ULE patch, call for testers

Andriy Gapon avg at FreeBSD.org
Mon Nov 5 09:14:01 UTC 2012

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
 		SDT_PROBE2(sched, , , off_cpu, td, td->td_proc);
 		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);
+		}
 		sched_pctcpu_update(newtd->td_sched, 0);
 		 * If DTrace has set the active vtime enum to anything
@@ -1990,6 +2004,7 @@ sched_wakeup(struct thread *td)
 	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_pctcpu_update(ts, 0);
 	/* Reset the slice value after we sleep. */
 	ts->ts_slice = sched_slice;
 	sched_add(td, SRQ_BORING);

Andriy Gapon

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