[lkml]   [2006]   [Jan]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: -mm seems significanty slower than mainline on kernbench
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
>>>>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 Williams

"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2006-01-12 02:31    [W:0.084 / U:2.760 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site