JEP 141: Increase the Client VM's Default Heap Size

Peter B. Kessler Peter.B.Kessler at Oracle.COM
Wed Feb 22 13:10:19 PST 2012


Michael Bien wrote:
> 
> On 02/22/2012 04:38 PM, Jon Masamitsu wrote:
>>
>>
>> On 2/22/2012 4:28 AM, Michael Bien wrote:
>>>  On 02/22/2012 12:48 AM, mark.reinhold at oracle.com wrote:
>>>> Posted: http://openjdk.java.net/jeps/141
>>>>
>>>> - Mark
>>>>
>>> additional risks:
>>>  - JVM may not be able to start
>>
>> Specifically, the JVM tries to reserve enough memory at start up
>> for the heap and fails to initialize if it cannot?
>>
>> Will add it.
>>
>>>  - could contribute to the false impression that java requires a lot 
>>> of memory to run (JVMs usually don't like to give memory back to the 
>>> OS once its in use)
>>
>> Is this an impression the user would get because the JVM would not 
>> start up as you
>> note above?   The JVM does not necessarily use all the memory it 
>> reserves.
> yep. However most GCs are quite lazy (by design) to reach good 
> throughput. My experience so far showed that GCs have the tendency to 
> expand the heap at a full GC if there is still room left. Shrinking 
> heaps happened to me only in constructed tests - never in real 
> applications (esp. not with default config ;)).
> 
> i am somehow not very comfortable with the fact that hello world or an 
> applet would request 1g from the OS. Are there usecases for applications 
> out there where the developer or admin doesn't know how much memory is 
> needed? If there are, than a default value would not help those much 
> anyway (-> expanding heap needed until system limit is reached). All 
> others set -Xmx  :)

The standard use case is an editor, where the heap needed depends on what's being edited.  To use a text example: a short note versus a book chapter.  As people's expectations grow for what the editor can do, they use it for larger projects.  Sure, at some point they won't have enough heap for some document, and you have to tell them how to work around that limit, but it would be nice if you didn't have to tell everyone how to set -Xmx.  Or, as the implementor, I don't want to ship my code with a script that sets -Xmx1g, because then people complain about reserving 1GB for editing a short note.  (That said, more efficient memory structures in the application are usually a better solution. :-)

This is a specific case of not knowing what the application is going to be used for before it starts up.  Short-lived applet in a browser or the 24x7x365 server running on big iron?  That's why the JVM has default values, and why they aren't always appropriate, and why we discuss the implications when we think about changing them.

			... peter

> just wanted to bring this up, you guys are the experts.
> 
> regards,
> michael
> 
>>
>>
>> Thanks.
>>
>> Jon
>>>
>>> best regards,
>>> michael
>>>
>>
>>
> 


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