Re: [Xen-devel] Question on xen schedulers

[Emmanuel Ackaouy <ack@xxxxxxxxxxxxx>]

On Wed, Nov 15, 2006 at 05:48:39PM +0100, Schmidt Werner wrote:
>    Now, 'credit' is designed for an optimum utilisation of a SMP system and
>    may be really the best scheduler for this purpose. But i think this is not
>    the only useful scenario for the usage of Xen; there maybe other scenarios
>    with other demands, e.g. 'soft' real-time systems and/or single processor
>    systems.

Hi.

Just to clarify, the credit CPU scheduler is designed with a
couple of goals in mind, not all of which have to do with
multi-processing:

1- Comparative weighted fairness among competing guests
    By default, all guests are equal. By increasing/decreasing
    a guest's weight compared to others, it can be made more/less
    important. This has effects on UP as well as SMP hosts.

2- Automatic preemption of CPU intensive guests by I/O generating ones
    Guests that sleep/wake frequently and do not consume their
    fair share of CPU resources will preempt CPU intensive domains
    automatically. The administrator will not need to tag domains as
    CPU intensive or I/O generating to achieve this result. Again,
    this feature is targeted at both UP and SMP hosts.

3- Work conserving on SMP systems
    No CPU shall ever go idle if there is runnable work. Load balancing
    is automatic and fine grain.


The credit scheduler attempts to solve common problems without
administrator involvement. Common problems targeted include:
- Scheduling competing I/O and CPU intensive domains.
- Load balancing VCPUs on an SMP system (My grandmother has an SMP
  desktop. Let's face it: this is a common scenario now).

I agree soft real time scheduling methods can also be useful to solve
other problems. I think it would be useful for people to explain what
problems they would like to solve so we can support the right type of
soft real time features to address them.

Would single processor strict priority scheduling solve most problems
people care about?

Are there scenarios where better per guest wake-to-run latency or CPU
reservations guarantees are required? What are they?

There is lots of work to do and lots of code to maintain. It would be
smart to prioritize things a bit. I think it would be useful to build
some concensus in the community with respect to this. Your post is a
good starting point. So, what scenarios do people out there care about?

Cheers,
Emmanuel.

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