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Date: Fri, 02 Oct 2009 19:45:08 +0200
From: =?ISO-8859-15?Q?Christian_St=F6ber?=
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To: Tomcat Users List <users@tomcat.apache.org>
Subject: Re: Memory behavior: Tomcat versus Jetty
References: <4AC3616E.9030706@googlemail.com> <4AC36661.2070105@apache.org>
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Tim, thanks for the hint with the BodyContent.
Mark, thanks for your hint to take heap dump.

On this morning the two Tomcat instances were using 200 MB more memory 
than the Jetty instances do. So I thought it's a good time to take a heap 
dump.

In the heap histogram in jhat I saw a big difference between the 
BodyContentImpl (class org.apache.jasper.runtime.BodyContentImpl) counts:

Jetty: 43
Tomcat: 768

Suspicious!

I've take a look at this code and the first thing I've seen was, that 
since Jasper 2.1 BodyContents won't reset its buffer by default. (see 
BodyContentImpl.clear()). The second thing - what's responsible - is the 
change at org.apache.jasper.runtime.JspFactoryImpl. JspFactoryImpl in 
Jasper 2.1 now caches the PageContext-objects per thread. A 
PageContext-object have an array of BodyContents and so the memory the 
buffers of these BodyContents reserved will newer be freed until the 
thread will be stopped and garbage collected.

Until today I were using the default connectors with the maxThreads 
parameter, which newer shutsdown a thread. So the BodyContent's buffer 
were growing and growing, because we have some tags which generates much 
output, and would never be cleared.

Now I am using a separate thread pool (executor), which cleans up the 
pool when a thread is idle. And now the memory behavior is almost equal 
to Jetty.              

Christian


Tim Funk schrieb:
> If you use JSP tags where the JSP body does not directly stream but 
> needs buffered for the tag to finish processing it (using BodyContent) 
> - then tomcat will allocate and reuse these. If you are creating pages 
> with large body contents - this can take *A LOT* of memory. The 
> rational is to reuse these instead of letting them go to the GC. But 
> there is a way to not have tomcat reuse these.
>
> http://tomcat.apache.org/tomcat-5.5-doc/jspapi/javax/servlet/jsp/tagext/BodyContent.html 
>
>
> -Tim
>
> Christian St�ber wrote:
>> Hello list,
>>
>> we are about to migrate our webserver cluster from Jetty 5.1.14 to 
>> Tomcat 6.0.20. Currently there are four servers in our cluster with 
>> simple load balancing and no session clustering.
>>
>> At the moment we are testing Tomcat on 2 nodes of our cluster (also 
>> no session clustering yet). 2 remains with Jetty. Same WAR file an 
>> the same environment. Our first test ends in an OutOfMemoryError 
>> after two days. The java vm of the Tomcat servers has the same memory 
>> parameters as the server with the Jetty: -Xmx512m. And no other 
>> special GC parameters.
>>
>> Apperently the Tomcat needs more memory than Jetty.
>>
>> So we are trying to give Tomcat more memory: -Xmx768m. After two days 
>> the Tomcat is still running fine.
>>
>> Obviously Tomcat needs just a little bit more memory than Jetty. 
>> After these two days we can state the following memory behavior.
>>
>> Node   Mem used   Mem reserved
>> 1 TC   400m       530m
>> 2 TC   350m       495m
>> 3 J    230m       350m
>> 4 J    290m       400m
>>
>> Averagely Tomcat is using round about 110m more memory than Jetty. 
>> But why?
>>
>> I have seen that the Tomcat DefaultServlet may cache content up to 
>> 10m per default. But that are only 10m.
>> Are there some other things Tomcat may cache and Jetty does not?
>>
>> Do you have any experiences?
>>
>> Thanks in advance
>> Christian
>>
>>
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