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    SubjectRe: [PATCH 07/41] cpuset: Set up interface for nohz flag
    From
    2012/5/9 Mike Galbraith <efault@gmx.de>:
    > 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.

    I don't think it's

    >> > > 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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