[patch] zfs livelock and thread priorities

Ben Kelly ben at wanderview.com
Tue May 19 02:13:04 UTC 2009

On May 18, 2009, at 1:38 PM, Attilio Rao wrote:
> OMG.
> This still doesn't explain priorities like 49 or such seen in the
> first report as long as we don't set priorities by hand,

I'm trying to understand why this particular priority value is so  
concerning, but I'm a little bit confused.  Can you elaborate on why  
you think its a problem?  From previous off-list e-mails I get the  
impression that you are concerned that it does not fall on an RQ_PPQ  
boundary.  Is this the case?  Again, I may be completely confused, but  
ULE does not seem to consider RQ_PPQ when it assigns priorities for  
interactive threads.  Here is how I came to this conclusion:

 From what I can tell a thread's priority might be adjusted for  
interactivity in sched_priority() around line 1421 of sched_ule:

>         score = imax(0, sched_interact_score(td) - td->td_proc- 
> >p_nice);
>         if (score < sched_interact) {
>                 pri = PRI_MIN_REALTIME;
>                 pri += ((PRI_MAX_REALTIME - PRI_MIN_REALTIME) /  
> sched_interact)
>                     * score;

One my machine (PRI_MAX_REALTIME - PRI_MIN_REALTIME) / sched_interact  
resolves to a value of 1.  So the priority is being set to  
PRI_MIN_REALTIME plus the value of score.  According to the comment on  
sched_interact_score() the returned value ranges from 0 to 100.  As  
far as I can tell from looking at the code the calculations based on  
the ration of ts_runtime to ts_sleeptime aren't guaranteed to return a  
value divisible by RQ_PPQ.  For example, on my machine tickincr turns  
out to be 8000.  So if ts_sleeptime is 16000 and ts_runtime is 8000  
sched_interact_score() will return a value of 25.  Which then means  
the thread will be assigned a priority of PRI_MIN_REALTIME + 25.

Also, looking at my currently idle system I actually have many  
priorities that do not fall on RQ_PPQ boundaries:

> ianto# ps ax -opri | grep 43 | wc -l
>       14
> ianto# ps ax -opri | grep 44 | wc -l
>       70
> ianto# ps ax -opri | grep 45 | wc -l
>        4
> ianto# ps ax -opri | grep 46 | wc -l
>        3
> ianto# ps ax -opri | grep 47 | wc -l
>        2
> ianto# ps ax -opri | grep 48 | wc -l
>        1
> ianto# ps ax -opri | grep 49 | wc -l
>        1

I'm running a non-SMP kernel with ULE last synced with subversion on  
March 16.   Full system info can be found here:


Anyway, I apologize if I'm missing something.  I'd be happy to do more  
investigation if you'd like.  Just let me know.


- Ben

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