How precisely do Java arrays use memory in HotSpot (i.e. how much slop)?

Question

C malloc implementations typically don't allocate the precise amount of memory requested but instead consume fixed-size runs of memory, eg with power-of-two sizes, so that an allocation of 1025 bytes actually takes a 2048-byte segment with 1023 bytes lost as slop.

Does HotSpot use a similar allocation mechanism for Java arrays? If so, what's the right way to allocate a Java array such that there's no slop? (Eg should the array length be a power of two or maybe power of two minus some fixed amount of overhead?)

Answer 1

If you're asking about the language, the answer is: Its not specified (same as for C)

If you're asking about a specific implementation, check out that implementation. I believe for Hotspot its 8 bytes granularity; that is object sizes are rounded up to the next granularity boundary. If the question is about the heap size increase when there isn't enough free heap, then it depends on implementation, GC settings, heap size params and so on; making it impractical to answer precisely.

EDIT: Using a small reflection hack, accessing the sun.misc.Unsafe class (Oracle JRE only), object references can be converted to memory addresses; output the addresses of two consecutively allocated arrays to check for yourself.

And I basically asked the same question here: Determine the optimal size for array with respect to the JVM's memory granularity (Answers include an example of using the Unsafe class to check for object size)