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Re: [Xen-devel] Some thoughts about the soft real time scheduler for Xen

Thanks Steven that really clarified things a lot.

Yan-Ching CHU

----- Original Message ----- 
From: "Steven Hand" <Steven.Hand@xxxxxxxxxxxx>
To: "Yan-Ching CHU" <cs0u210a@xxxxxxxxxxxxxxx>
Cc: "Ian Pratt" <Ian.Pratt@xxxxxxxxxxxx>; <xen-devel@xxxxxxxxxxxxxxxxxxxxx>
Sent: Wednesday, February 18, 2004 6:49 PM
Subject: Re: [Xen-devel] Some thoughts about the soft real time scheduler
for Xen

> >     Frist, a (partial) recap of what Ian said before:
> >
> > ======================================
> > We need a compile (or run time) option to completely replace the
> > current BVT scheduler with a soft real time scheduler that allows
> > domains to be given guarantees of the form "x microseconds every
> > y microseconds" (having a constraint that y must be a power of 2
> > or suchlike would be fine)
> >
> > If there's CPU time left over after meeting the guarantees of all
> > the runnable domains, it should be shared out in a proportional
> > manner between domains that have an 'eligible for best-effort
> > extra time' flag set.
> > ======================================
> >
> >     Some questions:
> >
> >     1. According to the "2003 Xenoserver Computing Infrastructure", in a
> > commercial production environment clients are supposed to "buy" the
> > computing time from Xenoserver, customers may not be happy with only
> > real time QoS?
> In the context of XenoServers, the notion is that you pay for e.g.
> S ms every P ms, where the total demand from all domains clearly needs
> to be less than e.g. 100% assuming EDF.
> However: if there is K% of the CPU remaining, we would like to be able
> to choose whether to dole this out to some subset of all domains, or
> to simply 'waste' it. The subset sharing the 'slack time' may or may
> not include any paying customers (e.g. it might involve running
> maintenance tasks for the XenoServer).
> In the non-XenoServer case (e.g. more traditional web hosting world),
> the use of a work conserving scheduler seems to make lots of sense.
> A scheduler which allows both these extremes (hard QoS & best effort)
> would hence we a very nice thing.
> >     2. I am working on a (simple) absolute share scheduler function in
> > which should provide the bottom line for what a customer buy from
> > Xenoserver. But I guess a hybrid scheduler combining these two is
> > in the future?
> Yup!
> >     3. For a Xenolinux (domain) to specify meaningful QoS requests, it
> > to gather information from application processes and inform them to Xen.
> > the literature there are serveral approaches such as directly modifying
> > kernel scheduler to be fully preemptible (preserving original
> > implementing new extension as module, using " dual kernels" by providing
> > thin layer between Linux kernel and interrupt control hardward (real
> > tasks interact with another [real time] kernel interface). Xen shows
> > properties like some of these in the way that it sits below standard
> > like "dual kernel", and, that application processes run unmodified.
> > Xen's scheduler, the schduler in Xenolinux needs to be changed. Any idea
> > this should be implemented in Xenolinux? Which approach is more
> So in principle a XenoLinux (or other guest) scheduler could certainly
> export some 'real-time' scheduling notions to its hosted processes.
> However this is not at all a requirement for us; in particular our
> experience with Nemesis showed that the sorts of QoS requirements
> people actually have in general are rather coarse... e.g. some notion
> of "an aggregate machine which is about 15% as powerful as a the
> real one". We'd like to support higher-level QoS specs (e.g. "a
> machine which is capable of scoring 225 on SPEC WEb99") and have a
> plan for this. But none of this involves tweaking any of the
> scheduling in XenoLinux or other guests.
> cheers,
> S.
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