7172708: 32/64 bit type issues on Windows

[Alan Bateman]

On 05/06/2012 15:29, Chris Dennis wrote:
> Alan,
>
> The 5g issue is kind of complicated.  The current behavior of 32bit JVMs as I understand it is that the 5g will get narrowed down to a 32bit value by the printf used to propagate the value across from the launcher and in to the system properties of the spawned JVM.  Consider the result of running the following test class on a 32bit and 64bit VM:
>
> public class TestMaxDirectMemory {
>
>      public static void main(String[] args) {
> 	System.out.println(sun.misc.VM.maxDirectMemory());
>      }
> }
>
> $ java -showversion -d32 -XX:MaxDirectMemorySize=5g TestMaxDirectMemory
> java version "1.6.0_31"
> Java(TM) SE Runtime Environment (build 1.6.0_31-b04-415-11M3635)
> Java HotSpot(TM) Client VM (build 20.6-b01-415, mixed mode)
>
> 1073741824
>
> $ java -showversion -d64 -XX:MaxDirectMemorySize=5g TestMaxDirectMemory
> java version "1.6.0_31"
> Java(TM) SE Runtime Environment (build 1.6.0_31-b04-415-11M3635)
> Java HotSpot(TM) 64-Bit Server VM (build 20.6-b01-415, mixed mode)
>
> 5368709120
>
> This is why in the LimitDirectMemory.java source I added some logic based on sun.arch.data.model to perform the equivalent narrowing conversion internally.
>
> It seems to me that we have a bunch of open and independent questions here:
>
> 1. What to do about the current MaxDirectMemorySize=-1 behavior.  Changing this might be a little tricky since -1 is currently used to propagate "unset" - causing the JDK code that consumes this value to use the default value.
>
> 2. What to do about the current error being thrown when trying to set MaxDirectMemorySize to an illegal value?
> 	$ ${JAVA_HOME_16}/bin/java -XX:MaxDirectMemorySize=foo
> 	Unrecognized VM option 'MaxDirectMemorySize=foo'
> 	Could not create the Java virtual machine.
> 	$ ${JAVA_HOME_17}/bin/java -XX:MaxDirectMemorySize=foo
> 	Error: Could not create the Java Virtual Machine.
> 	Error: A fatal exception has occurred. Program will exit.
>
> 3. Should we change the behavior for out of range values (e.g. 5G passed to a 32bit VM) passed to MaxDirectMemorySize?
>
> Once these are decided there is also the question of what bug-id each of these can or should be fixed under.
>
> Chris
>
I would suggest moving the thread to hotspot-dev or hotspot-runtime-dev.

On attempting to use -XX:MaxDirectMemorySize=5g on a 32-bit system then 
ideally this should be a fatal error, I don't think we would truncate 
the value to 32-bit. On specifying a value of -1 then I would suggest it 
should be consistent to other others, for example -XX:MaxHeapSize=-1 
will fail with an unrecognized VM option error.

-Alan.