Re: [Xen-users] very interesting Xen Disk I/O performance

[Peter Booth <peter_booth@xxxxxxx>]

You have eight cores and two VMs with 2vcpu. Can you try the  
experiment with 8 VMs?
On Mar 31, 2009, at 12:02 AM, Jia Rao wrote:

> 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 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, not pinned.
> Host: dell poweredge 1950: 8G, two quad-core Intel xeon.
>
> Thanks in advance,
> Jia.
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