Github user mridulm commented on a diff in the pull request:
https://github.com/apache/spark/pull/16603#discussion_r96581689
--- Diff: core/src/main/java/org/apache/spark/memory/TaskMemoryManager.java
---
@@ -144,8 +164,24 @@ public long acquireExecutionMemory(long required,
MemoryConsumer consumer) {
// spilling, avoid to have too many spilled files.
if (got < required) {
// Call spill() on other consumers to release memory
+ // Sort the consumers according their memory usage. So we avoid
spilling the same consumer
+ // which is just spilled in last few times and re-spilling on it
will produce many small
+ // spill files.
+ List<MemoryConsumer> sortedList = new
ArrayList<>(consumers.size());
for (MemoryConsumer c: consumers) {
if (c != consumer && c.getUsed() > 0 && c.getMode() == mode) {
+ sortedList.add(c);
+ }
+ }
+ Collections.sort(sortedList, new ConsumerComparator());
+ for (int listIndex = 0; listIndex < sortedList.size();
listIndex++) {
+ MemoryConsumer c = sortedList.get(listIndex);
+ // Try to only spill on the consumer which has the required size
of memory.
+ // As the consumers are sorted in descending order, if the next
consumer doesn't have
+ // the required memory, then we need to spill the current
consumer at least.
+ boolean doSpill = (listIndex + 1) == sortedList.size() ||
+ sortedList.get(listIndex + 1).getUsed() < (required - got);
+ if (doSpill) {
--- End diff --
I like the fact that this implementation does not need to incur the cost of
remove in a TreeMap.
Unfortunately, I dont think it is sufficient : the impl assumes that
spill() will actually always give you back getUsed - from the rest of the code
in the method, this does not look like a valid assumption to make.
This can resulting in spilling a large number of smaller blocks, and
potentially itself.
For example: required = 500MB, consumers = 1.5GB 1GB 500MB 2MB 1MB ..
If spilling 500MB resulted in (say) releasing 490MB, we might end up
spilling a large number of blocks and also (potentially) end up spilling itself
- also can end up returning less than requested while enough memory does exist
to satisfy the request.
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