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SubjectRe: [patch] CFS scheduler, v3
William Lee Irwin III wrote:
> On Fri, Apr 20, 2007 at 10:10:45AM +1000, Peter Williams wrote:
>> I have a suggestion I'd like to make that addresses both nice and
>> fairness at the same time. As I understand the basic principle behind
>> this scheduler it to work out a time by which a task should make it onto
>> the CPU and then place it into an ordered list (based on this value) of
>> tasks waiting for the CPU. I think that this is a great idea and my
>> suggestion is with regard to a method for working out this time that
>> takes into account both fairness and nice.
>
> Hmm. Let me take a look...
>
>
> On Fri, Apr 20, 2007 at 10:10:45AM +1000, Peter Williams wrote:
>> First suppose we have the following metrics available in addition to
>> what's already provided.
>> rq->avg_weight_load /* a running average of the weighted load on the CPU */
>> p->avg_cpu_per_cycle /* the average time in nsecs that p spends on the
>> CPU each scheduling cycle */
>
> I'm suspicious of mean service times not paired with mean inter-arrival
> times.
>
>
> On Fri, Apr 20, 2007 at 10:10:45AM +1000, Peter Williams wrote:
>> where a scheduling cycle for a task starts when it is placed on the
>> queue after waking or being preempted and ends when it is taken off the
>> CPU either voluntarily or after being preempted. So
>> p->avg_cpu_per_cycle is just the average amount of time p spends on the
>> CPU each time it gets on to the CPU. Sorry for the long explanation
>> here but I just wanted to make sure there was no chance that "scheduling
>> cycle" would be construed as some mechanism being imposed on the scheduler.)
>
> I went and looked up priority queueing queueing theory garbage and
> re-derived various things I needed. The basics check out. Probably no
> one cares that I checked.
>
>
> On Fri, Apr 20, 2007 at 10:10:45AM +1000, Peter Williams wrote:
>> We can then define:
>> effective_weighted_load = max(rq->raw_weighted_load, rq->avg_weighted_load)
>> If p is just waking (i.e. it's not on the queue and its load_weight is
>> not included in rq->raw_weighted_load) and we need to queue it, we say
>> that the maximum time (in all fairness) that p should have to wait to
>> get onto the CPU is:
>> expected_wait = p->avg_cpu_per_cycle * effective_weighted_load /
>> p->load_weight

You're right. The time that the task spent sleeping before being woken
should be subtracted from this value. If the answer is less than or
equal to zero pre-emption should occur.

>
> This doesn't look right, probably because the scaling factor of
> p->avg_cpu_per_cycle is the reciprocal of its additive contribution to
> the ->avg_weight_load as opposed to a direct estimate of its initial
> delay or waiting time before completing its current requested service.
>
> p->load_weight/effective_weighted_load more properly represents an
> entitlement to CPU bandwidth.

Yes. But expected_wait isn't entitlement it's its inverse.

> p->avg_cpu_per_cycle/(p->load_weight/effective_weighted_load)
> would be more like the expected time spent on the runqueue

When I went to school that would be just another way of expressing the
equation that I expressed.

> (whether
> waiting to run or actually running) for a time interval spent in a
> runnable state and the expected time runnable and waiting to run in such
> an interval would be
> p->avg_cpu_per_cycle*(1-effective_weighted_load/p->load_weight),
>
> Neither represents the initial delay between entering the runqeueue and
> first acquiring the CPU, but that's a bit hard to figure out without
> deciding the scheduling policy up-front anyway.
>
> This essentially doesn't look correct because while you want to enforce
> the CPU bandwidth allocation, this doesn't have much to do with that
> apart from the CPU bandwidth appearing as a term. It's more properly
> a rate of service as opposed to a time at which anything should happen
> or a number useful for predicting such. When service should begin more
> properly depends on the other tasks in the system and a number of other
> decisions that are part of the scheduling policy.

This model takes all of those into consideration. The idea is not just
to predict but to use the calculated time to decide when to boot the
current process of the CPU (if it doesn't leave voluntarily) and put
this one on. This more or less removes the need to give each task a
predetermined chunk of CPU when they go on to the CPU. This should, in
general, reduce the number context switches as tasks get to run until
they've finished what they're doing or another task becomes higher
priority rather than being booted off after an arbitrary time interval.
(If this ever gets tried it will be interesting to see if this
prediction comes true.)

BTW Even if Ingo doesn't choose to try this model, I'll probably make a
patch (way in the future after Ingo's changes are settled) to try it out
myself.

>
> If you want to choose a "quasi-inter-arrival time" to achieve the
> specified CPU bandwidth allocation, this would be it, but to use that
> to actually enforce the CPU bandwidth allocation, you would need to
> take into account the genuine inter-arrival time to choose an actual
> time for service to begin. In other words, this should be a quota for
> the task to have waited. If it's not waited long enough, then it should
> be delayed by the difference to achieve the inter-arrival time you're
> trying to enforce. If it's waited longer, it should be executed
> sooner modulo other constraints, and perhaps even credited for future
> scheduling cycles.

The idea isn't to enforce the bandwidth entitlement to the extent of
throttling tasks if they exceed their entitlement and there's no other
tasks ready to use the CPU. This is mainly because the bandwidth
entitlement isn't fixed -- it's changing constantly as the number and
type of runnable tasks changes.

>
> In order to partially avoid underallocating CPU bandwidth to p, one
> should track the time last spent sleeping and do the following:

Yes I made a mistake in omitting to take into account sleep interval.
See another e-mail to Ingo correcting this problem.

