2 * Copyright (c) 2016 Cisco Systems, Inc. and others. All rights reserved.
4 * This program and the accompanying materials are made available under the
5 * terms of the Eclipse Public License v1.0 which accompanies this distribution,
6 * and is available at http://www.eclipse.org/legal/epl-v10.html
9 package org.opendaylight.controller.cluster.access.client;
11 import com.google.common.base.Preconditions;
12 import javax.annotation.concurrent.NotThreadSafe;
13 import org.slf4j.Logger;
14 import org.slf4j.LoggerFactory;
17 * Base class for tracking throughput and computing delays when processing stream of tasks.
19 * <p>The idea is to improve throughput in a typical request-response scenario.
20 * A "frontend" is sending requests towards "backend", backend is sending responses back to fronted.
21 * Both frontend and backend may be realized by multiple Java threads,
22 * so there may be multiple requests not yet responded to.
23 * In terms of taks processing, frontend is "opening" tasks and backend is "closing" them.
24 * Latency of the backend may fluctuate wildly. To avoid backend running out of open tasks,
25 * there should be a queue of requests frontend can add to.
26 * In order to avoid excessive memore consumption, there should be a back-pressure mechanism
27 * which blocks the frontend threads for appropriate durations.
28 * Frontend can tolerate moderately delayed responses, but it only tolerates small block times.
30 * <p>An ideal back-pressure algorithm would keep the queue reasonably full,
31 * while fairly delaying frontend threads. In other words, backend idle time should be low,
32 * as well as frontend block time dispersion
33 * (as opposed to block time average, which is dictated by overall performance).
35 * <p>In order for an algorithm to compute reasonable wait times,
36 * various inputs can be useful, mostly related to timing of various stages of task processing.
37 * Methods of this class assume "enqueue and wait" usage.
38 * The delay computation is pessimistic, it expects each participating thread to enqueue another task
39 * as soon as its delay time allows.
41 * <p>This class is not thread safe, the callers are responsible for guarding against conflicting access.
42 * Time is measured in ticks (nanos), methods never look at current time, relying on {@code now} argument instead.
43 * Input data used for tracking is tightly coupled with TransitQueue#recordCompletion arguments.
45 * @author Vratko Polak
47 // TODO: Would bulk methods be less taxing than a loop of single task calls?
49 abstract class ProgressTracker {
50 private static final Logger LOG = LoggerFactory.getLogger(ProgressTracker.class);
53 * When no tasks has been closed yet, this will be used to estimate throughput.
55 private final long defaultTicksPerTask;
58 * Number of tasks closed so far.
60 private long tasksClosed = 0;
63 * Number of tasks so far, both open and closed.
65 private long tasksEncountered = 0;
68 * The most recent tick number when the number of open tasks has become non-positive.
70 private long lastIdle = Long.MIN_VALUE;
73 * The most recent tick number when a task has been closed.
75 private long lastClosed = Long.MIN_VALUE;
78 * Tick number when the farthest known wait time is over.
80 private long nearestAllowed = Long.MIN_VALUE;
83 * Number of ticks elapsed before lastIdle while there was at least one open task.
85 private long elapsedBeforeIdle = 0L;
90 * Construct an idle tracker with specified ticks per task value to use as default.
92 * @param ticksPerTask value to use as default
94 ProgressTracker(final long ticksPerTask) {
95 Preconditions.checkArgument(ticksPerTask >= 0);
96 defaultTicksPerTask = ticksPerTask;
100 * Construct a copy of an existing tracker, all future tracking is fully independent.
102 * @param tracker the instance to copy state from
104 ProgressTracker(final ProgressTracker tracker) {
105 this.defaultTicksPerTask = tracker.defaultTicksPerTask;
106 this.tasksClosed = tracker.tasksClosed;
107 this.tasksEncountered = tracker.tasksEncountered;
108 this.lastClosed = tracker.lastClosed;
109 this.lastIdle = tracker.lastIdle;
110 this.nearestAllowed = tracker.nearestAllowed;
111 this.elapsedBeforeIdle = tracker.elapsedBeforeIdle;
114 // Public shared access (read-only) accessor-like methods
117 * Get the value of default ticks per task this instance was created to use.
119 * @return default ticks per task value
121 public final long defaultTicksPerTask() {
122 return defaultTicksPerTask;
126 * Get number of tasks closed so far.
128 * @return number of tasks known to be finished already; the value never decreases
130 public final long tasksClosed() {
135 * Get umber of tasks so far, both open and closed.
137 * @return number of tasks encountered so far, open or finished; the value never decreases
139 public final long tasksEncountered() {
140 return tasksEncountered;
144 * Get number of tasks currently open.
146 * @return number of tasks started but not finished yet
148 public final long tasksOpen() {
149 // TODO: Should we check the return value is non-negative?
