Best practice for a web farm?

Russ Garrett russ at
Sat Feb 2 11:33:00 UTC 2008

Jed Reynolds wrote:
> Putting memcached on the same nodes as you put your apache workers 
> leaves you in a position to run your memcached into swap during a 
> request spike/flood and then you may as well just reboot your node 
> because the performance has fallen away badly.

At we run memcached on the same nodes as Apache, but all our 
webservers are diskless. Although that means that memcached can't swap 
out, it's not any better: if the node runs out of RAM without swap, the 
machine will lock up for several minutes while the OOM killer thinks 
about what to do - and that seriously affects any clients connected to 

Having said that, in normal use we don't hit this problem - we've found 
that it's possible to control the RAM usage of the machine to within a 
few tens of megabytes, even with PHP4 leaking memory like a sieve.

We only have ~6 Apache children per core though (between 12 and 48 
children per machine); request queuing happens with Perlbal, so we only 
have enough Apache processes as the machine's CPU can handle - this 
helps to control Apache memory usage.

Russ Garrett Ltd.
russ at

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