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Misconceptions regarding Java heap size arguments

05.06.2011
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There seem to be some misconceptions regarding the Java Heap size arguments.

For those who don't know, Java is peculiar in the sense that you have to specify the amount of memory you want your program to use, before you run your program. If you fail to do so then depending on the version and implementation of the JVM, your program will only be allowed to use a fraction of your computer's total RAM. This is the reason why you might get an OutOfMemoryError even if your machine has 24gb of ram and you know your Java program needs way below that amount of memory.

The way to mitigate this is to set the -Xms and -Xmx parameters when launching your Java program to explicitly set the initial and maximum amount of memory your Java program is allowed to use.

Note the use of the word 'allowed' in my previous statement. This is the misconception I am talking about.  Let's say you start Eclipse with a setting of -Xmx6g. This does not mean that magically eclipse will start allocating more memory. All this means is that you have allowed eclipse to use 6gb of memory and that if a situation does occur that eclipse maybe for a short time needs that extra amount of memory, it will be able to use it and wont crash by giving you an OutOfMemoryError.

The need to pre define the amount of memory usage allowed was a big problem with Chronon , since Chronon does make good use of the memory on your system and gets a huge performance boost the more memory you give it. However since eclipse by default only sets a -Xmx value of 384m, we would get a lot of complaints from people saying 'I have 4 gigs of ram on my machine, why is Chronon still running dog slow'. And it would always involve setting the -Xmx and -Xms to a higher value.

So we decided to use some of the functionalities of Eclipse p2 and now when you install the Chronon Time Travelling Debugger eclipse plugin, we automatically increase the -Xms and -Xmx to a much higher value in your eclipse.ini file. This has obviously put an end to all the performance complaints and I bet has solved a lot of non Chronon related performance problems for some eclipse users who are now actually able to use their machines to their full potential when running eclipse.

Of course, this has angered some people who when they see the heap size in their eclipse status bar set to a high value tend to freak out. "What, my eclipse is suddenly taking more memory!". Thus, the point of this post is to make it clear that Eclipse is not magically using more memory when you install Chronon, you are only allowing it to use all the memory of your system which you paid with your hard earned cash.

Published at DZone with permission of its author, Brian Noll.

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