Messages in this thread | | | Date | Fri, 07 Apr 2006 09:02:28 +1000 | From | Peter Williams <> | Subject | Re: RT task scheduling |
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Darren Hart wrote: > On Wednesday 05 April 2006 21:19, Peter Williams wrote: >> Darren Hart wrote: >>> My last mail specifically addresses preempt-rt, but I'd like to know >>> people's thoughts regarding this issue in the mainline kernel. Please >>> see my previous post "realtime-preempt scheduling - rt_overload behavior" >>> for a testcase that produces unpredictable scheduling results. >>> >>> Part of the issue here is to define what we consider "correct behavior" >>> for SCHED_FIFO realtime tasks. Do we (A) need to strive for "strict >>> realtime priority scheduling" where the NR_CPUS highest priority runnable >>> SCHED_FIFO tasks are _always_ running? Or do we (B) take the best effort >>> approach with an upper limit RT priority imbalances, where an imbalance >>> may occur (say at wakeup or exit) but will be remedied within 1 tick. >>> The smpnice patches improve load balancing, but don't provide (A). >>> >>> More details in the previous mail... >> I'm currently researching some ideas to improve smpnice that may help in >> this situation. The basic idea is that as well as trying to equally >> distribute the weighted load among the groups/queues we should also try >> to achieve equal "average load per task" for each group/queue. (As well >> as helping with problems such as yours, this will help to restore the >> "equal distribution of nr_running" amongst groups/queues aim that is >> implicit without smpnice due to the fact that load is just a smoothed >> version of nr_running.) > > Can you elaborate on what you mean by "average load per task" ?
It's the total weighted load on a run group/queue divided by the nr_running for that group/queue. If this is equal for all groups/queues and the total weighted load for them are also equal then the distribution of priorities in each group/queue should be similar and this will give a high probability that (for an N CPU system) the N highest priority tasks will be on different CPUs and hence the highest priority task on their CPU. But these are just tendencies not guarantees as it's a statistical process not a deterministic one.
> > Also, since smpnice is (correct me if I am wrong) load_balancing, I don't > think it will prevent the problem from happening, but rather fix it when it > does. If we want to prevent it from happening, I think we need to do > something like the rt_overload code from the RT patchset.
I agree. Changes to smpnice (as described above) would help with this problem (i.e. they'll make the distribution of tasks TEND towards the desired state) but would not provide the necessary determinism. I think special measures (such as rt_overload) are required if you want determinism.
Peter -- Peter Williams pwil3058@bigpond.net.au
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