[Xen-users] CPU assignment and sched-credit

[lee <lee@xxxxxxxxxxxxxxx>]

Hi,

looking at [1], I start to think that it would make a lot of sense to
overcommit CPUs on a server with very light load and let the scheduler
handle the assignment by time slices as demand requires.  I've been
reading that overcommitting CPUs is a bad idea, yet I don't understand
what the point is in letting N-1 CPUs idle while one of the N CPUs is
busy and the VM driven by it could benefit from being able to use more
CPUs.

IIUC, the scheduler will preempt a VM after so much time anyway and
allow other VMs to run.  When CPUs are not overcommitted, it wouldn't
need to preempt anything because all VMs can run simultaneously.  (I
don't know if does preempt a VM anyway.)

With CPUs overcommitted and all VMs busy, overall throughput can (will)
be diminished, and latency could become an issue.

With very little load, it seems to make sense to just let all VMs have
all CPUs.  There's probably a point at which it would be better to do
some fine tuning, based on the actual loads, and before that point is
reached, what could be better than letting all CPUs to all VMs?

Am I wrong or missing something?


[1]: http://wiki.xen.org/wiki/Credit_Scheduler

-- 
Knowledge is volatile and fluid.  Software is power.

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