Know your (u)limits

If you run a stress test on your new and shining tomcat box you may notice a bunch of nasty "Out of memory" errors and you may even be forced to restart you Tomcat service. I was.

What to do ?

First, check your memory settings in the /etc/inid.d/tomcat start-up file. Add the well known java memory switches in a line like this one :

JAVA_OPTS="-Xms256m -Xmx512m -Xss256k -server -Dfile.encoding=UTF-8"

Don't forget to change the values according to your case.

But the problem might not go away. Check the system limits for the user that run Tomcat.
Type, logged as the "tomcat" user :

ulimit -a

I was especially interested by the item max user processes because it was different between my remote hosted Linux box (showing out memory errors, with a value of 150) and my local "wmware" Centos (fine, no memory problems, the value was 4096).

On the other hand, in the Tomcat's server.xml, observe this line :

<connector port="80" maxhttpheadersize="8192" maxthreads="1500" minsparethreads="25" maxsparethreads="75" enablelookups="false" redirectport="8443" acceptcount="100" connectiontimeout="20000" disableuploadtimeout="true">

In my case, the value of the maxThreads attribute (1500) was way greater than the ulimit value on the remote box, but well below the value of the local "wmware" server.

So, to make a long story short, I changed the max user processes value on the remote box to a sensible value and all went fine.

There are the steps to accomplish this task :

• edit (as root) the file /etc/security/limits.conf
• add (or modify) the lines

  *   soft   nproc   4096 change this,
  *   hard   nproc   6000 and this value

  Eventually you will want to replace the stars * with the name of the "tomcat" user.
  If you delete (or omit) the lines, the value will be set to unlimited.
• Now, verify the settings by logging-in with the "tomcat" user and type the ulimit -a command and check if the value is the one you want (4096 in our case)
• Back to our server.xml, you can modify, if you want the maxThreads attribute to a greater value (but lesser or equal to the ulimit value).
• Restart Tomcat

And do the stress test again ....