Re: [patch] CFS scheduler, v3

[Willy Tarreau <w@1wt.eu>]

On Fri, Apr 20, 2007 at 10:10:45AM +1000, Peter Williams wrote:
> Ingo Molnar wrote:
> >
> > - bugfix: use constant offset factor for nice levels instead of 
> >   sched_granularity_ns. Thus nice levels work even if someone sets 
> >   sched_granularity_ns to 0. NOTE: nice support is still naive, i'll 
> >   address the many nice level related suggestions in -v4.
> 
> 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.
> 
> 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 */
> 
> 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.)
> 
> 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
> 
> Calculating p->avg_cpu_per_cycle costs one add, one multiply and one 
> shift right per scheduling cycle of the task.  An additional cost is 
> that you need a shift right to get the nanosecond value from value 
> stored in the task struct. (i.e. the above code is simplified to give 
> the general idea).  The average would be number of cycles based rather 
> than time based and (happily) this simplifies the calculations.
> 
> If the expected time to get onto the CPU (i.e. expected_wait plus the 
> current time) for p is earlier than the equivalent time for the 
> currently running task then preemption of that task would be justified.

I 100% agree on this method because I came to nearly the same conclusion on
paper about 1 year ago. What I'd like to add is that the expected wake up time
is not the most precise criterion for fairness. The expected completion
time is better. When you have one task t1 which is expected to run for T1
nanosecs and another task t2 which is expected to run for T2, what is
important for the user for fairness is when the task completes its work. If
t1 should wake up at time W1 and t2 at W2, then the list should be ordered
by comparing W1+T1 and W2+T2.

What I like with this method is that it remains fair with nice tasks because
because in order to renice a task tN, you just have to change TN, and if it
has to run shorter, it can be executed before CPU hogs and stay there for a
very short time.

Also, I found that if we want to respect interactivity, we must conserve a
credit for each task. It is a bounded amount of CPU time left to be used. When
the task t3 has the right to use T3 nsecs, and wakes up at W3, if it does not
spend T3 nsec on the CPU, but only N3<T3, then we have a credit C3 computed
like this :

  C3 = MAX(MAX_CREDIT, C3 + T3 - N3)

And if a CPU hog uses more than its assigned time slice due to scheduler
resolution, then C3 can become negative (and bounded too) :

  C3 = MAX(MIN_CREDIT, C3 + T3 - N3)

Now how is the credit used ? Simple: the credit is a part of a timeslice, so
it's systematically added to the computed timeslice when ordering the tasks.
So we indeed order tasks t1 and t2 by W1+T1+C1 and W2+T2+C2.

It means that an interactive task which has not eaten all of timeslice will
accumulate time credit have great chances of being able to run before others
if it wakes up again, and even use slightly more CPU time than others if it
has not used it before. Conversely, if a task eats too much CPU time, it will
be punished and wait longer than others, and run less, to compensate for the
advance it has taken.

Also, what I like with this method is that it can correctly handle the
fork/exit storms which quickly change the per-task allocated CPU time.
Upon fork() or exit(), it should not be too hard to readjust Tx and Cx
for each task and reorder them according to their new completion time.

I've not found a way to include a variable nr_running in the tree and
order the tasks according to an external variable, hence the need to
rescan the tree upon fork/exit for maximum precision. That's where I
stopped working on those ideas. If someone knows how to order the tree
by (Wx+(Tx+Cx)/nr_running) with nr_running which can change at any time
but which is common for everyone in the tree, that would be great.

Best regards,
Willy

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