The Java Buildpack Memory Calculator calculates a holistic JVM memory configuration with the goal of ensuring that applications perform well while not exceeding a container's memory limit and being recycled.

In order to perform this calculation, the Memory Calculator requires the following input:

• --total-memory: total memory available to the application, typically expressed with size classification (B, K, M, G, T)
• --loaded-class-count: the number of classes that will be loaded when the application is running
• --thread-count: the number of user threads
• --jvm-options: JVM Options, typically JAVA_OPTS
• --head-room: percentage of total memory available which will be left unallocated to cover JVM overhead

The Memory Calculator prints the calculated JVM configuration flags (excluding any that the user has specified in --jvm-options). If a valid configuration cannot be calculated (e.g. more memory must be allocated than is available), an error is printed and a non-zero exit code is returned. In order to override a calculated value, users should pass any of the standard JVM configuration flags into --jvm-options. The calculation will take these as fixed values and adjust the non-fixed values accordingly.

go get -v github.com/cloudfoundry/java-buildpack-memory-calculator

The following algorithm is used to generate the holistic JVM memory configuration:

1. Headroom is calculated as total memory * (head room / 100)
2. If -XX:MaxDirectMemorySize is configured it is used for the amount of direct memory. If not configured 10M (in the absence of any reasonable heuristic) is used.
3. If -XX:MaxMetaspaceSize is configured it is used for the amount of metaspace. If not configured then the value is calculated as (5800B * loaded class count) + 14000000b.
4. If -XX:ReservedCodeCacheSize is configured it is used for the amount of reserved code cache. If not configured 240M (the JVM default) is used.
5. If -Xss is configured it is used for the size of each thread stack. If not configured 1M (the JVM default) is used.
6. If -Xmx is configured it is used for the size of the heap. If not configured then the value is calculated as total memory - (headroom + direct memory + metaspace + reserved code cache + (thread stack * thread count)).

Broadly, this means that for a constant application (same number of classes), the non-heap overhead is a fixed value. Any changes to the total memory will be directly reflected in the size of the heap. Adjustments to the non-heap memory configuration (e.g. stack size, reserved code cache) can result in larger heap sizes but can also have negative runtime side-effects that must be taken into account.

The document Java Buildpack Memory Calculator v3 provides some rationale for the memory calculator externals.

According to the HotSpot GC Tuning Guide:

The MaxMetaspaceSize applies to the sum of the committed compressed class space and the space for the other class metadata.

Therefore the memory calculator does not set the compressed class space size (-XX:CompressedClassSpaceSize) since the memory for the compressed class space is bounded by the maximum metaspace size (-XX:MaxMetaspaceSize).

The Java Buildpack Memory Calculator is Open Source software released under the Apache 2.0 license.