150 return tasksEncountered - tasksClosed;
154 * When idle, there are no open tasks so no progress is made.
156 * @return {@code true} if every encountered task is already closed, {@code false} otherwise
158 public boolean isIdle() {
159 return tasksClosed >= tasksEncountered;
163 * Number of ticks elapsed (before now) since the last closed task while there was at least one open task.
165 * @param now tick number corresponding to caller's present
166 * @return number of ticks backend is neither idle nor responding
168 public long ticksStalling(final long now) {
169 return isIdle() ? 0 : Math.max(now, lastClosed) - lastClosed;
173 * Number of ticks elapsed (before now) while there was at least one open task.
175 * @param now tick number corresponding to caller's present
176 * @return number of ticks there was at least one task open
178 public long ticksWorked(final long now) {
179 return isIdle() ? elapsedBeforeIdle : Math.max(now, lastIdle) - lastIdle + elapsedBeforeIdle;
183 * One task is roughly estimated to take this long to close.
185 * @param now tick number corresponding to caller's present
186 * @return total ticks worked divided by closed tasks, or the default value if no closed tasks
188 public double ticksWorkedPerClosedTask(final long now) {
189 if (tasksClosed < 1) {
190 return defaultTicksPerTask;
192 return (double) ticksWorked(now) / tasksClosed;
196 * Give an estimate of openTask() return value.
198 * <p>When the returned delay is positive, the caller thread should wait that time before opening additional task.
200 * <p>This method in general takes into account previously assigned delays to avoid overlaps.
202 * @param now tick number corresponding to caller's present
203 * @return delay (in ticks) after which another openTask() is fair to be called by the same thread again
205 public long estimateDelay(final long now) {
206 return estimateAllowed(now) - now;
210 * Give an estimate of a tick number when there will be no accumulated delays.
212 * <p>The delays accumulated include one more open task.
213 * Basically, the return value corresponds to openTask() return value,
214 * but this gives an absolute time, instead of delay relative to now.
216 * @param now tick number corresponding to caller's present
217 * @return estimated tick number when all threads with opened tasks are done waiting
219 public long estimateAllowed(final long now) {
220 return Math.max(now, nearestAllowed) + estimateIsolatedDelay(now);
223 // State-altering public methods.
226 * Track a task is being closed.
228 * @param now tick number corresponding to caller's present
229 * @param enqueuedTicks see TransitQueue#recordCompletion
230 * @param transmitTicks see TransitQueue#recordCompletion
231 * @param execNanos see TransitQueue#recordCompletion
233 public void closeTask(final long now, final long enqueuedTicks, final long transmitTicks, final long execNanos) {
235 LOG.info("Attempted to close a task while no tasks are open");
237 protectedCloseTask(now, enqueuedTicks, transmitTicks, execNanos);
242 * Track a task that is being opened.
244 * @param now tick number corresponding to caller's present
245 * @return number of ticks (nanos) the caller thread should wait before opening another task
247 public long openTask(final long now) {
248 protectedOpenTask(now);
249 return reserveDelay(now);
252 // Internal state-altering methods. Protected instead of private,
253 // allowing subclasses to weaken ad-hoc invariants of current implementation.
256 * Compute the next delay and update nearestAllowed value accordingly.
258 * @param now tick number corresponding to caller's present
259 * @return number of ticks (nanos) the caller thread should wait before opening another task
261 protected long reserveDelay(final long now) {
262 nearestAllowed = estimateAllowed(now);
263 return nearestAllowed - now;
267 * Track a task is being closed.
269 * <p>This method does not verify there was any task open.
270 * This call can empty the collection of open tasks, that special case should be handled.
272 * @param now tick number corresponding to caller's present
273 * @param enqueuedTicks see TransitQueue#recordCompletion
274 * @param transmitTicks see TransitQueue#recordCompletion
275 * @param execNanos see TransitQueue#recordCompletion
277 protected void protectedCloseTask(final long now, final long enqueuedTicks, final long transmitTicks,
278 final long execNanos) {
282 elapsedBeforeIdle += now - lastIdle;
287 * Track a task is being opened.
289 * <p>This method does not aggregate delays, allowing the caller to sidestep the throttling.
290 * This call can make the collection of open tasks non-empty, that special case should be handled.
292 * @param now tick number corresponding to caller's present
294 protected void protectedOpenTask(final long now) {
296 lastIdle = Math.max(now, lastIdle);
302 * Give an estimate of a fair delay, assuming delays caused by other opened tasks are ignored.
304 * @param now tick number corresponding to caller's present
305 * @return delay (in ticks) after which another openTask() would be fair to be called by the same thread again
307 abstract long estimateIsolatedDelay(final long now);