Re: -mm seems significanty slower than mainline on kernbench

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

Con Kolivas wrote:
> On Thu, 12 Jan 2006 11:54 am, Peter Williams wrote:
> 
>>Peter Williams wrote:
>>
>>>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.
>>
>>Addressed in a separate e-mail.
>>
>>
>>>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.
> 
> 
> This is a shot in the dark. We haven't confirmed 1. there is a problem 2. that 
> this is the problem nor 3. that this patch will fix the problem.

I disagree.  I think that there is a clear mistake in my original patch 
that this patch fixes.

> I say we 
> wait for the results of 1. If the improved smp nice handling patch ends up 
> being responsible then it should not be merged upstream, and then this patch 
> can be tested on top.
> 
> Martin I know your work move has made it not your responsibility to test 
> backing out this change, but are you aware of anything being done to test 
> this hypothesis?

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

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