RFR: 8223837: Add -XX:MinHeapSize flag to set the minimum heap size
per.liden at oracle.com
Tue Jun 4 07:48:42 UTC 2019
On 5/27/19 9:53 AM, Stefan Karlsson wrote:
> Hi all,
> Please review this patch to add -XX:MinHeapSize flag to set the minimum
> heap size.
One minor nit, "MinimumHeapSizeConstraintFunc" should probably be
"MinHeapSizeConstraintFunc", to match names of the related functions.
> 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.
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