Re: [ANNOUNCE][RFC] PlugSched-6.2 for 2.6.16-rc1 and 2.6.16-rc1-mm1

[Peter Williams <pwil3058@bigpond.net.au>]

Paolo Ornati wrote:
> On Thu, 26 Jan 2006 12:09:53 +1100
> Peter Williams <pwil3058@bigpond.net.au> wrote:
> 
> 
>>I know that I've said this before but I've found the problem. 
>>Embarrassingly, it was a basic book keeping error (recently introduced 
>>and equivalent to getting nr_running wrong for each CPU) in the 
>>gathering of the statistics that I use. :-(
>>
>>The attached patch (applied on top of the PlugSched patch) should fix 
>>things.  Could you test it please?
> 
> 
> Ok, this one make a difference:
> 
> (transcode)
> 
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>  5774 paolo     34   0  116m  18m 2432 R 86.2  3.7   0:11.65 transcode
>  5788 paolo     32   0 51000 4472 1872 S  7.5  0.9   0:01.13 tcdecode
>  5797 paolo     29   0  4948 1468  372 D  3.2  0.3   0:00.30 dd
>  5781 paolo     33   0 19844 1092  880 S  1.0  0.2   0:00.10 tcdemux
>  5783 paolo     31   0 47964 2496 1956 S  0.7  0.5   0:00.08 tcdecode
>  5786 paolo     34   0 19840 1088  880 R  0.5  0.2   0:00.06 tcdemux
> 
> (sched_fooler)
> 
>  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>  5804 paolo     34   0  2396  292  228 R 35.7  0.1   0:12.84 a.out
>  5803 paolo     34   0  2392  288  228 R 30.5  0.1   0:11.49 a.out
>  5805 paolo     34   0  2392  288  228 R 30.2  0.1   0:10.70 a.out
>  5815 paolo     29   0  4948 1468  372 D  3.7  0.3   0:00.29 dd
>  5458 paolo     28   0 86656  21m  15m S  0.2  4.4   0:02.18 konsole
> 
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>  5804 paolo     34   0  2396  292  228 R 36.5  0.1   0:38.19 a.out
>  5803 paolo     34   0  2392  288  228 R 30.5  0.1   0:34.27 a.out
>  5805 paolo     34   0  2392  288  228 R 29.2  0.1   0:32.39 a.out
>  5829 paolo     34   0  4952 1472  372 R  3.2  0.3   0:00.35 dd
> 
> DD_TEST + sched_fooler: 512 MB --- ~20s instead of 16.6s
> 
> This is a clear improvement... however I wonder why DD priority
> fluctuate going up even to 34 (the range is something like 29 <--->
> 34).
> 

It's because the "fairness" bonus is still being done as a one shot 
bonus when a task's delay time become unfairly large.  I mentioned this 
before as possibly needing to be changed to a more persistent model but 
after I discovered the accounting bug I deferred doing anything about it 
in the hope that fixing the bug would have been sufficient.

I'll now try a model whereby a task's fairness bonus is increased 
whenever it has unfair delays and decreased when it doesn't.  Hopefully, 
with the right rates of increase/decrease, this can result in a system 
where a task has a fairly persistent bonus which is sufficient to give 
it its fair share.  One reason that I've been avoiding this method is 
that it introduces double smoothing: once in the calculation of the 
average delay time and then again in the determination of the bonus; and 
I'm concerned this may make it slow to react to change.  Any way I'll 
give it a try and see what happens.

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce