[Xen-users] Performance Issues: I/O Wait

["Nick Couchman" <Nick.Couchman@xxxxxxxxx>]

Hey, everyone,

I'm having some issues with a Xen DomU right related to performance.  We have an application that cross-compiles a Linux distribution from scratch for embedded systems.  We're attempting to run this application inside a Xen DomU (paravirtualized, modified guest kernel), and the performance is really bad.  The culprit seems to be high I/O wait times related to the network interface.

The host machine is a Dell PowerEdge 1950 with 2 x Dual-Core Xeon processors (Xeon 5150 @ 2.66GHz, 1333 FSB).  The domU is configured with 2 vCPUs and 1GB of RAM.  The Dom0 O/S is SuSE Linux Enterprise Server 10, and the DomU O/S is CentOS4U5.  For comparison, we're comparing against a physical machine that has a Pentium 4 3.2GHz processor on an 800MGz FSB with 4GB of RAM.  The RAM does not seem to be an issue - on the DomU, about half of the 1024MB is used by active processes and the other half is left to buffering and caching.  There are 4 kbytes of memory used on the swap partition.  Building these Linux distributions on the physical system takes 70-80 minutes (real time) - on the DomU system it takes 130-140 minutes.

The few times that I've see the system "lock up" the way the engineers who are using it claim it is doing, one of the DomU vCPUs goes to 100% wait for 20-30 seconds.  This seems to coincide with network operations in the DomU.  So, I'm wondering, what can I do to improve network performance and eliminate these I/O wait times on this CPU.  The server itself doesn't seem to be having any problems - while this occurs in the DomU, other physical systems can access the server just fine without any issues.  Any light that anyone can shed on the situation - how I can improve network performance and eliminate these I/O waits - would be most appreciated.  Here's the network config line from my Xen configuration file:

vif=[ 'mac=00:16:3e:75:0d:be,bridge=xenbr108', ]

And I found a suggestion for traffic control in the guest that I applied - maybe I did something wrong there, too.  Here's the output from tc qdisc show:

qdisc tbf 8001: dev eth0 rate 50Mbit burst 50Kb lat 20.0ms

Thanks,

Nick

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