[GitHub] [beam] reuvenlax commented on a change in pull request #14852: [BEAM-12378] GroupIntoBatches improvements

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reuvenlax commented on a change in pull request #14852:
URL: https://github.com/apache/beam/pull/14852#discussion_r682116285



##########
File path: runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/GroupIntoBatchesOverride.java
##########
@@ -87,14 +94,25 @@ private BatchGroupIntoBatches(long batchSize) {
                   new DoFn<KV<K, Iterable<V>>, KV<K, Iterable<V>>>() {
                     @ProcessElement
                     public void process(ProcessContext c) {
-                      // Iterators.partition lazily creates the partitions as they are accessed
-                      // allowing it to partition very large iterators.
-                      Iterator<List<V>> iterator =
-                          Iterators.partition(c.element().getValue().iterator(), (int) batchSize);
-
-                      // Note that GroupIntoBatches only outputs when the batch is non-empty.
-                      while (iterator.hasNext()) {
-                        c.output(KV.of(c.element().getKey(), iterator.next()));
+                      List<V> currentBatch = Lists.newArrayList();
+                      long batchSizeBytes = 0;
+                      for (V element : c.element().getValue()) {
+                        currentBatch.add(element);
+                        if (weigher != null) {
+                          batchSizeBytes += weigher.apply(element);
+                        }
+                        if (currentBatch.size() == maxBatchSizeElements
+                            || (maxBatchSizeBytes != Long.MAX_VALUE
+                                && batchSizeBytes >= maxBatchSizeBytes)) {
+                          c.output(KV.of(c.element().getKey(), currentBatch));
+                          // Call clear() since that allows us to reuse the array memory for
+                          // subsequent batches.
+                          currentBatch.clear();

Review comment:
       oof, this is a bug.
   
   It's probably ok in many cases, since the fused fn will be executed immediately when output is called. However the fused fn is allowed to store the element in memory and process it in finishBundle, and if that happens the data will be stomped. We need to remove this buggy optimization :P




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