Tuning varnish for high load

[Mark Smallcombe]

We are running varnish (default config) in production and are tuning
it to handle high load.

We have 4 load balanced varnish servers (64 bit RedHat 5.1, 4G of RAM,
everything on a gigabit network). We find that under high load varnish
is unable to serve pages or the keepalive pages to our load balancer
(so it takes the servers offline).

Running the following load test on a 65K, uncompressed static page
from a single load testing server stops the 4 varnish servers from
serving static web pages:

httperf --verbose --hog --timeout=25 --server=www.ourdomain.com
--port=80 --uri /pagetotest.html --rate=1000 --send-buffer=2000
--recv-buffer=2000 --num-conns=500000 --num-calls=1

What tuning recommendations do you have for varnish to help it handle high load?

Mark