Re: xen scheduler

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Mon, 2015-05-18 at 13:24 +0530, Rajendra Bele wrote:
> As per my knowledge.
> Credit scheduler sorts its queue of VCPUs with priority based on
> credit value.
>
Yes and no. :-)

This is probably formally correct, as:
 1. when sorting it, we do rearrange the runq in priority order
 2. the priority of a vCPU is _based_ on credits, as being in UNDER or
    in OVER state does depend on credits

However, as stated here:

/*
 * This is a O(n) optimized sort of the runq.
 *
 * Time-share VCPUs can only be one of two priorities, UNDER or OVER. We walk
 * through the runq and move up any UNDERs that are preceded by OVERS. We
 * remember the last UNDER to make the move up operation O(1).
 */
static void
csched_runq_sort(struct csched_private *prv, unsigned int cpu)

there are only two priorities, so, for Credit, "sorts its queue of VCPUs
with priority based on credit value" means "all the UNDER vCPUs come
before any OVER vCPU"... was that what you meant?

BTW, this is one of the differences between Credit and Credit2, as in
Credit2, the runqueues are kept sorted by credit order...

> It follows FCFS technique for equal priority if we apply SJF for equal
> priority
> will be helpful to reduce waiting time spend in the queue basically
> for the Under Priority (credits>0) VCPUs.
>
Yes, I think that treating the various vCPUs in UNDER differently,
basing on some parameter/state/etc. of them would be good... actually,
that's why I like Credit2, and why we're trying to make it usable in
production.

Doing the same in Credit is of course possible, but I fear it would
reveal really complex. Then, again, we already have Credit2 doing
something like that... So I think that anyone wanting a scheduler with a
similar property should invest time in Credit2, rather than trying to
tweak Credit1 into that.

But then, of course, I may be wrong, and you'll come up with a 15 lines
patch that does the trick! ;-P

Anyway, you're mentioning SJF, which would indeed be great, if it
weren't impossible to implement: "Another disadvantage of using shortest
job next is that the total execution time of a job must be known before
execution" (http://en.wikipedia.org/wiki/Shortest_job_next ) :-(

How where you thinking to approximate the execution time of upcoming
execution instance of a vCPU? I'm asking because, per my experience, the
method chosen for that purpose has quite a bit of influence in the
effectiveness of a particular SJF implementation.

> obliviously situation is rare but will make sense when large no of VM
> are active.
> 
I'm not sure I'm getting what you mean here. What's rare, that there are
many vCPUs in UNDER? I don't think it is. Or, in any case, it certainly
is the typical situation in which a scheduler is important (if there is
less work than CPUs, the scheduler does not count that much!), so it's a
good scenario to consider and try to improve... Or were you referring to
something else?

> If anybody working on this wants his/her comments on this idea
> 
I don't think there is anyone working on this particular item, but
scheduling is certainly receiving some attention, and we're always happy
to discuss potential new features, improvements, and alike! :-)

Regards,
Dario

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