RFR: 8223837: Add -XX:MinHeapSize flag to set the minimum heap size

[Stefan Karlsson]

Hi all,

Please review this patch to add -XX:MinHeapSize flag to set the minimum 
heap size.

http://cr.openjdk.java.net/~stefank/8223837/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8223837

Currently the -Xms flag is documented to set the initial heap size, but 
it also sets the minimum heap size. The initial heap size can be 
overridden with the -XX:InitialHeapSize flag, but there is no such flag 
for the minimum heap size.

To set the minimum heap size to a value different than the initial heap 
size the user must specify -Xms before -XX:InitialHeapSize.

For example:
-Xms4g -XX:InitialHeapSize=8g sets the minimum size to commit for the 
heap to 4g and the initially committed heap size to 8g.

On the other hand, the following command line:
-XX:InitialHeapSize=8g -Xms4g sets both the minimum and initial heap 
size to 4g.

This RFE proposes that we introduce a -XX:MinHeapSize flag to make it 
easier to independently set the minimum heap size and initial heap size.

The user will get the expected behavior from either of the following 
command lines:
-XX:MinHeapSize=4g -XX:InitialHeapSize=8g
-XX:InitialHeapSize=8g -XX:MinHeapSize=4g

The -Xms flag will keep it's old behavior and set both minimum and 
initial heap size, but when setting the minimum heap size it will change 
the MinHeapSize flag and not a JVM internal global variable. -Xms will 
behave as if both -XX:MinHeapSize and -XX:InitialHeapSize were set on 
the command line.

The VM will enforce the relationship:
MinHeapSize <= InitialHeapSize <= MaxHeapSize

I've created a CSR for this (JDK-8223852), but have not moved it out of 
draft yet. I'm also thinking about making MinHeapSize manageable, to 
allow the users to change the value the JVM is running, using our 
management APIs. Either I fold that into the same CSR, or I simply 
create a new CSR to cover that change.

Tested with updated version of TestMaxHeapSizeTools.java, to only cover 
cases where MinHeapSize is set. That tool is used by the tests in 
Test<GC>HeapSizeFlags.java. Other suggestions of testing for this would 
be appreciated.

Thanks,
StefanK