> > > > this_rq's length be the load_avg.  I suppose you could take even
> > > > less, but IMO, the /=2 is what I really don't like.  Perhaps:
> > > That is _exactly_ what I had before! Thats probably the way to go. Thanks
> > > for having a look at it.
> > >
> > >
> > > > *imbalance = min(this_load - load_avg, load_avg - max_load);
> > > > *imbalance = (*imbalance + FPT - 1) / FPT;
> > > >
> > > > This should work well for intranode balances, internode balances may
> > > > need a little optimization, since the load_avg really does not really
> > > > represent the load avg of the two nodes in question, just one cpu
> > > > from one of them and all the cpus from another.
> > Oh, actually, after my path, load_avg represents the load average of _all_
> > the nodes. Have a look at find_busiest_node. Which jogs my memory of why
> > its not always a good idea to do your *imbalance min(...) thing (I actually
> > saw this happening).
>
> Oops, I meant avg_load, which you calculate in find_busiest_queue on the fly.  

OK

> > 5 CPUs, 4 processes running on one cpu. load_avg would be 0.8 for all cpus.
> > balancing doesn't happen. I have to think about this a bit more...
>
> Actually, if we use avg_load, I guess it would be 0, since this is an unsigned 
> long.  Maybe avg_load needs to have a min value of 1.  Then if we apply:

Well its got a fixed point scaling factor.

> *imbalance = min(max_load - avg_load, avg_load - this_load)
> 	     min(4 - 1, 1 - 0)
> 	     	

I think you want:

*imbalance = min(max_load - avg_load, avg_load - this_load)
if ( (*imbalance < 1*FPT) &&
       (max_load - this_load) > 1*FPT )
   *imbalance = 1*FPT;

So if there is a total imbalance of more than 1 task, at least one
will be moved.



> And imbalance looks a lot better.  Only concern would be an idle cpu stealing 
> from another, leaving the other cpu idle.  I guess a check could be put 
> there.	
> 	     

pull_task won't pull a running task, so you get some protection there.


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