[Xen-users] Poor performance running apache under xen

[Richard@xxxxxxxxxxxxxxx, Heycock@xxxxxxxxxxxxxxx]

We've been seeing a fairly significant slow down of a fairly large, in
house java application based on apache tomcat, when running under xen.
In order to gain some insight into this I've run similar tests using
Apache2 and ApacheBench and we are seeing almost exactly the same slow
down of about 24% (23.8% for our application and 23.5% for apache).

The test consisted of sending one million reqests to the web server and
was run from a different machine over gigbit ethernet. During the tests
the driving machine did not go over 15% cpu load and the 1 min load avg
did not go over 2. The machine under test had nothing else running. The
results given are for the second of two runs after a reboot. The cpu
usage is the average over the entire run obtained from sar with a four
second average (sar 4 0). The cpu figure is of the machine running the
test. The /lib/tls directory was removed.

The tests where carried out using the following versions of software:

* Ubuntu 5.10 (Breezy Badger) using Debian 32bit kernels
* Xen 3.0.2
* Apache 2.0.54 using both the mpm-prefork (multi-process worker) and the
  mpm-worker (hybrid multi-threaded multi-process worker)
* ApacheBench 2.0.41-dev

The different kernel configurations are:

    Stock kernel: 2.6.16-2-686-smp #1 SMP
    dom0 kernel: 2.6.16-1-xen-686 #1 SMP
    domU kernel: 2.6.16-1-xen-686 #1 SMP

The throughput is:

        Kernel                  throughput  % cpu (user + system)
    stock kernel (prefork):     5019.42     24.31
    stock kernel (worker):      4800.20     30.37
    dom0 kernel (prefork):      5029.60     22.37
    dom0 kernel (worker):       4843.59     29.97
    domU kernel (prefork):      3726.44     38.80
    domU kernel (worker):       3704.95     41.32

The reason I tested both the mpm-prefork and mpm-worker is test to see
if the threading concurrency model made a big difference -- it didn't!

Can anyone shed any light on these results? Is this the normal
performance that is to be expected from this type of application?

I've put all the results (ApacheBench results and the sar outputs) on
http://twosecondmemory.org/xen-performance/

rgh


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