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From "Craig R. McClanahan" <craig...@apache.org>
Subject Re: Thread handling in Tomcat 4.x
Date Wed, 04 Dec 2002 20:40:02 GMT


On Wed, 4 Dec 2002, Dunlop, Aaron wrote:

> Date: Wed, 4 Dec 2002 09:32:56 -0800
> From: "Dunlop, Aaron" <aaron.dunlop@transcore.com>
> Reply-To: Tomcat Users List <tomcat-user@jakarta.apache.org>
> To: "'tomcat-user@jakarta.apache.org'" <tomcat-user@jakarta.apache.org>
> Subject: Thread handling in Tomcat 4.x
>
> Is there any scenario in which Tomcat 4.0 or 4.1 will kill off processor
> threads?
>

There is no such mechanism currently -- all you can is set the minimum and
maximum thread counts (minProcessors and maxProcessors).

> I know that under apache, it is possible to limit the number of spare
> processes, in which case the controller process will kill off extras when
> load decreases.
>
> Alternatively, is there any error condition under which a thread will be
> considered hung or otherwise corrupted and killed off?
>
> Our application maps certain objects on a per-thread basis, and I'm worried
> that we may end up with memory leaks when objects remain referenced in our
> map even though the thread they are assigned to is no longer valid. (And yes
> - I know this is probably a bad design, and we're looking into some
> refactoring there, but not a week before a release if we can avoid it...)

You're right -- this (persistent storage on a per-thread basis) is not a
good design for a web application, although temporary storage (for the
lifetime of a request in a ThreadLocal) would probably be fine.

The threads belong to the container, not to the app -- and, in fact, they
are shared across apps if you're running more than one.

The best place to cache per-app persistent objects, if you need them to
stay in memory, is in servlet context attributes (if they are shared) or
in a session (if they are specific to a user).

>
> Either there hasn't been any discussion of this topic in the archives or
> (more likely) I just didn't hit on the right combination of keywords to
> search on. And I can't convince myself one way or that other from the code.
> ;)
>

Short of refactoring, you'd need to patch the way that Coyote manages its
thread pool in order to implement a cleanup.  I think your time would be
better spent on refactoring, though.

> Thanks in advance,
> Aaron
>

Craig


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