[Xen-devel] Strange Xen Disk I/O performance among DomUs

[Jia Rao <rickenrao@xxxxxxxxx>]

Hi all,

I tested the xen vm disk I/O this weekend and had some interesting observations:

I
ran TPC-C benchmarks (mostly random small disk read) within two VMs
(PV) with the exactly the same resource and software configuration. I
started the benchmarks in the two VMs at the same time (started with a
script, the time difference is within several ms). The Xen VM scheduler
seems always favor one VM, which results in a 50% better performance
over the other VM.  I changed the seqence of the VM creation and
application starting order, the specific VM always got better
performance, 30%-50% better. 

What could be the reason that xen always favor a specific VM?

I
ran the above test for several more times. Between each run, I purged
the cached data within each VM to make the I/O demand always the same.
It is interesting that the performance gap between the two VM becomes
smaller and smaller. After 6 runs, the performance almost the same. 

Anyone has any idea? Does the VM scheduler scheduling VMs based on history?

I have already tried file-based, physical partition and LVM VMDs, similar results.

I am using Xen 3.3.1, CentOS 5.1, linux 2.8.18-x86_64
Each VM has 512M, 2-VCPU not pinned. Dom0 with 512M, 8-VCPU not pinned.
Host: dell poweredge 1950: 8G, two quad-core Intel xeon.

Thanks in advance,
Jia.
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