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    SubjectRe: [PATCH 07/41] cpuset: Set up interface for nohz flag
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    On Tue, 2012-05-08 at 15:45 -0500, Christoph Lameter wrote: 
    > On Tue, 8 May 2012, Mike Galbraith wrote:
    >
    > > On Tue, 2012-05-08 at 18:16 +0200, Peter Zijlstra wrote:
    > > > On Tue, 2012-05-08 at 10:57 -0500, Christoph Lameter wrote:
    > >
    > > > isolcpus is a very limited hack that adds more pain that its worth. Its
    > > > yet another mask to check and its functionality is completely available
    > > > through cpusets.
    > >
    > > Agreed.
    >
    > How would that work? By creating cpusets that only have a single cpu in
    > them?

    No, just turn load balancing off for exclusive set, domains go poof.

    > > > You cannot cree multi-cpu partitions using isolcpus, you cannot
    > > > dynamically reconfigure it.
    > >
    > > Big plus for cpusets.
    >
    > Why would you want to do anything like it? cpusets are confusing. You can
    > have a cpu be part of multiple cpusets. Which nohz setting applies for a
    > particular cpu then? If any of the cpusets have nohz set then it applies
    > to the cpu? And thus someone in a cpuset that does not has nohz set will
    > find that a cpu will have nohz functionality?

    nohz has to be at least an exclusive set property.

    > Its not a good match for this. You would want a per cpu attribute for
    > nohz.

    Or per cpuset, which can be the same thing as per cpu if you want.

    > > > And on the scheduler side cpusets doesn't add runtime overhead to normal
    > > > things, only sched_setaffinity() and a few other rare operations get
    > > > slightly more expensive. And it allows to reduce runtime overhead by
    > > > making the load-balancer domains smaller.
    > >
    > > Very big deal if you have a load that doesn't do all the performance 'i'
    > > dotting and 't' crossing it maybe could have, but ends up on a big box.
    >
    > isolcpus are not part of load balancer domains.

    Yup, so if you have an application with an RT component, somewhat
    sensitive, needs isolation from rest of a big box, but app also has
    SCHED_OTHER components. isolcpus is a pain, everything has to be static
    and nailed to the floor. Load just works when plugged into a cpuset.

    -Mike



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