Github user tgravescs commented on a diff in the pull request:
https://github.com/apache/spark/pull/22173#discussion_r216063759
--- Diff:
common/network-common/src/main/java/org/apache/spark/network/server/ChunkFetchRequestHandler.java
---
@@ -0,0 +1,136 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.network.server;
+
+import java.net.SocketAddress;
+
+import com.google.common.base.Throwables;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelFuture;
+import io.netty.channel.ChannelFutureListener;
+import io.netty.channel.ChannelHandlerContext;
+import io.netty.channel.SimpleChannelInboundHandler;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import org.apache.spark.network.buffer.ManagedBuffer;
+import org.apache.spark.network.client.TransportClient;
+import org.apache.spark.network.protocol.ChunkFetchFailure;
+import org.apache.spark.network.protocol.ChunkFetchRequest;
+import org.apache.spark.network.protocol.ChunkFetchSuccess;
+import org.apache.spark.network.protocol.Encodable;
+
+import static org.apache.spark.network.util.NettyUtils.*;
+
+
+/**
+ * A dedicated ChannelHandler for processing ChunkFetchRequest messages.
When sending response
+ * of ChunkFetchRequest messages to the clients, the thread performing the
I/O on the underlying
+ * channel could potentially be blocked due to disk contentions. If
several hundreds of clients
+ * send ChunkFetchRequest to the server at the same time, it could
potentially occupying all
+ * threads from TransportServer's default EventLoopGroup for waiting for
disk reads before it
+ * can send the block data back to the client as part of the
ChunkFetchSuccess messages. As a
+ * result, it would leave no threads left to process other RPC messages,
which takes much less
+ * time to process, and could lead to client timing out on either
performing SASL authentication,
+ * registering executors, or waiting for response for an OpenBlocks
messages.
+ */
+public class ChunkFetchRequestHandler extends
SimpleChannelInboundHandler<ChunkFetchRequest> {
+ private static final Logger logger =
LoggerFactory.getLogger(ChunkFetchRequestHandler.class);
+
+ private final TransportClient client;
+ private final StreamManager streamManager;
+ /** The max number of chunks being transferred and not finished yet. */
+ private final long maxChunksBeingTransferred;
+
+ public ChunkFetchRequestHandler(
+ TransportClient client,
+ StreamManager streamManager,
+ Long maxChunksBeingTransferred) {
+ this.client = client;
+ this.streamManager = streamManager;
+ this.maxChunksBeingTransferred = maxChunksBeingTransferred;
+ }
+
+ @Override
+ public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause)
throws Exception {
+ logger.warn("Exception in connection from " +
getRemoteAddress(ctx.channel()),
+ cause);
+ ctx.close();
+ }
+
+ @Override
+ protected void channelRead0(ChannelHandlerContext ctx,
+ final ChunkFetchRequest msg) throws
Exception {
+ Channel channel = ctx.channel();
+ if (logger.isTraceEnabled()) {
+ logger.trace("Received req from {} to fetch block {}",
getRemoteAddress(channel),
+ msg.streamChunkId);
+ }
+ long chunksBeingTransferred = streamManager.chunksBeingTransferred();
+ if (chunksBeingTransferred >= maxChunksBeingTransferred) {
+ logger.warn("The number of chunks being transferred {} is above {},
close the connection.",
+ chunksBeingTransferred, maxChunksBeingTransferred);
+ channel.close();
+ return;
+ }
+ ManagedBuffer buf;
+ try {
+ streamManager.checkAuthorization(client, msg.streamChunkId.streamId);
+ streamManager.registerChannel(channel, msg.streamChunkId.streamId);
+ buf = streamManager.getChunk(msg.streamChunkId.streamId,
msg.streamChunkId.chunkIndex);
+ } catch (Exception e) {
+ logger.error(String.format("Error opening block %s for request from
%s",
+ msg.streamChunkId, getRemoteAddress(channel)), e);
+ respond(channel,
+ new ChunkFetchFailure(msg.streamChunkId,
+ Throwables.getStackTraceAsString(e)));
+ return;
+ }
+
+ streamManager.chunkBeingSent(msg.streamChunkId.streamId);
+ respond(channel, new ChunkFetchSuccess(msg.streamChunkId,
buf)).addListener(
+ (ChannelFutureListener) future ->
streamManager.chunkSent(msg.streamChunkId.streamId));
+ }
+
+ /**
+ * The invocation to channel.writeAndFlush is async, and the actual I/O
on the
+ * channel will be handled by the EventLoop the channel is registered
to. So even
+ * though we are processing the ChunkFetchRequest in a separate thread
pool, the actual I/O,
+ * which is the potentially blocking call that could deplete server
handler threads, is still
+ * being processed by TransportServer's default EventLoopGroup. In order
to throttle the max
+ * number of threads that channel I/O for sending response to
ChunkFetchRequest, the thread
+ * calling channel.writeAndFlush will wait for the completion of sending
response back to
+ * client by invoking sync(). This will throttle the rate at which
threads from
+ * ChunkFetchRequest dedicated EventLoopGroup submit channel I/O
requests to TransportServer's
+ * default EventLoopGroup, thus making sure that we can reserve some
threads in
+ * TransportServer's default EventLoopGroup for handling other RPC
messages.
+ */
+ private ChannelFuture respond(final Channel channel,
+ final Encodable result) throws
InterruptedException {
+ final SocketAddress remoteAddress = channel.remoteAddress();
+ return
channel.writeAndFlush(result).sync().addListener((ChannelFutureListener) future
-> {
--- End diff --
do we actually want to use await() here instead of sync(). Sync says
"Waits for this future until it is done, and rethrows the cause of the failure
if this future failed."
I'm not sure we want a rethrow here since we have the addListener to handle
the future failure.
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