Re: Questions regarding Xen Credit Scheduler

[Zhiyuan Shao <zyshao.maillist@gmail.com>]

In your case, I think, you can try Boost-Credit. Your I/O intensive 
domain will benefit for that.

Best,
Zhiyuan

On 07/09/2010 05:14 AM, Gaurav Dhiman wrote:
> Hi All,
>
> I am using Xen 3.3.2 for some of my experiments, and have been
> consistently observing some sub-optimal results in our experiments
> with latency sensitive I/O intensive and compute bound VMs running
> together on a physical machine. We are observing latency issues in
> cases with enough CPU resources available for both the VMs to co-exist
> well, even if we give much higher weight to the latency sensitive VM.
> I suspect it is due to the way the Xen credit scheduler works. In this
> context, I have some questions regarding the scheduler:
>
> 1) In the sched_acct function, the credit cap is set to 300, enough to
> survive one time slice. But if some VCPU crosses that cap, it is set
> to 0, and marked inactive. Why is there no concept of a ceiling (like
> that of a floor for the VCPUs going over the credit line), i.e. why is
> it not set to 300? Is there some fundamental reason for setting it to
> 0? I believe this is resulting in a lot of times when our latency
> sensitive VCPUs have to wait for maybe a time slice, when they can
> immediately run. This might happen if they run with BOOST priority and
> get interrupted by a timer tick, which takes that priority away.
>
> 2) Why is the runq sorted by just priority (which is very coarse
> grained: BOOST, UNDER and OVER), and not the credit? This can result
> in VCPUs with higher credit getting starved for CPU if we have batch
> and latency sensitive VCPUs in the system.
>
> 3) Is there some patch, which makes the current credit scheduler
> fairer to the latency sensitive VCPUs? I see that the sched_credit2
> scheduler addresses these issues, but right now it has just one global
> runq and no load balancing features.
>
> Any advice/inputs here will be extremely valuable!
>
> Thanks in advance,
> -Gaurav
>
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