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

[Mark Watts <m.watts@xxxxxxxxxxxxxxxxx>]

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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 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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