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Re: [Xen-devel] [PATCH v1 1/4] xen: add real time scheduler rt

2014-09-03 10:08 GMT-04:00 George Dunlap <George.Dunlap@xxxxxxxxxxxxx>:
On Fri, Aug 29, 2014 at 10:05 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>> On 28.08.14 at 18:06, <xumengpanda@xxxxxxxxx> wrote:
>> As to your concern "I still don't see why such wouldn't be
>> applicable uniformly to all schedulers.", are you suggesting that the
>> credit and credit2 scheduler could also allow users to set/get each VCPU's
>> parameters? (I think that could be possible but this should be some design
>> decision made by credit and credit2 scheduler developer?)
> Perhaps. But I think a more uniform interface to the individual
> schedulers would help on the tools side too. But in the end I'm
> just raising questions/concerns here, it's George who needs to
> be okay with your approach.

Well the domctls aren't part of the stable ABI, so we can always
refactor things if we want to.

One could imagine restructuring the hypercall so that you could
specify a list of vcpus at the top level; but given that none of the
other schedulers actually want that at the moment, it seems like a lot
of faff for no good reason.

>>> I think anything going into the seconds, not to speak of minutes or
>>> hours, range is already beyond boundaries of being reasonable /
>>> useful.
>> Hmm, that's fair. If we want to limit the range to seconds, then uint32 is
>> enough. However, what if some user really want to set the period/budget to
>> hours/days? Then we couldn't support it if we use uint32 type for
>> period/budget. (This is just my subtle concern. I'm not arguing that we
>> have to use uint64 or uint32. As long as everyone agrees with a type, I can
>> just change it to the agreed type very quickly.) Do you have any suggestion
>> of how we can reach an agreement and get the range finalized?
> Let's see what others, mainly George and Dario, think.

Under what circumstances would anyone want a period of an hour?
â â
this point we're talking about a vcpu being allowed to run
continuously without pre-emption for half an hour, and then not
*allowed* to run at all for another half hour. That just seems really

âHonestly speaking, I don't think setting period and budget to hours is a good idea
â either. â
âActually, if users want to run a vcpu for half hour every hours, they could also set it to run 5milliseconds every 10 milliseconds. So any large period and budget can be scaled to smaller period and budget. The domain's performance will not degrade when the period and budget of each vcpu of this domain is scaled to a smaller number.Â

How about this:
I change the type to uint32 bit and use 2^32us ~= 1.19h as the upper bound of period and budget.Â
âI document the range of period and budget on the website and also let users know how to scale down the parameters of vcpus in case they have a very large period and budget.

What do you guys think?




Meng Xu
PhD Student in Computer and Information Science
University of Pennsylvania
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