timarmstrong commented on a change in pull request #34186:
URL: https://github.com/apache/spark/pull/34186#discussion_r728282858
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File path: core/src/main/java/org/apache/spark/memory/TaskMemoryManager.java
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@@ -149,95 +149,100 @@ public long acquireExecutionMemory(long required,
MemoryConsumer consumer) {
// Try to release memory from other consumers first, then we can reduce
the frequency of
// 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.
+ logger.debug("Task {} need to spill {} for {}", taskAttemptId,
+ Utils.bytesToString(required - got), requestingConsumer);
+ // We need to call spill() on consumers to free up more memory. We
want to optimize for two
+ // things:
+ // * Minimize the number of spill calls, to reduce the number of spill
files and avoid small
+ // spill files.
+ // * Avoid spilling more data than necessary - if we only need a
little more memory, we may
+ // not want to spill as much data as possible. Many consumers spill
more than the
+ // requested amount, so we can take that into account in our
decisions.
+ // We use a heuristic that selects the smallest memory consumer with
at least `required`
+ // bytes of memory in an attempt to balance these factors. It may work
well if there are
+ // fewer larger requests, but can result in many small spills if there
are many smaller
+ // requests.
+
+ // Build a map of consumer in order of memory usage to prioritize
spilling. Assign current
+ // consumer a nominal memory usage of 0 so that it is always last in
priority order.
TreeMap<Long, List<MemoryConsumer>> sortedConsumers = new TreeMap<>();
for (MemoryConsumer c: consumers) {
Review comment:
You are right - it will not be in there if this is the first time it has
called `acquireExecutionMemory()`, but in that case we don't need to consider
spilling it. I'll extend the comment to clarify.
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