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From Daniel Mikusa <dmik...@gopivotal.com>
Subject Re: Tomcat 6.0.26 on Windows server - sessions are not expired
Date Mon, 03 Feb 2014 12:10:00 GMT
On Feb 3, 2014, at 3:30 AM, Maor Yosef <maoryosef@gmail.com> wrote:

> Hi.
> 
> 1. We are aware that 6.0.26 is old, but since there is a large operational
> impact, we wont upgrade the tomcat until we will know its definetly an
> issue in this specific version

While I understand what you’re saying, I disagree.  If you need to sell the change to management,
you should take a look at the security problems that have been fixed since 6.0.26 and weigh
the cost of upgrading versus a security breach or manually applying mitigations, if that’s
even possible.

  http://tomcat.apache.org/security-6.html

> 2. You are right, it was my mistake, it causes OOM and not stack overflow,
> when we see high sessions count we get exceptions saying "unable to create
> new native thread”

This is telling you that there’s not enough memory to allocate any more threads.  

1.) Have you tried setting “-Xss”?  This will set the thread stack size, i.e. how much
memory each thread has available for its stack.  Often times you don’t need nearly as much
as the default allocated by the JVM, so you can lower it and get more threads out of the same
available memory.  Maybe try 256k or even lower.  You’ll know you went too low if you see
StackOverflowErrors.

2.) How many threads are being created / used?  Perhaps you’re creating too many threads?
 You’ll probably want to do some monitoring and see how many the Tomcat is creating / using
and how many your application is creating / using.  Perhaps you’ve got a problem where too
many threads are being created or where threads are being created and not properly cleaned
up.  Tools that could help here:  jconsole / jvisualvm or thread dumps

> 3. Looking at the sessions manager, we see a lot of sessions with a
> negative TTL - meaning they werent expired, if I manually expire them then
> the sessions count decreases.

This doesn’t sound related, although it’s hard to say.  Might be helpful if you can include
your configuration, minus comments.

> 4. Can you point me to an article on how to configure different background
> thread for each container? is it configured in tomcat or should be
> implemented in the application?

What exactly do you mean here?  You can control Tomcat’s thread usage with an Executor [1]
or directly on the connector [2].

Dan

[1] - http://tomcat.apache.org/tomcat-6.0-doc/config/executor.html
[2] - http://tomcat.apache.org/tomcat-6.0-doc/config/http.html


> 
> Thanks,
> 
> 
> On Sun, Feb 2, 2014 at 7:38 PM, Konstantin Kolinko
> <knst.kolinko@gmail.com>wrote:
> 
>> 2014-02-02 Maor Yosef <maoryosef@gmail.com>:
>>> Hi,
>> 
>> 1. 6.0.26 is old.
>> 
>>> We are facing issues where the sessions are not being expired
>>> and eventually causing a stack overflow.
>> 
>> 2. Non-expiring sessions may cause OOM, but they cannot cause a stack
>> overflow.
>> 
>> What are your evidences?
>> 
>>> We see that at some point the sessions get pilled up and we need to
>>> manually expire those sessions through the manager application, or
>>> until we will restart tomcat but after a few hours / days, sessions
>>> will start to get stuck again
>> 
>> 3. Increasing count of session does not mean that sessions do not expire.
>> 
>> Your evidence = ?
>> 
>>> We see it in all the applications that are deployed on tomcat,
>>> including the manager application - this rules out (in my opinion) the
>>> possibility that its an issue with our application.
>> 
>> 4. Sessions are expired by a background thread. If the thread is stuck
>> somewhere (e.g. doing auto-deployment work) it will affect expiration
>> of sessions.
>> 
>> Your thread dump = ?
>> 
>> By default there is one background thread that is shared by all
>> container levels in Tomcat,  but you can configure a separate one in
>> each container (container = Context, Host or Engine).
>> 
>> 5. Web bots that do not support cookies may generate multiple sessions.
>> 
>> See CrawlerSessionManagerValve
>> 
>> Best regards,
>> Konstantin Kolinko
>> 
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