GEOS SDK TechDocs
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4 VM Chains
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5.1 Structure of a Huge Array
Sometimes a geode needs to work with an array that can get very large. Chunk arrays are very convenient, but they are limited to the size of an LMem heap, which is slightly less than 64K; furthermore, their performance begins to degrade when they get larger than 6K. Similarly, if a geode uses raw memory for an array, it is limited to the maximum size of a global memory block, again 64K.
For this reason, GEOS provides the Huge Array library. A Huge Array is stored in a VM file. All the elements are stored in VM blocks, as is the directory information. The application can specify an array element by its index, and the Huge Array routine will lock the block containing that element and return its address. Array indices are dword-sized, meaning a Huge Array can have up to 232 elements. Since elements are stored in VM blocks, each element has a maximum size of 64K; however, the size of the entire array is limited only by the amount of disk space available. The blocks in a Huge Array are linked together in a VM chain, so the VM chain utilities can be used to duplicate, free, and compare Huge Arrays.
There are a couple of disadvantages to using Huge Arrays. The main disadvantage is that it takes longer to access an element: the routine has to lock the directory block, look up the index to find out which block contains the element, lock that block, calculate the offset into that block for the element, and return its address. (However, elements are consecutive within blocks; thus, you can often make many array accesses with a single Huge Array lookup command.) There is also a certain amount of memory overhead for Huge Arrays. Thus, if you are confident that the array size will be small enough for a single block, you are generally better off with a Chunk Array (see the Local Memory chapter).
Huge arrays may have fixed-size or variable-sized elements. If elements are variable-sized, there is a slight increase in memory overhead, but no significant slowdown in data-access time.
GEOS SDK TechDocs
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| 4 VM Chains
| 5.1 Structure of a Huge Array