ULE patch, call for testers
David Xu
davidxu at freebsd.org
Tue Nov 6 01:28:32 UTC 2012
On 2012/11/05 17:13, Andriy Gapon wrote:
> on 05/11/2012 04:41 David Xu said the following:
>> Another problem I remembered is that a thread on runqueue may be starved
>> because ULE treats a sleeping thread and a thread waiting on runqueue
>> differently. If a thread has slept for a while, after it is woken up,
>> its priority is boosted, but for a thread on runqueue, its priority
>> will never be boosted. In essential, they should be same becase both of
>> them are waiting for cpu. If I am a thread, I'd like to wait on sleep
>> queue rather than on runqueue, since in former case, I will get
>> bonus, while in later case, I'll get nothing. Under heavy load,
>> there are many runnable threads, this unfair can cause a very low priority
>> thread on runqueue to be starved. 4BSD seems not suffer from
>> this problem, because it also decay cpu time of thread on runqueue.
>> I think ULE needs some anti-starvation code to give thread a shot
>> if it is waiting on runqueue too long time.
>
> I also noticed this issue and I've been playing with the following patch.
> Two points:
> o I am not sure if it is ideologically correct
> o it didn't improve much the behavior of my workloads
> In any case, here it is:
>
> - extend accounted interactive sleep time to a point where a thread runs
> (as opposed to be added to runq)
>
> --- a/sys/kern/sched_ule.c
> +++ b/sys/kern/sched_ule.c
> @@ -1898,8 +1899,21 @@ sched_switch(struct thread *td, struct thread *newtd, int
> flags)
> SDT_PROBE2(sched, , , off_cpu, td, td->td_proc);
> lock_profile_release_lock(&TDQ_LOCKPTR(tdq)->lock_object);
> TDQ_LOCKPTR(tdq)->mtx_lock = (uintptr_t)newtd;
> +#if 1
> + /*
> + * If we slept for more than a tick update our interactivity and
> + * priority.
> + */
> + int slptick;
> + slptick = newtd->td_slptick;
> + newtd->td_slptick = 0;
> + if (slptick && slptick != ticks) {
> + newtd->td_sched->ts_slptime +=
> + (ticks - slptick) << SCHED_TICK_SHIFT;
> + sched_interact_update(newtd);
> + }
> +#endif
> sched_pctcpu_update(newtd->td_sched, 0);
> -
> #ifdef KDTRACE_HOOKS
> /*
> * If DTrace has set the active vtime enum to anything
> @@ -1990,6 +2004,7 @@ sched_wakeup(struct thread *td)
> THREAD_LOCK_ASSERT(td, MA_OWNED);
> ts = td->td_sched;
> td->td_flags &= ~TDF_CANSWAP;
> +#if 0
> /*
> * If we slept for more than a tick update our interactivity and
> * priority.
> @@ -2001,6 +2016,7 @@ sched_wakeup(struct thread *td)
> sched_interact_update(td);
> sched_pctcpu_update(ts, 0);
> }
> +#endif
> /* Reset the slice value after we sleep. */
> ts->ts_slice = sched_slice;
> sched_add(td, SRQ_BORING);
>
>
What I want is fairness between waiting on runqueue and waiting on
sleepqueue. Supports you have N threads on runqueue:
T1,T2,T3...Tn.
and a thread T(n+1) on sleepqueue.
If CPU runs threads T1...Tn in round-robin fashion, and suppose at
time n, the thread Tn is run, this means total time of n-1 is passed,
and at the time, thread T(n+1) is woken up, and scheduler's
sched_interact_score() will give it higher priority over Tn, this is
unfair because both threads have spent same total time to waiting for
cpu. Do your patch fix the problem ?
Regards,
David Xu
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