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Re: [Xen-users] Linux DomU vs Bare Metal performance issues

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] Linux DomU vs Bare Metal performance issues
From: Mark Watts <m.watts@xxxxxxxxxxxxxxxxx>
Date: Sun, 05 Dec 2010 23:25:36 +0000
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On 04/12/2010 08:10, Mark Watts wrote:
> On 03/12/2010 22:18, Jeff Sturm wrote:
>>> -----Original Message-----
>>> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-
>>> bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Mark Watts
>>> Sent: Friday, December 03, 2010 2:40 PM
>>> To: xen-users@xxxxxxxxxxxxxxxxxxx
>>> Subject: [Xen-users] Linux DomU vs Bare Metal performance issues
>>> Can anyone shed any light on why I see such a drop in performance
>> between these
>>> two servers? I could understand, to an extent, seeing the DomU getting
>> 3/4 of the
>>> performance of the bare metal server given it has one CPU less, but
>> this is less than
>>> 1/2 the performance.
>> Understand what you're really testing here is the performance of the
>> virtual network driver, since Apache does almost no work to serve a
>> simple static page.  You're exercising the TCP stack and network drivers
>> more than anything else.
> Sure, but iperf shows near-enough line speed in both directions.
> Shouldn't this 'prove' the virtual network driver is up to scratch for
> this system?
>> Out of curiosity, can you easily run the same test against dom0?  The
>> dom0 runs a Xen kernel, on top of the hypervisor, but with a physical
>> network interface (rather than virtual).
> I can, certainly.
>> One thing you can try is to pin the dom0 and/or domU to a physical core,
>> using "xm vcpu-pin".  That gave me better throughput on some of my
>> tests.
> Dom-0 is already pinned to one CPU with the Dom-U given the remaining three.
>> (I have doubts that hyperthreading scales as well as multicore
>> processors, however I don't have any hyperthreaded systems of my own to
>> test.)
> I can certainly try this again with HT switched off on both boxes,
> although that'll have to wait 'till monday.

Some updates:

Running the same ab test against the Dom-0 gets me around 4,400 req/sec
- - a modest increase over the Dom-U (~3,800).
Rebooting my Dom-0 system into the standard CentOS 5.5 kernel sees the
same 10,200 req/sec, so its clearly something related to this kernel
which causes the slow down.

Any ideas?


- -- 
Mark Watts, BSc RHCE
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