Re: [Xen-users] Questions about sedf scheduler

[Stephan Diestelhorst <sd386@xxxxxxxxx>]

Hi Amitayu,

> In my setup, I'd two DomUs and Dom0 was running. I got the following 
> output while running the program:
> # ./sedf
> Domina#0: slice = 15000000, period = 20000000, extratime = 1, latency 
> = 0, weight = 0
> Domina#18: slice = 0, period = 100000000, extratime = 1, latency = 0, 
> weight = 0
> Domina#19: slice = 0, period = 100000000, extratime = 1, latency = 0, 
> weight = 0
> I did not understand few things and would appreciate very much if 
> someone can clarify the following doubts:
> 1. Why the period-length is different for Dom0 and DomU's?
Because that's the default ;-) Domain 0 is running with a guaranteed 
time slice of 15ms every 20ms and gets also some of the available 
sparetime, whereas both domu's are by default set to get only time that 
is leftover inside the system. Their period (100ms) is (pretty much) 
meaningless to standard operation.
> 2. Does period-length mean the total time within which *all* domains 
> are supposed to be scheduled *at least once*?
>     What does it mean exactly? What does slice-length mean exactly?
Hopefully the above explanation clarifies things. Slices are meaningful 
as a guarantee, that means your domain d will get slice ms cpu-time 
every period ms. Please note that this guarantee is much stronger than 
domain d gets say 50% of available CPU time, as this could mean: every 
year your domain runs for half a year (and then exclusively...). 
Specifiying periods is exactly as you assume a way of stating the 
*at-least-once* property you are refering to.
> 3. Can a domU have a slice-length as 0? If so, how can it be 
> scheduled? I presume that value of extratime flag will make it 
> scheduled, if set. Please clarify.
Inded. Please see above. For more detailed comments give me some time, I 
need to check my own code again ;-)
> 4. If extratime flag was not set, do we need to set non-zero 
> slice-length explicitly to schedule a DomU?
Exactly. Otherwise the domain will simply not get any cputime, though 
existing in the system.
Hope that helps,

Stephan

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