Memory leak in 1.5.0 JVM

[Mika Nystrom]

Hi Nick,

Thanks for the explanation!  I guess I can just chalk up another
reason on my list of reasons "why I would never use Java for anything
I care about [given the choice]".  Better garbage collectors were
described by Dijkstra et al... now 30 years ago.

This is what my NOP-program does after allocating a gigabyte (only
takes a couple of minutes on a Q6600):

Exception in thread "main" java.lang.OutOfMemoryError: unable to create new native thread
        at java.lang.Thread.start0(Native Method)
        at java.lang.Thread.start(Thread.java:574)
        at Leakq.main(Leakq.java:14)

    Mika

Nick Johnson writes:
>On Thu, 21 Feb 2008, Mika Nystrom wrote:
>
>> Out-running the garbage collector?  I'm not used to Java, but in
>> Modula-3, the garbage collector gets called when you allocate memory,
>> is that not how it works in Java?
>
>In a word, no.  Garbage collection in Java is asynchronous.  In the Sun 
>JVM there are several different garbage collection strategies.  The 
>default strategy uses multiple "generations"-- a "new" generation for 
>short-lived objects; those objects that live through a new generation 
>collection get promoted to one of many "survivor" spaces, and those that 
survive collection there (either because of time or exhausting the size of 
>the survivor space) get promoted to the "tenured" or "old" generation.  
>Then there's a "permanent" generation for classes and other stuff.
>
>If you just sit and watch the memory size of a JVM process, it might 
>appear to be "leaking" memory, but what's happening under the hood is far 
>more complicated than that.  The JVM will also grow to accomodate the 
>amount of memory it has needed to allocate, but won't necessarily -shrink- 
>again once those objects are garbage collected.  In a long-running 
>process, chances are that if a program needed 256M once, it's going to 
>need it again.
>
>To get a real good idea of what's going on under the hood, look at tools 
>like jvmstat and visualgc.
>
>This document will give you a much more detailed view than my rough 
>outline here:
>
>	http://java.sun.com/docs/hotspot/gc5.0/gc_tuning_5.html
>
>   Nick
>
>
>-- 
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> afraid and doing what you have to do anyway."
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