Re: -mm seems significanty slower than mainline on kernbench

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

Con Kolivas wrote:
> On Wednesday 11 January 2006 23:24, Peter Williams wrote:
> 
>>Martin J. Bligh wrote:
>>
>>>That seems broken to me ?
>>
>>But, yes, given that the problem goes away when the patch is removed
>>(which we're still waiting to see) it's broken.  I think the problem is
>>probably due to the changed metric (i.e. biased load instead of simple
>>load) causing idle_balance() to fail more often (i.e. it decides to not
>>bother moving any tasks more often than it otherwise would) which would
>>explain the increased idle time being seen.  This means that the fix
>>would be to review the criteria for deciding whether to move tasks in
>>idle_balance().
> 
> 
> Look back on my implementation. The problem as I saw it was that one task 
> alone with a biased load would suddenly make a runqueue look much busier than 
> it was supposed to so I special cased the runqueue that had precisely one 
> task.

OK.  I'll look at that.

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.

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

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