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SubjectRe: CFS review

* Roman Zippel <zippel@linux-m68k.org> wrote:

> > i tried your fl.c and if sched_clock() is high-resolution it's scheduled
> > _perfectly_ by CFS:
> >
> > PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
> > 5906 mingo 20 0 1576 244 196 R 71.2 0.0 0:30.11 l
> > 5909 mingo 20 0 1844 344 260 S 9.6 0.0 0:04.02 lt
> > 5907 mingo 20 0 1844 508 424 S 9.5 0.0 0:04.01 lt
> > 5908 mingo 20 0 1844 344 260 S 9.5 0.0 0:04.02 lt
> >
> > if sched_clock() is low-resolution then indeed the 'lt' tasks will
> > "hide":
> >
> > PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
> > 2366 mingo 20 0 1576 248 196 R 99.9 0.0 0:07.95 loop_silent
> > 1 root 20 0 2132 636 548 S 0.0 0.0 0:04.64 init
> >
> > but that's nothing new. CFS cannot conjure up time measurement
> > methods that do not exist. If you have a low-res clock and if you
> > create an app that syncs precisely to the tick of that clock via
> > timers that run off that exact tick then there's nothing the
> > scheduler can do about it. It is false to charachterise this as
> > 'sleeper starvation' or 'rounding error' like you did. No amount of
> > rounding logic can create a high-resolution clock out of thin air.
>
> [...] I didn't say 'sleeper starvation' or 'rounding error', these are
> your words and it's your perception of what I said.

Oh dear :-) It was indeed my preception that yesterday you said:

| A problem here is that this can be exploited, if a job is spread over
| ^^^^^^^^^^^^^^^^^^^^^
| a few threads, they can get more time relativ to other tasks, e.g. in
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| this example there are three tasks that run only for about 1ms every
| 3ms, but they get far more time than should have gotten fairly:
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
| 4544 roman 20 0 1796 520 432 S 32.1 0.4 0:21.08 lt
| 4545 roman 20 0 1796 344 256 R 32.1 0.3 0:21.07 lt
| 4546 roman 20 0 1796 344 256 R 31.7 0.3 0:21.07 lt
| 4547 roman 20 0 1532 272 216 R 3.3 0.2 0:01.94 l

[ http://lkml.org/lkml/2007/7/31/668 ]

( the underlined portion, in other words, is called 'starvation'.)

And again today, i clearly perceived you to say:

| > in that case 'top' accounting symptoms similar to the above are not
| > due to the scheduler starvation you suspected, but due the effect of
| > a low-resolution scheduler clock and a tightly coupled
| > timer/scheduler tick to it.
|
| Well, it magnifies the rounding problems in CFS.

[ http://lkml.org/lkml/2007/8/1/153 ]

But you are right, that must be my perception alone, you couldnt
possibly have said any of that =B-)

Or are you perhaps one of those who claims that saying something
analogous to sleeper starvation does not equal to talking about 'sleeper
starvation' and saying something about 'rounding problems in CFS' does
in no way mean you were talking about rounding errors? :-)

Ingo
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