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    SubjectRe: [PATCH 07/32] cpuset: Set up interface for nohz flag
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    On Tue, 2012-03-27 at 13:22 +0200, Frederic Weisbecker wrote: 
    > On Thu, Mar 22, 2012 at 05:03:53AM +0100, Mike Galbraith wrote:
    > > On Wed, 2012-03-21 at 09:50 -0500, Christoph Lameter wrote:
    > > > On Wed, 21 Mar 2012, Frederic Weisbecker wrote:
    > > >
    > > > > Prepare the interface to implement the nohz cpuset flag.
    > > > > This flag, once set, will tell the system to try to
    > > > > shutdown the periodic timer tick when possible.
    > > > >
    > > > > 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
    > > had to make isolated cores immune from playing load balancer. Even so,
    > > to achieve target latency, I had to hack up cpusets to let the user
    > > dynamically switch nohz off for specified sets (and the tick has to be
    > > skewed in both cases or you can just forget it). With nohz, I can't
    > > quite achieve 30us jitter target, turn it off, I get single digit. Out
    > > 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.
    >
    > But this is because of waking up from CPU low power mode, right? If so
    > then busy tickless shouldn't be concerned. We can certainly have
    > configurations where the tick is not stopped in idle but can be elsewhere.

    Boxen are restricted to C1 (even at that Q6600 _sucks rocks_, but more
    modern CPUs don't). ATM, ticked is cheaper, I can't get there from here
    with nohz.

    -Mike



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