Re: smp 'nice' bias support breaks scheduler behavior

["Siddha, Suresh B" <suresh.b.siddha@intel.com>]

On Fri, Jan 27, 2006 at 10:36:36AM +1100, Peter Williams wrote:
> Con Kolivas wrote:
> > On Thursday 26 January 2006 21:52, Siddha, Suresh B wrote:
> >>>[PATCH] sched: implement nice support across physical cpus on SMP
> >>
> >>I don't see imbalance calculations in find_busiest_group() take
> >>prio_bias into account. This will result in wrong imbalance value and
> >>will cause issues.
> > in 2.6.16-rc1:
> > 
> > find_busiest_group(....
> > 
> > 	load = __target_load(i, load_idx, idle);
> > else
> > 	load = __source_load(i, load_idx, idle);
> > 
> > where __target_load and __source_load is where we take into account prio_bias.

We take that into consideration only while calculating the loads.. But we
don't scale it down while calculating imbalance, resulting in the problem
I mentioned.

> > 
> > I'm not sure which code you're looking at, but Peter Williams is working on 
> > rewriting the smp nice balancing code in -mm at the moment so that is quite 
> > different from current linus tree.
> > 

Peters changes in -mm fix this issue. Will this be pushed to Linus tree
before 2.6.16 comes out?

> 
> Yes, indeed.  And it would be very helpful if people interested in this 
> topic (and that have test suites designed to test whether "niceness" is 
> being well balanced across CPUs) could test it.  This is especially the 
> case for larger systems as I do not have ready access for testing on them.

I don't have any test suites for testing "niceness". But I can def check
more to make sure that it doesn't cause any regression :)

thanks,
suresh

> 
> > 
> > 
> >>For example on a DP system with HT, if there are three runnable processes
> >>(simple infinite loop with same nice value), this patch is resulting in
> >>bouncing of these 3 processes from one processor to another...Lets assume
> >>if the 3 processes are scheduled as 2 in package-0 and 1 in package1..
> >>Now when the busy processor on package-1 does load balance and as
> >>imbalance doesn't take "prio_bias" into account, this will kick active
> >>load balance on package-0.. And this is continuing for ever, resulting
> >>in bouncing from one processor to another.
> >>
> >>Even when the system is completely loaded and if there is an imbalance,
> >>this patch causes wrong imabalance counts and cause unoptimized
> >>movements.
> >>
> >>Do you want to look into this and post a patch for 2.6.16?
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