When running Apache Tomcat 5.0.28 configured with Apache Commons Logging
and Log4J there appears to be a serious resource leak which results from
the per thread Nested Diagnostic Context created by Log4J.
Consider the following usage scenario;
A Tomcat-5.0.28 server configured with 50 threads min, and 100 threads
max. And a VM heap of 400mb. The commons logging has been configured to
use Log4j-1.2.15
A Servlet which handles a reasonable volume of data (like a large sql
query or a big xml file) is written and emplaced at "/test"
A testing application can then be written which will deliberately create
100 simultaneous connections to Tomcat, each querying the "/test"
Servlet.
Upon hitting the thread limit of 50, Tomcat will create more threads to
reach its max of 100, each of these threads will in turn make a call to
org.apache.log4j.NDC.push(..) to create a new logging context per
thread. The testing application will then pause for a short while to
allow Tomcat to reduce the Thread count back to 50. When Tomcat reaches
50 threads, references to the previously created 50 temporary threads
are still held in the NDC hashtable. These threads are never freed
unless a call to org.apache.log4j.remove() is made either explicitly per
thread, or once to allow the lazy evaluation and cleanup.
As far as I can tell, in the way that Log4J is used within Tomcat and
Apache Commons Logging described here, this never happens, and the
Servlet developer certainly isn't responsible for calling
org.apache.log4j.remove() as they don't necessarily know they are even
using or deploying on Log4J.
The above scenario can be made to cause Tomcat to run out of memory in
as little as 15 minutes, with a 400mb heap.
As far as I know, this will affect all current and pervious versions of
Tomcat using Log4J and Apache Commons Logging, and any other application
servers with similar logging, i.e. JBoss.
The above results can be verified using a heap profiler in Java 1.6, and
many references to similar problems can be found online.
Emerson
_______________________________________________
This e-mail may contain information that is confidential, privileged or
otherwise protected from disclosure. If you are not an intended recipient of
this e-mail, do not duplicate or redistribute it by any means. Please delete it
and any attachments and notify the sender that you have received it in error.
Unless specifically indicated, this e-mail is not an offer to buy or sell or a
solicitation to buy or sell any securities, investment products or other
financial product or service, an official confirmation of any transaction, or
an official statement of Barclays. Any views or opinions presented are solely
those of the author and do not necessarily represent those of Barclays. This
e-mail is subject to terms available at the following link:
www.barcap.com/emaildisclaimer. By messaging with Barclays you consent to the
foregoing. Barclays Capital is the investment banking division of Barclays
Bank PLC, a company registered in England (number 1026167) with its registered
office at 1 Churchill Place, London, E14 5HP. This email may relate to or be
sent from other members of the Barclays Group.
_______________________________________________
