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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