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From Wade Chandler <hwadechandler-apa...@yahoo.com>
Subject Re: heap size problems (speed) [2]
Date Mon, 28 Nov 2005 18:55:48 GMT
--- Cristian <cs@nightwish.eu.org> wrote:

> 
> ----- Original Message ----- 
> From: "Duan, Nick" <NDuan@mcdonaldbradley.com>
> To: "Tomcat Users List" <users@tomcat.apache.org>
> Sent: Monday, November 28, 2005 7:30 PM
> Subject: RE: heap size problems (speed) [2]
> 
> 
> > Were you using the right JVM?  There were two JDK
> download for LINUX
> on
> > Sun's web site.
> 
> I have downloaded : J2SE(TM) Development Kit 5.0
> Update 5
> Linux self extracting file (although I think I
> should have get the rpm).
> 
> I've extrated it in a separate directory and set
> JAVA_HOME in tomcat to
> point to that dir.
> 
> Does it make any difference other than the fact that
> this way it is not
> rpm compliant?
> 
> > The main problem may be your AMD Duron processor
> which has a very
> small
> > cache size (I believe it's about 512K).  Once the
> processor cache
> > reaches its limit, increasing the JVM heap size
> won't be very
> effective.
> 
> This is even worse for my AMD Duron
> cache size      : 64 KB
> 
> but on the other machine I have :
> cache size      : 1024 KB
> and still get the same results.
> 
> Cristian
> 
> 
> 
>
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> 

I don't think your CPU L caches are going to influence
the issues you are seeing...not noticeably..not with
todays hardware.  Cristian is resizing the java heap
to allow it to get larger than it was able before
(default or the Tomcat default setting).  In this case
heap being the amount of memory the java process is
going to be able to use for the process aside from the
static space which can not be controlled by the heap
setting being used.  

How much memory does your computer physically have
Cristian?  Are you also using -Xms512m?  I'll wait to
here about your memory first before making other
comments on that.  

The next thing I'm going to ask deals with another
issue we had on the list.  Do you also write to the
HashMap from your application?  Are you reading and
writing to this cache or your application loads this
cached data at start up and it is merely read from
that point on?  If you are writing to this data you
need to be sure to synchronize the HashMap because if
you don't you can get into some serious problems with
concurrency issues and HashMap resizing (synchronize
reads and writes).  If you load all of the data up
then only read from the application then synchronizing
the reads to the hash map won't matter (unless you
allow reading while the loading is occuring).  

The next question would be: Even if only reading and
you are are currently synchronizing the calls to this
"cached" data are the operations long?  If the
operations to a synchronized resource are very long
then other threads (requests in this case) will be
delayed while trying to access the HashMap.

Wade

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