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    SubjectRe: [PATCH 07/32] cpuset: Set up interface for nohz flag
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    On Thu, 2012-03-22 at 11:26 -0500, Christoph Lameter wrote: 
    > On Thu, 22 Mar 2012, Mike Galbraith wrote:
    >
    > > > > We use here a per cpu refcounter. As long as a CPU
    > > > > is contained into at least one cpuset that has the
    > > > > nohz flag set, it is part of the set of CPUs that
    > > > > run into adaptive nohz mode.
    > > >
    > > > What are the drawbacks for nohz?
    > >
    > > For nohz in general, latency. To make it at all usable for rt loads, I
    >
    > Well nohz while a process is running on a dedicated cpu means the cpu is
    > running full power and no disruptions occur. This is a tremendous benefit.

    In the context of single task burning in userspace, you bet.

    > Less than 10us jitter can alrady be accomplished by building a kernel with
    > certain options off (like for example preemption...) and ensuring that
    > stuff stays off certain processors. Lets not confuse realtime with low
    > latency. Real time in the sense of deterministic execution is bad for
    > latency because overhead is added to ensure the determinism which
    > increases latency.

    Yeah, I know RT pays heavily for determinism. It loses on best case.

    > > of the current box, triple digit for simple synchronized frame timers +
    > > compute worker-bees load on 64 cores. Patch 4 probably helps that, but
    > > don't _think_ it'll fix it. If you (currently) ever become balancer,
    > > you're latency target is smoking wreckage.
    >
    > Yes so we need something to tell the system which cpu is the sacrificial
    > lamb that will not run low latency applications.

    Definitely a lamb is required.

    (This set is targeted at HPC, so I'll shut up now.. but RT is HPC too)

    -Mike



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