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

["Yan-Ching CHU" <cs0u210a@xxxxxxxxxxxxxxx>]

Hi Xen guys,

    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 soft
real time QoS?

    2. I am working on a (simple) absolute share scheduler function in Xen,
which should provide the bottom line for what a customer buy from
Xenoserver. But I guess a hybrid scheduler combining these two is desirable
in the future?

    3. For a Xenolinux (domain) to specify meaningful QoS requests, it has
to gather information from application processes and inform them to Xen. In
the literature there are serveral approaches such as directly modifying the
kernel scheduler to be fully preemptible (preserving original interface),
implementing new extension as module, using " dual kernels" by providing a
thin layer between Linux kernel and interrupt control hardward (real time
tasks interact with another [real time] kernel interface). Xen shows
properties like some of these in the way that it sits below standard Linux
like "dual kernel", and, that application processes run unmodified. Besides
Xen's scheduler, the schduler in Xenolinux needs to be changed. Any idea how
this should be implemented in Xenolinux? Which approach is more appropriate?

    Any comments are welcomed.

Thanks,
Yan-Ching CHU




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