RE: [Xen-users] CPU intensive VM starves IO intensive VMs

["Apparao, Padmashree K" <padmashree.k.apparao@xxxxxxxxx>]

Question:

I think these numbers will depend very much on the physical cpus
assigned to the vms. 

What are the number of vcpus so for each VM and what physical cpus are
the VMs on.  I have run similar tests and seen different results from
what you see. 

You can get the vvpu/pcpu by doing an "xm vcpu-list" 
As Dom0 does the interrupt processing for the network intensive app, the
network intensive app will starve if dom0 does not get enough cpu due to
scheduling. It is necessary to give dom0 enough cpu to work on. 


- Padma


-----Original Message-----
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Tim Wood
Sent: Tuesday, August 29, 2006 10:32 AM
To: Xen Users
Subject: [Xen-users] CPU intensive VM starves IO intensive VMs

Hi,
I'm noticing very bad performance when one VM is running a CPU
intensive job and another VM is doing a network intensive task.

For example:

I run Iperf and measure the attained bandwidth with and without
running a CPU Hog application at the same time. The hog app just runs
in an infinite loop performing calculations.

When I do this in Dom0 I get essentially the same bandwidth: 921
Mb/sec without the hog, 920 Mb/sec with.  This is on a gigabit
network, so that seems right.  It makes sense that running the cpu hog
doesn't really affect bandwidth since the IO intensive job shouldn't
require much real computation other than negotiating protocols.

If I do this inside a VM, without the hog I see 447 Mb/sec, and with
the hog I see 109 Mb/sec.  I can understand that there is a difference
between dom0 and the VM without the hog app running due to Xen
overhead, but it doesn't seem right that there should be such a drop
when the hog application is running.

If the hog app is running in a separate VM, performance is even worse
- only 97 Mb/sec.

In all of these examples I am using the sedf scheduler with equal CPU
weights for dom0 and all VMs.  Desite this, in the 2 VM scenario, the
scheduler ends up giving 99% of the cpu to the VM running the hog app,
practically starving the IO intensive VM.

I am aware that the next version of Xen uses the new credit scheduler
- does anyone know if that scheduler tries to deal with these kinds of
issues?  The changes I had heard mostly regarded better supporting
SMP.

-Tim

_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users

_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users