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SubjectRe: O(1) scheduler starvation
At 04:22 PM 6/18/2003 +0200, Felipe Alfaro Solana wrote:
>On Wed, 2003-06-18 at 14:04, Mike Galbraith wrote:
> > At 09:53 AM 6/18/2003 +0200, Felipe Alfaro Solana wrote:
> > >Hi!
> > >
> > >I've been poking around and found the following link on O(1) scheduler
> > >starvation problems:
> > >
> > >http://www.hpl.hp.com/research/linux/kernel/o1-starve.php
> > >
> > >The web page contains a small test program which supposedly is able to
> > >make two processes starvate. However, I've been unable to reproduce what
> > >is described in the above link. In fact, the CPU usage ratio ranges
> > >between 60-40% or 70-30% in the worst cases.
> >
> > (you're talking about with my monotinic_clock() diff in your kernel right?)
>
>Don't know exactly its name. wli posted to LKLM a few days ago. In a few
>words, the patch creates
>
>+inline void __scheduler_tick(runqueue_t *rq, task_t *p)
>
>and
>
>+#define SCHED_NANOSECOND 1
>+#define SCHED_SECOND (1000000000 * SCHED_NANOSECOND)
>+#define SCHED_TICK (SCHED_SECOND / HZ)
>+#define TICKS_PER_SECOND (SCHED_SECOND / SCHED_TICK)
>
>Don't know if this is the patch you're talking about. It's not thud,
>anyways.

Yeah, that's the diff. (thud is a starvation testcase posted a while back)

> > If you examine the priorities vs cpu usage, therein lies a big fat bug.
> >
> > I think the fundamental problem is that you can only execute in series,
> but
> > can sleep in parallel, which makes for more sleep time existing than all
> > execution time combined. If you're running test-starve with my
> > monotonic_clock() diff, you should notice that one task is at maximum
> > priority and eating ~75% cpu, while the other is at minumum and getting
> the
> > rest minus what top gets. In a sane universe, that should be
> > impossible. In my current tree, this _is_ now impossible, but I haven't
> > worked out some nasty kinks.
>
>Exactly. This is more or less what was happening, roughly a 70-30
>balance of CPU usage.
>
> > >I'm running 2.5.72-mm1 with Mike Galbraith's scheduler patches and a
> > >small patch I made myself to improve interactivity (mainly, to stop XMMS
> > >from skipping by adjusting some scheduler parameters).
> >
> > Please show me your xmms hack, and show me how you make xmms skip without
> > doing something that puts tons of stress on the cache. I built xmms here,
> > and the only time the audio thread gets starved is when starting a new
> > song. That's because of CHILD_PENALTY when starting a new copy of xmms
> > while something of prio < 20 is hogging cpu (process_load <grrrr>). Once
> > playing, it's rock solid here.
>
>No, I wasn't talking about an XMMS hack. I was talking about changing
>scheduler defaults, as shown in the following patch:

Ah, ok (darn, was hoping for fresh idea)

>--- old/kernel/sched.c 2003-06-17 21:04:21.240902000 +0200
>+++ new/kernel/sched.c 2003-06-17 20:58:54.840902000 +0200
>@@ -66,14 +66,14 @@
> * they expire.
> */
> #define MIN_TIMESLICE ( 10 * HZ / 1000)
>-#define MAX_TIMESLICE (200 * HZ / 1000)
>+#define MAX_TIMESLICE ( 20 * HZ / 1000)

(tiny slices make for much context switching, but [alone] doesn't help low
prio tasks get cpu)

> #define CHILD_PENALTY 50
> #define PARENT_PENALTY 100
> #define EXIT_WEIGHT 3
>-#define PRIO_BONUS_RATIO 25
>+#define PRIO_BONUS_RATIO 20
> #define INTERACTIVE_DELTA 2
>-#define MAX_SLEEP_AVG (10*HZ)
>-#define STARVATION_LIMIT (10*HZ)
>+#define MAX_SLEEP_AVG (2*HZ)

(this will increase cpu multiplexing drastically, but will also make the
desktop feel any background load pretty much instantly. btdt too, ok for
very light load, but bad for balls-to-the-wall throughput here)

>+#define STARVATION_LIMIT (2*HZ)
> #define NODE_THRESHOLD 125
> #define SCHED_NANOSECOND 1
> #define SCHED_SECOND (1000000000 * SCHED_NANOSECOND)
>
>To make XMMS skip, just force the X server to do a lot of repainting,
>for example, by dragging a big window slowly enough over another one
>which requires a lot of painting (Evolution, for example, is a good
>candidate as it requires a lot of CPU to repaint uncovered areas). It's
>easy to reproduce just after launching XMMS. However, after a while, it
>gets difficult to make XMMS to skip sound (it seems the scheduler
>adjusts priorities well enough). This is on a PIII 700Mhz laptop with no
>niced processes at all.

Thanks. I don't have evolution on my linux box (pine/vi/procmail
rules). ImageMagic ought to give X more than enough spurts of frenetic
activity though. Do you have that, and does image manipulation make xmms
stutter as well? Just moving windows around and changing backgrounds
doesn't do anything here. (500mhz piii/128mb ram btw)

>With your patch and my scheduler changes, I've been unable to make XMMS
>skip audio, even when the X server is reniced to -20, or another CPU hog
>is running.

<g> I can make it do a skip so big that SysRq-E or BRB are the only
recovery options.

-Mike

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