FAQ
Hello All:

I've been requested to add some TCP tuning parameters to some CentOS
5.4 systems. These tunings are for the TCP receive buffer windows:

net.core.rmem_max
net.core.wmem_max

Information on this tuning is broadly available:
http://fasterdata.es.net/TCP-tuning/linux.html
http://www.speedguide.net/read_articles.php?id1

Potential downsides are available:
http://www.29west.com/docs/THPM/udp-buffer-sizing.html
From the above, the rmem size is a function of the number of listening
processes times the size of the receive buffer.

Apache can initially create a bunch of server processes (StartServers
directive, for example). In our environment we may spawn off 50 or
more httpd listeners. Does each listener allocate its own buffer or is
the parent process the only one?

The tunable seems innocuous but I am concerned about the impact to
memory given that there will be many listener processes. We are
running fairly tight already and because these are virtual
environments in a cluster, adding a meg or so per listener can equate
to multiple gigs across all the machines in the virtual host cluster.

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postedMay 3, '10 at 2:13p
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