Analysis of Xms and InitialHeapSize parameters in the VM

-Xms and -XX:InitialHeapSize

This page tries to summarize the changes of the relation between -Xms and -XX:InitialHeapSize in the changeset for 8006088.

Generally -Xms sets both minimum and initial heap size, while -XX:InitialHeapSize only sets initial heap size. The table below shows for all input combinations of -Xms and -XX:InitialHeapSize in the first two columns the resulting minimum and initial heap size in a current patch compared to the current (old) behavior.

The input values have the following meaning:

Input value meaning
not set	the value has not been set on the command line
0	value=0 set on the command line
S	value < NewSize + OldSize
X	value >= NewSize + OldSize

The output values have the following meaning:

Output value meaning
Ergo	value has been chosen ergonomically. The ergonomics for minimum and initial heap size are different. For minimum heap size, the VM typically chooses NewSize + OldSize, for initial heap size a fraction of physical memory or maximum heap size. The string may contain additional constraints for the value. The ergonomics algorithm is the same in both the new and old behavior shown below.
S, X	the value S or X
Error	the VM prints some error message

Here is the table.

Bold cells indicate that the behavior seems unexpected.

Input values		New behavior		Old behavior
-Xms	-XX:InitialHeapSize	Minimum Heap Size	Initial Heap Size	Minimum Heap Size	Initial Heap Size
not set	not set	Ergo	Ergo	Ergo	Ergo
not set	S	S	S	Error	Error
not set	X	Ergo <= X	X	Ergo <= X	X
not set	0	Ergo	Ergo	Ergo	NewSize + OldSize
0	not set	Ergo	Ergo	Error	Error
0	S	S	S	Error	Error
0	X	Ergo <= X	X	Error	Error
0	0	Ergo	Ergo	Error	Error
S	not set	S	S	S	S
S_1	S_2	S_1	S_2	S_1	S_2
S	X	S	X	S	X
S	0	S	Ergo	S	Ergo
X	not set	X	X	X	X
X	S	Error	Error	Error	Error
X_1	X_2	X_1	X_2	X_1	X_2
X	0	X	Ergo >= X	Error	Error

Additional notes:

the S_1/S_2 and X_1/X_2 rows only give that result if S_1 <= S_2 or X_1 <= X_2 respectively. Otherwise they give a VM error.
the relative positions of -Xms and -XX:InitialHeapSize matters, as -Xms sets both minimum and initial heap size. This apparently cannot be changed as -Xms is a cross-VM switch, and there does not seem to be a common definition for that switch.
you can set only minimum heap size by providing -Xms<minimum> and -XX:InitialHeapSize=0 afterwards on the command line