[Xen-devel] The caculation of the credit in credit_scheduler

["Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>]

When reading the credit scheduler code and doing experiment, I notice one thing 
interesting in current credit scheduler. For example, in following situation:

Hardware:
A powerful system with 64 CPUs.

Xen Environment:
Dom0 with 8 vCPU bound to CPU (0, 16~24)

3 HVM domain, all with 2 vCPUS, all bound as vcpu0->pcpu1, vcpu1->pcpu2. Among 
them, 2 are CPU intensive while 1 is I/O intensive.

The result shows that the I/O intensive domain will occupy more than 100% cpu, 
while the two cpu intensive domain each occupy 50%. 

IMHO it should be 66% for all domain.

The reason is how the credit is caculated. Although the 3 HVM domains is pinned 
to 2 PCPU and share the 2 CPUs, they will all get 2* 300 credit when credit 
account. That means the I/O intensive HVM domain will never be under credit, 
thus it will preempt the CPU intensive whenever it is boost (i.e. after I/O 
access to QEMU), and it is set to be TS_UNDER only at the tick time, and then, 
boost again.

I'm not sure if this is meaningful usage model and need fix, but I think it is 
helpful to show this to the list.

I didn't try credit2, so no idea if this will happen to credit2 also.

Thanks
--jyh


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