Re: [Xen-devel] [PATCH v2] xen/credit scheduler; Use delay to control scheduling frequency

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Dec 19, 2011 at 12:05 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>> The way it stands now, the ratelimit value will override the timeslice
>> value.  It had to be one way or the other; do you think the timeslice
>> value should be the priority?
>
> The minimum of both should be used, I would think.

What do you mean?  You mean in the assignment above?

That won't have any effect other than increasing interrupts and trips
through the scheduler.  Suppose the following set of events:
* timeslice is set to 1ms, ratelimit_us to 5000.
* a vcpu V is chosen; it's set to run with 1ms timeout.
* 1ms later, we go through the scheduler; the ratelimit code
determines that it hasn't run for long enough (only 1ms), so choses it
to run again (with a 1ms timeslice)
* Repeat until V has run for 5ms.

So although the timeslice is set to 1ms, and interrupts are happening
after 1ms, the ratelimit is overriding the 1ms of the timeslice and
making it 5ms.  Fundamentally, one of the two parameters must override
the other.  With this patch the way it is now, ratelimit will override
timeslice.  if you want the timeslice to override ratelimit, then
there will have to be more code to make that happen.

 -George

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