[Xen-devel] credit scheduler

[Karl Rister <kmr@xxxxxxxxxx>]

While doing some performance analysis of Xen I have noticed some interesting 
behavior of the credit scheduler that I would like to see discussed.

In my most basic test I have a 4 socket dual core Intel box (Paxville) with a 
uniprocessor dom0 and 7 uniprocessor domUs.  Each of the domUs is pinned on 
its own core with the first core of the system left for dom0.  When running 
the credit scheduler the dom0 VCPU will bounce around the system, sometimes 
landing on the same thread as one of the domUs or sometimes on one of the 
sibling hyperthreads (this appears to happen a majority of the time it 
moves).  This is less than ideal when considering cache warmth and the 
sharing of CPU resources when the first core of the system is always 
available in this configuration.  Does the credit scheduler have any 
awareness of cache warmth or CPU siblings when balancing?

I have also seen similar behavior when running tests in the domUs such that 
each has its VCPU running at 100% utilization so I believe this behavior to 
be fairly uniform.

In my testing I am looking for uniform behavior so I have just been setting 
sched=sedf but I would like to move to credit scheduler since it is the new 
default. However, until I sort out this behavior I cannot.

Attached is a snapshot of "xm vcpu-list" taken every 5 seconds for 5 
measurements to show the dom0 VCPU migration pattern on a system with idle 
domains.

-- 
Karl Rister
IBM Linux Performance Team
kmr@xxxxxxxxxx

Attachment: xm.vcpu-list
Description: Text document

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