> last_sleep = now - p->last_went_to_sleep;
> wait = p->avg_cpu_per_cycle*effective_weighted_load/p->load_weight;
> wait = wait > last_sleep ? wait - last_sleep : 0;
>
> By and large the scheduler can deterministically choose waits on the
> runqueue but it doesn't actually do that. Some additional corrections
> for delays beyond those decided up-front while on the runqueue or
> getting scheduled early on the runqueue may also help, though I'm not
> as interested in them as I am the one for sleep:
> last_wait = now - p->last_taken_off_the_cpu;
> wait = p->avg_cpu_per_cycle*effective_weighted_load/p->load_weight;
> if (wait > last_wait) /* didn't wait long enough? */
> wait += wait - last_wait; /* wait the difference longer */
> else if (wait < last_wait) /* waited too long? */
> /* wait less to make up the difference */
> wait -= wait > last_wait - wait ? last_wait - wait : wait;
> Where ->last_taken_off_the_cpu represents the time the task was last
> taken off the CPU for whatever reason (this may need to be done
> differently to handle time variables).
>
> In order to do better, longer-term history is required,

The half life of the Kalman filter (roughly equivalent to a running
average) used to calculate the averages determines how much history is
taken into account. It could be made configurable (at least, until
enough real life experience was available to decide on the best value to
use).

> but it can't be
> a truly infinite history.

I agree.

> To attempt to maintain an infinite history of
> bandwidth underutilization to be credited too far in the future would
> enable potentially long-term overutilization when such credits are
> cashed en masse for a sustained period of time. At some point you have
> to say "use it or lose it;" over a shorter period of time some smoothing
> is still admissible and even desirable.

Yes, that's why I suggest a running average over the last few scheduling
cycles for the task. But thinking about it some more I'm now not so
sure. The lack of apparent "smoothness" when I've done this sort of
thing with raw rather than smooth data (in modifications to the current
dynamic priority based scheduler model) is usually noticed by running
top and seeing wildly fluctuating dynamic priorities. I'm not sure that
the actual responsiveness of the system reflects this. So I'm now
willing to reserve my judgement on this issue.

>
> One might also consider debits owing to non-preemptibility or other
> sorts of cpu bandwidth overutilization with similar caveats. A half-
> life to an otherwise infinite history of credits and/or debits
> specified in absolute time may be appropriate, though it's not quite as
> computationally cheap as the above. The accounting for these credits
> and debits would take the place of ->last_taken_off_the_cpu above.

Credits shouldn't be necessary with this model if, instead of using the
average "on CPU" time per cycle for a pre-empted task when requeuing it,
you use the time period that it was "an CPU" before it was booted off as
this should compensate it correctly.

>
> Another attack on the problem of CPU bandwidth misallocation would be
> to further credit or debit the task according to the ratio of actual
> CPU bandwidth to allocated CPU bandwidth in order to compensate
> for prior failures to enforce the allocation.

The fact that the allocated CPU bandwidth is changing continuously makes
this not a good idea. On a system where allocations were made directly
rather being a side effect of "nice" and the number of runnable
processes it might be the way to go.

> Unfortunately, the result
> of scaling the artificial inter-arrival time in the obvious way is
> degenerate (independent of priority) so it can't be done that simply.
> Perhaps it could be used solely to adjust the credit and debit history
> contribution to the quasi-inter-arrival time or the difference used
> somehow, but I don't care to make more complex proposals than the
> first-order correction above (if it's even worthy of being called a
> proposal or a correction) for either this or a time-decaying credit
> and debit history or even debit accounting at all.
>
> I'd be happy enough to see the correction term to subtract out sleeping
> time (i.e. the code snippet with last_sleep).

Yes, this is THE important deficiency in my proposal.

> The rest and/or stronger
> attempts to factor in sleeping time or bandwidth misallocation I don't
> consider so significant, at least not without some demonstration there
> is CPU bandwidth misallocation happening.
>
>
> On Fri, Apr 20, 2007 at 10:10:45AM +1000, Peter Williams wrote:
>> I appreciate that the notion of basing the expected wait on the task's
>> average cpu use per scheduling cycle is counter intuitive but I believe
>> that (if you think about it) you'll see that it actually makes sense.
>
> I don't know. It sounds rather standard to me.

That's good to hear.

I also think that (as Ingo would probably prefer) that this mechanism
might work without using averages i.e. just use the last "on CPU" time.
If it does work it would have the advantage of being impervious to the
possible bad effects of CPU speed changes on those systems where this is
a feature. When using averages, I think that special consideration
would need to be made for variable cpu speeds -- probably not that
difficult but additional overhead anyway.
I've been giving some thought into a mechanism for pre-empting the
current task when another task's expected "on CPU" time arrives. I
think that it would be sufficient for scheduler_tick() to compare the
current time (via sched_clock() or whatever) with the "on CPU" for the
next task on the queue and initiation pre-emption if it's later than
that time. This increases the granularity a little but is compensated
for by the fact it will help with the situation where more than one task
on the queue has the same expected "on CPU" time in that each of them
would get at least a tick (if they needed it).

If some form of precise timer was used (instead) to trigger pre-emption
then, where there is more than one task with the same expected "on CPU"
time, only the last would get any CPU (and that's only the case if some
infinite loop doesn't get created).
Peter
--
Peter Williams pwil3058@bigpond.net.au

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