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RE: [Xen-devel] RE: [RFC][PATCH 0/4] Modification of credit scheduler rev2

>From: NISHIGUCHI Naoki [mailto:nisiguti@xxxxxxxxxxxxxx] 
>Sent: Thursday, January 15, 2009 12:43 PM
>Hi, Kevin
>Tian, Kevin wrote:
>>> From:NISHIGUCHI Naoki
>>> Sent: Thursday, January 15, 2009 10:05 AM
>>>>    4. issues left:
>>>>            a. Abrupt glitches are still generated when the 
>>> QEMU emulated mouse being used and moving mouse quickly in 
>>> guest A. Passing-through USB mouse/keyboard to guest A, then 
>>> no glitches.
>>> I also noticed that. Though I don't know the precise cause, I 
>>> found that 
>>> dom0 and guest A would consume largely CPU time (hundreds of 
>>> milliseconds) in such situation. In this case, the priority of 
>>> dom0 and 
>>> guest A falls rapidly, then guest B runs until the priority of 
>>> dom0 and 
>>> guest A becomes BOOST. In worst case, it will take about 120ms.
>> I remember that Disheng once told me that BOOST only happens
>> when vcpu is waken up and its current priority is UNDER. In your
>> case guest A should be in OVER after running hundreds of ms, 
>> and then it waits enough long time to become UNDER and then 
>> BOOST. If this is the case, your enhancement on BOOST level
>> seems only solving part of the latency issue. Here either assigning
>> a static priority, or adding more BOOST source (like event, intr,
>> etc) seems more complete solution.
>In my case, though the vcpu should be switched to other vcpu in time 
>slice, the cpu running the vcpu doesn't schedule during 
>hundreds of ms. 
>I don't know why this happens.

What's running within your guest B? Unless full cpu intensive workload
happens within guest B, there's chance for guest B to issue block 
hypercall once it enters idle loop, and then once it's blocked, Xen 
credit scheduler can pick dom0 or guest A anyway. So 1st thing you
could figure out the activity within guest B.

If guest B does be always busy, then you may need to check the 30ms
credit allocation algorithm in credit scheduler. It looks like some sequence
that guest A may be always granted as OVER priority due to its earlier
overrun, until guestB also overruns a similar length. Then in this punish
period, guest A has no chance to be boosted with all cycles granted to
guest B instead. if it's intended for fairness p.o.v, it may not suit for rt

>In credit scheduler, credit consumed by the vcpu must be subtracted. 
>Therefore I think it is correct that dom0 and guest A are OVER because 
>my approach is to boost the vcpu within the range of weight.
>I think assigning a static priority is one solution. However, I think 
>that it affects credit accounting because we don't know how long the 
>domain with the static priority (probably highest priority) is run.

It could be one configurable option for some client usages, where 
a coarse-level static priority could better ensure the deterministic
to satisfy specific rt requirement.

>About adding more BOOST source, could you explain more to me?

Current the only source for boost is the wakeup event on a vcpu
with UNDER priority to catch up which is simply from fairness p.o.v
But for vcpu with RT requirement, more boost sources can be added.
E.g. when audio interrupt (either emulated, or passthrough), boost 
target vcpu and trigger a reschedule softirq immediately to reduce 
uncertainty of schedule latency. We need such a manual boost 
interface which is then inserted into some critical event paths where
we believe immediate schedule is necessary. Disheng is working on
this area now, I think. :-)

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