[Xen-devel] Re: credit scheduler error rates as reported by HP and UCSD

[Emmanuel Ackaouy <ackaouy@xxxxxxxxx>]

I've been away from my computer for a while on holiday and now
in the middle of moving so I've not had a chance to comment on
this thread until now. I apologize for reviving an old topic.

I should also say I've not read Lucy and Diwaker's paper nor have
I attended the latest Xen Summit but based on prior email
conversations with them I suspect I may know enough to insert
one comment here:

The credit scheduler's cap mechanism is not a reservation or
allocation system. If I remember I threw it in there because Ian
thought some host administrators may have wanted to prevent
customers paying for SLAs from consuming idle CPU resources
when they were available. You wouldn't want them to get used
to freebies and complain when they were "only" getting what
they were actually paying for.

The design principle for the caps was to add as few new lines
of code as possible as it was deemed to be a secondary feature.
I'm not surprised it's not very precise with small caps. I wasn't
even surprised when Lucy and Diwaker found bugs with it
after the scheduler was released. Caps are enforced more or
less at the accounting period of the credit scheduler which is
30milliseconds. Resource consumption is also calculated
by looking at which VCPU is running in the 10ms clock handler.
That's not a serious way to do soft real time scheduling.

I think adding actual allocations or some other form of soft
real time guarantees to work side by side with the credit
scheduler would be a neat idea. Personally I don't see the
point of running experiments or writing papers to understand
or show this but, if it convinces someone to do the work, then
I'm certainly for it.

Emmanuel.

On Apr 12, 2007, at 18:22, Lucy Cherkasova wrote:

> Hi Mike,
>
> > My first observation is that the credit scheduler will select a vcpu
> > that has exceeded its credit when there is no other work to be done on
> > any of the other physical cpus in the system.
> In the version of the paper that you read and refer to, we consciously
> considered  the three scheduler comparison using 1 CPU machine:
> the goal was to compare the "BASIC" scheduler functionality.
> I will present a bit more results for 2-CPU case during the Xen Summit.
>
> > In light of the paper, with very low allocation targets for vcpus, it
> > is not surprising that the positive allocation errors can be quite
> > large. It is also not surprising that the errors (and error
> > distribution) decrease with larger allocation targets.
> Because of 1-CPU machine, the explanation of this phenomena is 
> different
> (it is not related to load balancing of VCPUs) and the Credit scheduler
> can/should  be made more precise.
> What our paper does not show is the original error distribution for 
> Credit
> (original -- means after it was released). The resulst that you see in
> the paper are with the next, significantly improved version by 
> Emmanuel.
> I beleive that there is still a significant room for improvement.
>
> > None of this explains the negative allocation errors, where the vcpu's
> > received less than their pcpu allotments. I speculate that a couple of
> > circumstances may contribute to negative allocation errors:
> >
> > very low weights attached to domains will cause the credit scheduler
> > to attempt to pause vcpus almost every accounting cycle. vcpus may
> > therefore not have as many opportunities to run as frequently as
> > possible. If the ALERT measument method is different, or has a
> > different interval, than the credit schedulers 10ms tick and 30ms
> > accounting cycle, negative errors may result in the view of ALERT.
> ALERT benchmark is setting the allocation of a SINGLE domain (on 1 CPU 
> machine,
> no other competing domains while running this benchmark) to a chosen
> target CPU allocation, e.g., 20%, in the non-work-conserving mode.
> It means that the CPU allocation is CAPPED by 20%. This single domain 
> runs
> "slurp" (a tight CPU loop, 1 process) to consume the allocated CPU 
> share.
> The monitoring part of ALERT just collects the measurements from the 
> system
> using both XenMon and xentop with 1 second reporting granularity
> Since 1 sec is so much larger than 30 ms slices, there should be 
> possible
> to get a very accurate CPU allocation for larger CPU allocation 
> targets.
> However, for 1% CPU allocation you have an immediate error, because
> Credit will allocate 30ms slice (that is 3% of 1 sec). If Credit
> would use 10 sec slices than the error will be (theoretically) bounded
> to 1%.
>
> The expectations are that each 1 sec measurements should show 20% CPU
> utilization for this domain.
>
> We run ALERT for different CPU allocation targets from 1% to 90%.
> The reported error is the error between the targetted CPU allocation 
> and
> the measured CPU allocation at 1 sec granularity.
>
> > I/O activity: if ALERT performans I/O activity the test, even though
> > it is "cpu intensive" may cause domu to block on dom0 frequently,
> > meaning it will idle more, especially if dom0 has a low credit
> > allocation.
> There are no I/O activities, ALERT functionality is very special as
> described above: nothing else is happening in the system.
>
>
> > Questions: how does ALERT measure actual cpu allocation? Using Xenmon?
> As, I've mentioned above we have measurements from both XenMon and 
> xentop,
> they are very close for these experiments.
>
> > How does the ALERT exersize the domain?
> ALERT runs "slurp", a cpu-hungry loop, which will "eat"
> as much CPU as you allocate to it. It is a single process application.
>
>
> The paper didn't mention the
> > actual system calls and hypercalls the domains are making when running
> > ALERT.
> There is none of such: it is a pure user space benchmark.
>
>
> Best regards, Lucy
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