Re: -mm seems significanty slower than mainline on kernbench

[Peter Williams <pwil3058@bigpond.net.au>]

Peter Williams wrote:
> Martin Bligh wrote:
>
> > > > But I was thinking more about the code that (in the original) 
> > > > handled the case where the number of tasks to be moved was less than 
> > > > 1 but more than 0 (i.e. the cases where "imbalance" would have been 
> > > > reduced to zero when divided by SCHED_LOAD_SCALE).  I think that I 
> > > > got that part wrong and you can end up with a bias load to be moved 
> > > > which is less than any of the bias_prio values for any queued tasks 
> > > > (in circumstances where the original code would have rounded up to 1 
> > > > and caused a move).  I think that the way to handle this problem is 
> > > > to replace 1 with "average bias prio" within that logic.  This would 
> > > > guarantee at least one task with a bias_prio small enough to be moved.
> > > >
> > > > I think that this analysis is a strong argument for my original 
> > > > patch being the cause of the problem so I'll go ahead and generate a 
> > > > fix. I'll try to have a patch available later this morning.
> > >
> > >
> > >
> > >
> > > Attached is a patch that addresses this problem.  Unlike the 
> > > description above it does not use "average bias prio" as that 
> > > solution would be very complicated.  Instead it makes the assumption 
> > > that NICE_TO_BIAS_PRIO(0) is a "good enough" for this purpose as this 
> > > is highly likely to be the median bias prio and the median is 
> > > probably better for this purpose than the average.
> > >
> > > Signed-off-by: Peter Williams <pwil3058@bigpond.com.au>
> >
> >
> >
> > Doesn't fix the perf issue.
>
>
> OK, thanks.  I think there's a few more places where SCHED_LOAD_SCALE 
> needs to be multiplied by NICE_TO_BIAS_PRIO(0).  Basically, anywhere 
> that it's added to, subtracted from or compared to a load.  In those 
> cases it's being used as a scaled version of 1 and we need a scaled 

This would have been better said as "the load generated by 1 task" 
rather than just "a scaled version of 1".  Numerically, they're the same 
but one is clearer than the other and makes it more obvious why we need 
NICE_TO_BIAS_PRIO(0) * SCHED_LOAD_SCALE and where we need it.

> version of NICE_TO_BIAS_PRIO(0).  I'll have another patch later today.

I'm just testing this at the moment.

Peter
--
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
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