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From André Warnier>
Subject Re: Error 503 ocurring when server under load
Date Thu, 07 Oct 2010 17:31:24 GMT
Rob G wrote:
> Hey all,
> Recently migrated a production site (mixture of Servlets and JSPs)
> from Oracle Application Server to Apache/Tomcat. Since then we have
> seen numerous HTTP Error 503 - Service unavailable errors at peak
> times when site is under load. mod_jk.log has the following error
> message(s):
> [2184:1952] [error] jk_lb_worker.c (1473): All tomcat instances
> failed, no more workers left
> I'm looking for help in trying to tweak settings to prevent this, or
> confirmation that I've configured the setup correctly.
> Many thanks
> Rob
> Platform:
> Windows Server 2003 SP2
> Setup:
> Two tomcat instances with a single Apache front end, all on the same server
> Versions
> Tomcat: 6.0.24
> Apache: 2.2.16
> mod SSL: 2.2.16
> Open SSL: 0.9.8
> mod_JK:1.2.30

Your configuration looks very clean to me (no unnecessary settings etc.), which in this 
case is a plus (a good base to start tuning).
You may want to upgrade Tomcat to the latest version (6.0.29).

But before you start tuning, you should get some idea of what is actually going on.

For example, at the moment these errors happen, what are these Tomcats really doing ?
Are they really busy each processing 200 requests, with 200 threads running and actually 
doing something ? (200 is the default for the "maxThreads" attribute of the AJP Connector).

If yes, then you may just need a leaner application, or a bigger system (more RAM, faster

CPU), or more systems.  What does the Task Manager tell you about the total system load ?

If not, and many of these threads are waiting, then you may have an issue with a keepAlive

that is too long.
See, for the connectionTimeout and

keepAliveTimeout attributes.

Whatever you do, first get an idea of the starting situation.  Then modify one setting at

a time, and observe (and note) the effects.

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