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From GitBox <...@apache.org>
Subject [GitHub] [zookeeper] hanm commented on a change in pull request #986: ZOOKEEPER-3243: Add server-side request throttling
Date Sat, 22 Jun 2019 04:28:09 GMT
hanm commented on a change in pull request #986: ZOOKEEPER-3243: Add server-side request throttling
URL: https://github.com/apache/zookeeper/pull/986#discussion_r296433301
 
 

 ##########
 File path: zookeeper-server/src/main/java/org/apache/zookeeper/server/RequestThrottler.java
 ##########
 @@ -0,0 +1,273 @@
+/**
+ * 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.zookeeper.server;
+
+import java.util.concurrent.LinkedBlockingQueue;
+
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import org.apache.zookeeper.common.Time;
+
+/**
+ * When enabled, the RequestThrottler limits the number of outstanding requests
+ * currently submitted to the request processor pipeline. The throttler augments
+ * the limit imposed by the <code>globalOutstandingLimit</code> that is enforced
+ * by the connection layer ({@link NIOServerCnxn}, {@link NettyServerCnxn}).
+ *
+ * The connection layer limit applies backpressure against the TCP connection by
+ * disabling selection on connections once the request limit is reached. However,
+ * the connection layer always allows a connection to send at least one request
+ * before disabling selection on that connection. Thus, in a scenario with 40000
+ * client connections, the total number of requests inflight may be as high as
+ * 40000 even if the <code>globalOustandingLimit</code> was set lower.
+ *
+ * The RequestThrottler addresses this issue by adding additional queueing. When
+ * enabled, client connections no longer submit requests directly to the request
+ * processor pipeline but instead to the RequestThrottler. The RequestThrottler
+ * is then responsible for issuing requests to the request processors, and
+ * enforces a separate <code>maxRequests</code> limit. If the total number of
+ * outstanding requests is higher than <code>maxRequests</code>, the throttler
+ * will continually stall for <code>stallTime</code> milliseconds until
+ * underlimit.
+ *
+ * The RequestThrottler can also optionally drop stale requests rather than
+ * submit them to the processor pipeline. A stale request is a request sent
+ * by a connection that is already closed, and/or a request whose latency
+ * will end up being higher than its associated session timeout. The notion
+ * of staleness is configurable, @see Request for more details.
+ *
+ * To ensure ordering guarantees, if a request is ever dropped from a connection
+ * that connection is closed and flagged as invalid. All subsequent requests
+ * inflight from that connection are then dropped as well.
+ */
+public class RequestThrottler extends ZooKeeperCriticalThread {
+    private static final Logger LOG = LoggerFactory.getLogger(RequestThrottler.class);
+
+    private final LinkedBlockingQueue<Request> submittedRequests =
+        new LinkedBlockingQueue<Request>();
+
+    private final ZooKeeperServer zks;
+    private volatile boolean stopping;
+    private volatile boolean killed;
+
+    /**
+     * enabled can only be set via system properties and not at runtime to
+     * ensure requests are never sent sometimes to the RequestThrottler and
+     * other times directly to a request processor, thus potentially reodering
+     * requests.
+     *
+     * This setting is designed as a code kill switch. In normal operation,
+     * enabled should be set to true. When enabled, maxRequests can be adjusted
+     * to enable/disable throttling at runtime.
+     */
+    private static final boolean enabled =
+        ZooKeeperServer.getBooleanProp("zookeeper.request_throttle", true);
+
+    /**
+     * The total number of outstanding requests allowed before the throttler
+     * starts stalling.
+     *
+     * When maxRequests = 0, throttling is disabled.
+     */
+    private static volatile int maxRequests =
+        Integer.getInteger("zookeeper.request_throttle_max_requests", 0);
+
+    /**
+     * The time (in milliseconds) this is the maximum time for which throttler
+     * thread may wait to be notified that it may proceed processing a request.
+     */
+    private static volatile int stallTime =
+        Integer.getInteger("zookeeper.request_throttle_stall_time", 100);
+
+    /**
+     * When true, the throttler will drop stale requests rather than issue
+     * them to the request pipeline. A stale request is a request sent by
+     * a connection that is now closed, and/or a request that will have a
+     * request latency higher than the sessionTimeout. The staleness of
+     * a request is tunable property, @see Request for details.
+     */
+    private static volatile boolean dropStaleRequests =
+        ZooKeeperServer.getBooleanProp("zookeeper.request_throttle_drop_stale", true);
+
+    public RequestThrottler(ZooKeeperServer zks) {
+        super("RequestThrottler", zks.getZooKeeperServerListener());
+        this.zks = zks;
+        this.stopping = false;
+        this.killed = false;
+    }
+
+    public static int getMaxRequests() {
+        return maxRequests;
+    }
+
+    public static void setMaxRequests(int requests) {
+        maxRequests = requests;
+    }
+
+    public static int getStallTime() {
+        return stallTime;
+    }
+
+    public static void setStallTime(int time) {
+        stallTime = time;
+    }
+
+    public static boolean getDropStaleRequests() {
+        return dropStaleRequests;
+    }
+
+    public static void setDropStaleRequests(boolean drop) {
+        dropStaleRequests = drop;
+    }
+
+    public static boolean enabled() {
+        return enabled;
+    }
+
+    @Override
+    public void run() {
+        try {
+            while (true) {
+                Request request = submittedRequests.take();
+                if (Request.requestOfDeath == request) {
+                    break;
+                }
+
+                if (request.mustDrop()) {
+                    continue;
+                }
+
+                // Throttling is disabled when maxRequests = 0
+                if (maxRequests > 0) {
+                    while (!killed) {
+                        if (dropStaleRequests && request.isStale()) {
+                            // Note: this will close the connection
+                            dropRequest(request);
+                            ServerMetrics.getMetrics().STALE_REQUESTS_DROPPED.add(1);
+                            request = null;
+                            break;
+                        }
+                        if (zks.getInProcess() < maxRequests) {
+                            break;
+                        }
+                        throttleSleep(stallTime);
+                    }
+                }
+
+                if (killed) {
 
 Review comment:
   this could be elevated to the top of the first while block, so we can break early and start
draining the queue sooner. consider one case - we have a big queue of submitted requests and
most of them must be dropped. If we leave the check here, we'll not break the loop until processing
the last request (the request of death.).

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