When an Aggregator is used with ExternalSorter in a task, spark will create many small files and could cause too many files open error during merging.
Currently, ShuffleMemoryManager does not work well when there are 2 spillable objects in a thread, which are ExternalSorter and ExternalAppendOnlyMap(used by Aggregator) in this case. Here is an example: Due to the usage of mapside aggregation, ExternalAppendOnlyMap is created first to read the RDD. It may ask as much memory as it can, which is totalMem/numberOfThreads. Then later on when ExternalSorter is created in the same thread, the ShuffleMemoryManager could refuse to allocate more memory to it, since the memory is already given to the previous requested object(ExternalAppendOnlyMap). That causes the ExternalSorter keeps spilling small files(due to the lack of memory)
I'm currently working on a PR to address these two issues. It will include following changes:
1. The ShuffleMemoryManager should not only track the memory usage for each thread, but also the object who holds the memory
2. The ShuffleMemoryManager should be able to trigger the spilling of a spillable object. In this way, if a new object in a thread is requesting memory, the old occupant could be evicted/spilled. Previously the spillable objects trigger spilling by themselves. So one may not trigger spilling even if another object in the same thread needs more memory. After this change The ShuffleMemoryManager could trigger the spilling of an object if it needs to.
3. Make the iterator of ExternalAppendOnlyMap spillable. Previously ExternalAppendOnlyMap returns an destructive iterator and can not be spilled after the iterator is returned. This should be changed so that even after the iterator is returned, the ShuffleMemoryManager can still spill it.
Currently, I have a working branch in progress: https://github.com/tsdeng/spark/tree/enhance_memory_manager. Already made change 3 and have a prototype of change 1 and 2 to evict spillable from memory manager, still in progress. I will send a PR when it's done.
Any feedback or thoughts on this change is highly appreciated !