Re: [PATCH 1/2] sched: fix and clean up calculate_imbalance
From: Rik van Riel
Date: Tue Jul 29 2014 - 11:18:15 EST
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On 07/29/2014 10:59 AM, Peter Zijlstra wrote:
> On Tue, Jul 29, 2014 at 11:04:50AM +0200, Vincent Guittot wrote:
>>> In situations where all the domains are overloaded, or where
>>> only the busiest domain is overloaded, that code is also
>>> superfluous, since the normal env->imbalance calculation will
>>> figure out how much to move. Remove the load_above_capacity
>> IMHO, we should not remove that part which is used by
>> Originally, we had 2 type of busiest group: overloaded or
>> imbalanced. You add a new one which has only a avg_load higher
>> than other so you should handle this new case and keep the other
>> ones unchanged
> Right, so we want that code for overloaded -> overloaded migrations
> such as not to cause idle cpus in an attempt to balance things.
> Idle cpus are worse than imbalance.
> But in case of overloaded/imb -> !overloaded migrations we can
> allow it, and in fact want to allow it in order to balance idle
In case the destination is over the average load, or the source is under
the average load, fix_small_imbalance() determines env->imbalance.
The "load_above_capacity" calculation is only reached when busiest is
busier than average, and the destination is under the average load.
In that case, env->imbalance ends up as the minimum of busiest - avg
and avg - target.
Is there any case where limiting it further to "load - capacity" from
the busiest domain makes a difference?
All rights reversed
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