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    SubjectRe: scheduler scalability - cgroups, cpusets and load-balancing

    On Tue, 2008-01-29 at 06:03 -0600, Paul Jackson wrote:
    > 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.

    Ok, I'll take a stab at understanding that code. Perhaps it seems to me
    a lot of confusion could be solved by getting a more level playing
    ground :-)

    > > > 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.'

    Great :-)

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