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    SubjectRe: scheduler scalability - cgroups, cpusets and load-balancing
    Paul, responding to Peter:
    > > We now have a per-cpuset Boolean flag file called 'sched_load_balance'.
    > SD_LOAD_BALANCE, right?

    No. SD_LOAD_BALANCE is some attribute of sched domains.

    The 'sched_load_balance' flag is an attribute of cpusets.

    The mapping of cpusets to sched domains required several pages of 'fun
    to write' code, which had to go through a couple of years of fixing and
    one major rewrite before it (knock on wood) worked correctly. It's not
    a one-to-one relation, in other words. See my earlier messages for
    further explanation of how this works.

    I'm not sure what SD_LOAD_BALANCE does ... I guess from a quick
    read that it just optimizes the recognition of singleton sched
    domains for which load balancing would be a wasted effort.

    > > This 'sched_load_balance' flag is, thus far, "the" cpuset hook
    > > supporting realtime. One can use it to configure a system so that
    > > the kernel does not do normal load balancing on select CPUs, such
    > > as those CPUs dedicated to realtime use.
    > Ah, here I disagree, it is possible to do (hard) realtime scheduling
    > over multiple cpus, the only draw back is that it requires a very strong
    > load-balancer, making it unsuitable for large number of cpus.

    I don't think we are disagreeing. I was speaking of "normal"
    load balancing (what the mainline kernel/sched.c code normally
    does). You're speaking of hard realtime load balancing.

    I think we agree that these both exist, and require different
    load balancing code, the latter 'very strong.'

    I won't rest till it's the best ...
    Programmer, Linux Scalability
    Paul Jackson <> 1.940.382.4214

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