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Re: [Xen-devel] Question about the ability of credit scheduler to handle

To: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Question about the ability of credit scheduler to handle I/O and CPU intensive VMs
From: Yuehai Xu <yuehaixu@xxxxxxxxx>
Date: Thu, 30 Sep 2010 08:28:47 -0400
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, yhxu@xxxxxxxxx
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On Tue, Sep 14, 2010 at 5:22 AM, George Dunlap
<George.Dunlap@xxxxxxxxxxxxx> wrote:
> Credit2 development is mostly stalled; I've just got too many other
> things to do at the moment.  If you know someone good at hypervisor
> development that wants to move to Cambridge to help me out, I think we
> have some open positions... :-)
> The problem you describe, which I call the "mixed workload" problem,
> is something that I'd like to try to solve with credit2.  The actual
> problem with credit1, at the moment, is that when a vcpu is scheduled
> to run, it can always run for 30ms if it wants to.  So if it's a CPU
> burner, in order to give it 50%, you have to keep it from running for
> 30ms before letting it run for 30ms again.
> I agree, letting a VM with an interrupt run for a short period of time
> makes sense.  The challenge is to make sure that it can't simply send
> itself interrupts every 50us and get to run 100% of the time. :-)

I am afraid I don't really understand the challenge is, or, in another
word, this method is good principally, but in practice, it is hard to
implement? As I know, the OS should always schedules I/O related
processes once they are in runnable queue, so, as long as we give even
a very short period of time to the waken up guest VM, the I/O process
in it should be scheduled at once. In that case, this problem should
be solved. Of course, I don't do experiments, saying is always much
easier than doing.

> I don't have time to work on this right now, but if you work up some
> patches, I can give you feedback.  Be advised, that getting this stuff
> to work right is not easy.
>  -George

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