Messages in this thread | | | Subject | Re: [PATCH 07/32] cpuset: Set up interface for nohz flag | From | Mike Galbraith <> | Date | Tue, 27 Mar 2012 13:53:03 +0200 |
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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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