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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-9913) Improve output serialization only once in RecordWriter
Date Tue, 28 Aug 2018 15:07:01 GMT

    [ https://issues.apache.org/jira/browse/FLINK-9913?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16595110#comment-16595110
] 

ASF GitHub Bot commented on FLINK-9913:
---------------------------------------

zhijiangW commented on a change in pull request #6417: [FLINK-9913][runtime] Improve output
serialization only once in RecordWriter
URL: https://github.com/apache/flink/pull/6417#discussion_r213343581
 
 

 ##########
 File path: flink-runtime/src/main/java/org/apache/flink/runtime/io/network/api/writer/RecordWriter.java
 ##########
 @@ -87,62 +86,71 @@ public RecordWriter(ResultPartitionWriter writer, ChannelSelector<T>
channelSele
 
 		this.numChannels = writer.getNumberOfSubpartitions();
 
-		/*
-		 * The runtime exposes a channel abstraction for the produced results
-		 * (see {@link ChannelSelector}). Every channel has an independent
-		 * serializer.
-		 */
-		this.serializers = new SpanningRecordSerializer[numChannels];
+		this.serializer = new SpanningRecordSerializer<T>();
 		this.bufferBuilders = new Optional[numChannels];
 		for (int i = 0; i < numChannels; i++) {
-			serializers[i] = new SpanningRecordSerializer<T>();
 			bufferBuilders[i] = Optional.empty();
 		}
 	}
 
 	public void emit(T record) throws IOException, InterruptedException {
+		serializer.serializeRecord(record);
+
 		for (int targetChannel : channelSelector.selectChannels(record, numChannels)) {
-			sendToTarget(record, targetChannel);
+			copyToTarget(targetChannel);
 		}
+
+		// Make sure we don't hold onto the large intermediate serialization buffer for too long
+		serializer.prune();
 	}
 
 	/**
 	 * This is used to broadcast Streaming Watermarks in-band with records. This ignores
 	 * the {@link ChannelSelector}.
 	 */
 	public void broadcastEmit(T record) throws IOException, InterruptedException {
+		serializer.serializeRecord(record);
+
 		for (int targetChannel = 0; targetChannel < numChannels; targetChannel++) {
-			sendToTarget(record, targetChannel);
+			copyToTarget(targetChannel);
 		}
+
+		serializer.prune();
 	}
 
 	/**
 	 * This is used to send LatencyMarks to a random target channel.
 	 */
 	public void randomEmit(T record) throws IOException, InterruptedException {
-		sendToTarget(record, rng.nextInt(numChannels));
-	}
+		serializer.serializeRecord(record);
 
-	private void sendToTarget(T record, int targetChannel) throws IOException, InterruptedException
{
-		RecordSerializer<T> serializer = serializers[targetChannel];
+		copyToTarget(rng.nextInt(numChannels));
 
-		SerializationResult result = serializer.addRecord(record);
+		serializer.prune();
+	}
 
+	private void copyToTarget(int targetChannel) throws IOException, InterruptedException {
+		// We should reset the initial position of the intermediate serialization buffer before
+		// copying, so the serialization results can be copied to multiple target buffers.
+		serializer.reset();
+
+		BufferBuilder bufferBuilder = getBufferBuilder(targetChannel);
+		SerializationResult result = serializer.copyToBufferBuilder(bufferBuilder);
 		while (result.isFullBuffer()) {
-			if (tryFinishCurrentBufferBuilder(targetChannel, serializer)) {
-				// If this was a full record, we are done. Not breaking
-				// out of the loop at this point will lead to another
-				// buffer request before breaking out (that would not be
-				// a problem per se, but it can lead to stalls in the
-				// pipeline).
-				if (result.isFullRecord()) {
-					break;
-				}
+			tryFinishCurrentBufferBuilder(targetChannel);
 
 Review comment:
   You pointed out a good question!
   1. Considering `tryFinishCurrentBufferBuilder()`, the logic is somewhat different from
before. In the past, the buffer builder may be empty when calling `tryFinishCurrentBufferBuilder()`,
then it returns a boolean value to indicate the result. But now, we know the buffer builder
is always present when calling `tryFinishCurrentBufferBuilder`, so we may change it to `finishCurrentBufferBuilder()`
seems more appropriate. And adds the check code instead as following:
     ```
   private void finishCurrentBufferBuilder(int targetChannel) {
   		checkState(bufferBuilders[targetChannel].isPresent());
   
   		BufferBuilder bufferBuilder = bufferBuilders[targetChannel].get();
   		bufferBuilders[targetChannel] = Optional.empty();
   		numBytesOut.inc(bufferBuilder.finish());
   	}
   ```
   2. But I think we still need call `finishCurrentBufferBuilder()`  after checking every
full buffer. Because in the while loop the serialized record may be copied into several different
buffer builders, and each buffer build referenced by the `bufferBuilders ` needs to be finished
when full.
   
   3. It actually seems better than before, as the loop exit not rely on the return value
of `tryFinishCurrentBufferBuilder()`.

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> Improve output serialization only once in RecordWriter
> ------------------------------------------------------
>
>                 Key: FLINK-9913
>                 URL: https://issues.apache.org/jira/browse/FLINK-9913
>             Project: Flink
>          Issue Type: Improvement
>          Components: Network
>    Affects Versions: 1.6.0
>            Reporter: zhijiang
>            Assignee: zhijiang
>            Priority: Major
>              Labels: pull-request-available
>             Fix For: 1.7.0
>
>
> Currently the {{RecordWriter}} emits output into multi channels via {{ChannelSelector}} 
or broadcasts output to all channels directly. Each channel has a separate {{RecordSerializer}}
for serializing outputs, that means the output will be serialized as many times as the number
of selected channels.
> As we know, data serialization is a high cost operation, so we can get good benefits
by improving the serialization only once.
> I would suggest the following changes for realizing it.
>  # Only one {{RecordSerializer}} is created in {{RecordWriter}} for all the channels.
>  # The output is serialized into the intermediate data buffer only once for different
channels.
>  # The intermediate serialization results are copied into different {{BufferBuilder}}s
for different channels.
> An additional benefit by using a single serializer for all channels is that we get a
potentially significant reduction on heap space overhead from fewer intermediate serialization
buffers (only once we got over 5MiB, these buffers were pruned back to 128B!).



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