Many Unity projects are found to operate with several tens or hundreds of kilobytes of temporary data being allocated to the managed heap each frame. If a program’s available memory address space is exhausted, the operating system will terminate the program.įor 64-bit programs, the address space is sufficiently large that this is extremely unlikely to occur for programs whose running time does not exceed the average human lifespan. The address space used by the managed heap is never returned to the operating system.įor 32-bit programs, this can lead to address space exhaustion if the managed heap expands and contracts many times. The interval at which this occurs is not guaranteed and should not be relied upon. On most platforms, Unity eventually releases the pages used by empty portions of the managed heap back to the operating system. This is to prevent the need to re-expand the heap should further large allocations occur. Unity does not often release the memory pages allocated to the managed heap when it expands it optimistically retains the expanded heap, even if a large portion of it is empty. The core issues with managed heap expansion are twofold: The specific amount that the heap expands is platform-dependent however, most Unity platforms double the size of the managed heap. If, after the GC runs, there is still not enough contiguous space to fit the requested amount of memory, the heap must expand.
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This attempts to free up enough space to fulfill the allocation request.
However, if a large object is allocated and there is insufficient contiguous free space to accommodate the object, as illustrated above, the Unity memory manager performs two operations.įirst, if it has not already done so, the garbage collector runs. In this case, even though there may be enough total space to accommodate a certain allocation, the managed heap cannot find a large enough block of contiguous memory in which to fit the allocation. This leads to the core problem of memory fragmentation: while the overall amount of space available in the heap may be substantial, it is possible that some or all of that space is in small “gaps” between allocated objects. When allocating an object, remember that the object must always occupy a contiguous block of space in memory. The newly-freed space can therefore only be used to store data of identical or lesser size than the freed object. Because of this, the freed space is a “gap” between other segments of memory (this gap is indicated by the red circle in the diagram). The objects on either side of the freed object may still be in use. However, the freed space does not become part of a single large pool of “free memory”. When an object is released, its memory is freed. The above diagram shows an example of memory fragmentation. “Non-compacting” means that objects in memory are not relocated in order to close gaps between objects. “Non-generational” means that the GC must sweep through the entire heap when performing a collection pass, and its performance therefore degrades as the heap expands. Unreferenced objects are then deleted, freeing up memory.Ĭrucially, Unity’s garbage collection – which uses the Boehm GC algorithm – is non-generational and non-compacting. This sweeps through all objects on the heap, marking for deletion any objects that are no longer referenced. The garbage collector runs periodically(3) ( Note: The exact timing is platform-dependent). When additional values are needed, more space is allocated from within the managed heap.
In the above diagram, the white box represents a quantity of memory apportioned to the managed heap, and the colored boxes within it represent data values stored within the managed heap’s memory space.
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All objects created in managed code must be allocated on the managed heap(2) ( Note: Strictly speaking, all non-null reference-typed objects and all boxed value-typed objects must be allocated on the managed heap). The “managed heap” is a section of memory that is automatically managed by the memory manager of a Project’s scripting runtime (Mono or IL2CPP).
How the managed heap operates and why it expands Furthermore, Unity’s garbage collection strategy tends to fragment memory, which can prevent a large heap from shrinking. In Unity, the managed heap expands much more readily than it shrinks. Another common problem faced by many Unity developers is the unexpected expansion of the managed heap.
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