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Re: [Xen-users] Questions about sedf scheduler

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,


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