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    SubjectRe: [PATCH 4/4] sched: bias to target cpu load to reduce task moving
    On Tue, Jan 07, 2014 at 12:55:18PM +0000, Morten Rasmussen wrote:
    > My understanding is that should_we_balance() decides which cpu is
    > eligible for doing the load balancing for a given domain (and the
    > domains above). That is, only one cpu in a group is allowed to load
    > balance between the local group and other groups. That cpu would
    > therefore be reponsible for pulling enough load that the groups are
    > balanced even if it means temporarily overloading itself. The other cpus
    > in the group will take care of load balancing the extra load within the
    > local group later.

    Correct.

    > I may have missed something, but I don't understand the reason for the
    > performance improvements that you are reporting. I see better numbers
    > for a few benchmarks, but I still don't understand why the code makes
    > sense after the cleanup. If we don't understand why it works, we cannot
    > be sure that it doesn't harm other benchmarks. There is always a chance
    > that we miss something but, IMHO, not having any idea to begin with
    > increases the chances for problems later significantly. So why not get
    > to the bottom of the problem of cleaning up cpu_load?
    >
    > Have you done more extensive benchmarking? Have you seen any regressions
    > in other benchmarks?

    I only remember hackbench numbers and that generally fares well with a
    more aggressive balancer since it has no actual work to speak of the
    migration penalty is very low and because there's a metric ton of tasks
    the aggressive leveling makes for more coherent 'throughput'.


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