Re: xen 4.5.0 rtds scheduler perform poorly with 2vms

[Ian Campbell <ian.campbell@citrix.com>]

On Tue, 2015-12-01 at 23:54 -0600, Meng Xu wrote:
> Hi Lars and Dario,
> 
> 2015-12-01 4:11 GMT-06:00 Lars Kurth <lars.kurth.xen@gmail.com>:
> > 
> > I wonder whether we need to add some health warnings and recommended
> > background reading to http://wiki.xenproject.org/wiki/RTDS-Based-Schedu
> > ler
> 
> 
> Maybe we could add some health warning and add a link to this discussion?
> Misconfiguration of the system will usually cause performance
> degradation, even for the other schedulers, such as ARINC653, credit,
> credit2.
>
> What I'm thinking is how much expert information we should expose to
> users. Sometimes, exposing too much information may not be so helpful.
> Sometimes, more information just  cause more confusion.
> 
> What do you guys think which type of information we should include?

I think there is an important distinction between credit2/credit and RT
schedulers such as arinc/rtds etc, which is that the former should just
work out of the box with no tweaking at all whereas the latter in general
need some sort of "intelligent input/configuration" to even begin using
them and have GIGO properties wrt their parameters.

(That's not to say you can't tweak credit* etc and break it if you want, but one typically doesn't need to start doing so just to get something running at all).

And AIUI the "intelligent input/configuration" requires some amount of background in RT scheduling, else you can get pathological results and think the scheduler and/or Xen is broken.

So I think some sort of warning that the RT schedulers do not "just work" and require one to understand the properties of your workloads and the schedulers and to feed them non-garbage inputs would be a useful to people who might otherwise expect to just "xl create" (maybe with some random inputs to the required settings) and have a useful result.

Having given that warning I don't think some links to some relevant background RT stuff would be too much info, neither would the inclusion of some specifics about the specific algorithm. After all that background and info is critical to being able to run a system using those schedulers, isn't it?

Ian.

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