Metal: implement more accurate buffer tracking #7839
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pixelflinger
requested changes
May 10, 2024
| NSHashTable* hashTable = aliveBuffers[toIndex(type)]; | ||
| // Caution! We can't simply use hashTable.count here, which is inaccurate. | ||
| // See http://cocoamine.net/blog/2013/12/13/nsmaptable-and-zeroing-weak-references/ | ||
| return hashTable.objectEnumerator.allObjects.count; |
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everything hinges on this working, which looks like magic to me. do you have high confidence this is indeed working?
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I do. Objective-C guarantees that weak pointers are set to nil automatically when whatever they're referencing is deallocated. So essentially the hashTable simply needs to tally which pointers aren't nil.
pixelflinger
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May 10, 2024
bejado
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This PR updates our Metal buffer tracking logic to use a
NSHashTableto track alive buffers.NSHashTableis configured to hold weak references to the buffers we add to it. When a buffer is deallocated, it is automatically removed from the hash table. By counting the number of buffers in the hash table, we get an accurate count of the number of alive buffers.The previous implementation had the limitation that once we pass a buffer off to Metal (when a command buffer becomes the sole owner of it, for example), we assume Metal will eventually release it and and decrement our count. In reality, that buffer could stay alive for a frame or two while the command buffer executes. Using the hash table should give us a more accurate tally of the alive buffer count.