On Tue, Jul 29, 2014 at 11:15:49AM -0400, Rik van Riel wrote: > > 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 > > cpus. > > 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?
sadly yes; suppose 8 cpus in 2 groups and 9 tasks, 8 tasks of weight 10, 1 of 1024. The local group will have 5 tasks of 10, the busiest will have the remaining 4. The sd avg is 138, local avg is 12, busiest avg is 263. This gives: busiest-avg = 122, avg - local = 110 So an imbalance of 110. Without limiting it further, we would migrate all 3 10 tasks over to local and run 3 cpus idle. Now running all 8 10 tasks on a single cpu and the 1 1024 task on another and keeping 6 cpus idle is the 'fairest' solution, but that's not the only goal, we also try and be work-conserving, iow. keep as many cpus busy as possible. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
