Messages in this thread | | | Subject | Re: [PATCH] Load balancing problem in 2.6.2-mm1 | Date | Fri, 06 Feb 2004 15:11:39 -0800 | From | Rick Lindsley <> |
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OK, but do you agree that the rate we rebalance things like 2 vs 1 should be slower than the rate we rebalance 3 vs 1 ? Fairness is only relevant over a long term imbalance anyway, so there should be a big damper on "fairness only" rebalances.
I think, given the precision we're granted via SCHED_LOAD_SCALE, in combination with the new "load average" (cpu_load) code, that we can achieve what we want.
If cpu0 has 2 runnable tasks and cpu1 has 1 runnable task, won't we see the "load average" of cpu0 slowly approach 2, but not jump there?
Right now, we round up on all fractions and Martin has proposed a patch which takes it the other way and rounds down. What if in marginal cases like this where this is a small but persistent difference, we could bump the task to another cpu when it reaches (say) 1.8 or 1.9? That would keep it there longer for shorter-lived tasks, but for those long-runners, they'd eventually spread the pain around a little.
And yes, a cpu_load of even 1.0 should *never* get migrated to a cpu with a load 0.0. Instead of
*imbalance = (*imbalance + SCHED_LOAD_SCALE - 1) >> SCHED_LOAD_SHIFT;
how about, for instance,
if (max_load <= SCHED_LOAD_SCALE) *imbalance = 0; else *imbalance = (*imbalance + (SCHED_LOAD_SCALE / 6) - 1) >> SCHED_LOAD_SHIFT;
The intent is to never move anything if max_load is 1 or less (what advantage is there?) and to create a slight tendency to round up at loads greater than that, which would still tend to leave things where they were until they'd been there a while. In fact the "bonus" (SCHED_LOAD_SCALE / 6 - 1) could be another configurable in the scheduling domain so that at some level you're not interested in fairness and they just don't bounce at all.
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