Messages in this thread | | | From | "Chen, Kenneth W" <> | Subject | RE: + sched-use-tasklet-to-call-balancing.patch added to -mm tree | Date | Sun, 12 Nov 2006 22:40:51 -0800 |
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Christoph Lameter wrote on Sunday, November 12, 2006 9:45 PM > > (2) we should initiate load balance within a domain only from least > > loaded group. > > This would mean we would have to determine the least loaded group first.
Well, find_busiest_group() scans every single bloody CPU in the system at the highest sched_domain level. In fact, this function is capable to find busiest group within a domain, it should be capable to determine least loaded group for free because it already scanned every groups within a domain.
> > Part of all this problem probably stemmed from "load balance" is incapable > > of performing l-d between arbitrary pair of CPUs, and tightly tied load scan > > and actual l-d action. And on top of that l-d is really a pull operation > > to current running CPU. All these limitations dictate that every CPU somehow > > has to scan and pull. It is extremely inefficient on large system. > > Right. However, if we follow this line of thought then we will be > redesigning the load balancing logic.
It won't be a bad idea to redesign it ;-)
There are number of other oddity beside what was identified in it's design:
(1) several sched_groups are statically declared and they will reside in boot node. I would expect cross node memory access to be expansive. Every cpu will access these data structure repeatedly.
static struct sched_group sched_group_cpus[NR_CPUS]; static struct sched_group sched_group_core[NR_CPUS]; static struct sched_group sched_group_phys[NR_CPUS];
(2) load balance staggering. Number of people pointed out that it is overly done.
(3) The for_each_domain() loop in rebalance_tick() looks different from idle_balance() where it will traverse entire sched domains even if lower level domain succeeded in moving some tasks. I would expect we either break out of the for loop like idle_balance(), or somehow update load for current CPU so it gets accurate load value when doing l-d in the next level. Currently, It is doing neither. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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