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Can I allocate objects contiguously in java?

Asked 2012-03-09T10:12:53.937
9

Assume I have a large array of relatively small objects, which I need to iterate frequently.
I would like to optimize my iteration by improving cache performance, so I would like to allocate the objects [and not the reference] contiguously on the memory, so I'll get fewer cache misses, and the overall performance could be segnificantly better.

In C++, I could just allocate an array of the objects, and it will allocate them as I wanted, but in java - when allocating an array, I only allocate the reference, and the allocation is being done one object at a time.

I am aware that if I allocate the objects "at once" [one after the other], the jvm is most likely to allocate the objects as contiguous as it can, but it might be not enough if the memory is fragmented.

My questions:

  1. Is there a way to tell the jvm to defrag the memory just before I start allocating my objects? Will it be enough to ensure [as much as possible] that the objects will be allocated continiously?
  2. Is there a different solution to this issue?
java performance caching memory garbage-collection
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-4

Doesn't work that way in Java. Iteration is not a matter of increasing a pointer. There is no performance impact based on where on the heap the objects are physically stored.

If you still want to approach this in a C/C++ way, think of a Java array as an array of pointers to structs. When you loop over the array, it doesn't matter where the actual structs are allocated, you are looping over an array of pointers.

I would abandon this line of reasoning. It's not how Java works and it's also sub-optimization.

answered 2012-03-09T10:21:42.243

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