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From Stuart Still <stust...@gmail.com>
Subject Re: Introduction, and a question
Date Tue, 17 May 2005 13:27:26 GMT
This sounds like how java works under OS X.

Newbie question: What is to stop us from caching JITed code?  .NET/ 
mono does this as far as I know?

On 17 May 2005, at 06:05, Nick Lothian wrote:

>> El lun, 16-05-2005 a las 16:08 +0530, Subramanian, Sundar escribió:
>> (...)
>>> I guess what Brad is asking is for a snapshot of the state of JVM.
>>> This
>>> would be really useful to migrate stuff from one environment to
>>> another preserving the underlying state.
>> I have mixed feelings about having a "save image" __a la__
>> Smalltalk, if this is what you are meaning. While delivering
>> an image looks tempting, state gets corrupt in unpredictable
>> ways, and having ways to track changes becomes a nightmare.
>> The Smalltalk community has worked hard in this (hard)
>> problem, but I'm still not sure if it would make sense in the
>> java world. Java is a system-oriented language, and the
>> ability to save the whole VM state and recover from this
>> saved image would bring additional constraints to the Virtual
>> Machine implementation. For instance, machine specific JIT
>> code should be invalidated upon save, negating a substantial
>> part of the advantages of a saved image (faster startup).
>> This said, if VM implementors out there find easy ways to
>> meet these constraints w/o burdening runtime or memory
>> requirements, it could be an area for experimenting.
> This looks like it would be related to the stuff Sun has done with  
> class data sharing in the 1.5 JVMs:
> "When the JRE is installed on 32-bit platforms using the Sun  
> provided installer, the installer loads a set of classes from the  
> system jar file into a private internal representation, and dumps  
> that representation to a file, called a "shared archive". Class  
> data sharing is not supported in Microsoft Windows 95/98/ME. If the  
> Sun JRE installer is not being used, this can be done manually, as  
> explained below. During subsequent JVM invocations, the shared  
> archive is memory-mapped in, saving the cost of loading those  
> classes and allowing much of the JVM's metadata for these classes  
> to be shared among multiple JVM processes." [1]
> This isn't quite the same as saving JIT'ed code, but it sounds like  
> it is saving the pre-parsed and verified class files.
> Nick
> [1] http://java.sun.com/j2se/1.5.0/docs/guide/vm/class-data- 
> sharing.html
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