Messages in this thread | | | Subject | Re: [PATCH 07/32] cpuset: Set up interface for nohz flag | From | Mike Galbraith <> | Date | Thu, 22 Mar 2012 20:20:30 +0100 |
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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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