Peter Williams wrote:
Ingo Molnar wrote:
* Peter Williams <[EMAIL PROTECTED]> wrote:
I retract this suggestion as it's a very bad idea. It introduces the
possibility of starvation via the poor sods at the bottom of the
queue having their "on CPU" forever postponed and we all know that
even the smallest possibility of starvation will eventually cause
problems.
I think there should be a rule: Once a task is on the queue its "on
CPU" time is immutable.
Yeah, fully agreed. Currently i'm using the simple method of
p->nice_offset, which plainly just moves the per nice level areas of
the tree far enough apart (by a constant offset) so that lower nice
levels rarely interact with higher nice levels. Lower nice levels
never truly starve because rq->fair_clock increases deterministically
and currently the fair_key values are indeed 'immutable' as you suggest.
In practice they can starve a bit when one renices thousands of tasks,
so i was thinking about the following special-case: to at least make
them easily killable: if a nice 0 task sends a SIGKILL to a nice 19
task then we could 'share' its p->wait_runtime with that nice 19 task
and copy the signal sender's nice_offset. This would in essence pass
the right to execute over to the killed task, so that it can tear
itself down.
This cannot be used to gain an 'unfair advantage' because the signal
sender spends its own 'right to execute on the CPU', and because the
target task cannot execute any user code anymore when it gets a SIGKILL.
In any case, it is clear that rq->raw_cpu_load should be used instead
of rq->nr_running, when calculating the fair clock, but i begin to
like the nice_offset solution too in addition of this: it's effective
in practice and starvation-free in theory, and most importantly, it's
very simple. We could even make the nice offset granularity tunable,
just in case anyone wants to weaken (or strengthen) the effectivity of
nice levels. What do you think, can you see any obvious (or less
obvious) showstoppers with this approach?
I haven't had a close look at it but from the above description it
sounds an order of magnitude more complex than I thought it would be.
The idea of different nice levels sounds like a recipe for starvation to
me (if it works the way it sounds like it works).
I guess I'll have to spend more time reading the code because I don't
seem to be able to make sense of the above description in any way that
doesn't say "starvation here we come".
I'm finding it hard to figure out what the underling principle for the
way you're queuing things by reading the code (that's the trouble with
reading the code it just tells you what's being done not why -- and
sometimes it's even hard to figure out what's being done when there's a
lot of indirection). sched-design-CFS.txt isn't much help in this area
either.
Any chance of a brief description of how it's supposed to work?
Key questions are:
How do you decide the key value for a task's position in the queue?
Is it an absolute time or an offset from the current time?
How do you decide when to boot the current task of the queue? Both at
wake up of another task and in general play?
Peter
PS I think that you're trying to do too much in one patch.
--
Peter Williams [EMAIL PROTECTED]
"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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