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

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Fri, Dec 03, 2010 at 07:40:23PM +0000, Mark Watts wrote:
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> Hi, I'm trying to diagnose a reasonably large performance drop between
> two ostensibly similar servers; one running CentOS 5.5 on bare metal,
> the other running Fedora 14 as a Xen DomU under CentOS 5.5 (Xen
> 3.1.2-194.26.1.el5)
> 
> I have a pair of older Dell PowerEdge 1750 servers, each with 4GB ram
> and 2x 2.4GHz Xeon CPU's with HyperThreading enabled. CPU's do not
> support any VT. Disks are 3x U320 10k SCSI disks in software RAID-5.
> Both servers are connected to a Cisco 2950 100Mbit switch
> 
> Server A is running kernel-2.6.18-194.26.1.el5
> 
> Server B is running kernel-xen-2.6.18-194.26.1.el5.
> Dom0 is pinned to one CPU, with 512MB ram.
> I have one DomU;
> - - B1 - Fedora 14 allocated the remaining 3 CPU's and 3400MB ram.
> 
> I have Apache httpd installed on Server A and B1. Both instances are
> running the same configuration based on mpm-worker and supporting
> keep-alives.
> 

I can't remember from top of my head.. does that create a lot of new
processes, or does it re-use the same existing processes? 

-- Pasi

> I have a third box, also Dual-Xeon/HT from which I'm running the ab
> benchmark; ab -c40 -n100000 -k http://<ip>/index.html
> The index.html file simply contains the hostname of the server, so
> should be cached once read.
> 
> iperf reports a healthy 95Mbit in each direction between the Client and
> DomU.
> 
> Server A can support around 10,200 requests/second.
> The DomU only reaches a maximum of 3,800 requests/second.
> 
> During the test, xentop reports no disk activity but CPU usage going up
> to 300%; consistent with 3 CPU's.
> 
> 
> 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.
> 
> Regards,
> 
> Mark.
> 
> - -- 
> Mark Watts, BSc RHCE
> http://www.linux-corner.info/
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