src/java/org/apache/cassandra/concurrent/SEPExecutor.java - cassandra - Git at Google

 /*
  * 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.cassandra.concurrent;

 import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.ConcurrentLinkedQueue;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicLong;

 import org.apache.cassandra.metrics.SEPMetrics;
 import org.apache.cassandra.utils.concurrent.SimpleCondition;
 import org.apache.cassandra.utils.concurrent.WaitQueue;

 import static org.apache.cassandra.concurrent.SEPWorker.Work;

 public class SEPExecutor extends AbstractLocalAwareExecutorService
 {
     private final SharedExecutorPool pool;

     public final int maxWorkers;
     public final String name;
     private final int maxTasksQueued;
     private final SEPMetrics metrics;

     // stores both a set of work permits and task permits:
     //  bottom 32 bits are number of queued tasks, in the range [0..maxTasksQueued]   (initially 0)
     //  top 32 bits are number of work permits available in the range [0..maxWorkers]   (initially maxWorkers)
     private final AtomicLong permits = new AtomicLong();

     // producers wait on this when there is no room on the queue
     private final WaitQueue hasRoom = new WaitQueue();
     private final AtomicLong completedTasks = new AtomicLong();

     volatile boolean shuttingDown = false;
     final SimpleCondition shutdown = new SimpleCondition();

     // TODO: see if other queue implementations might improve throughput
     protected final ConcurrentLinkedQueue<FutureTask<?>> tasks = new ConcurrentLinkedQueue<>();

     SEPExecutor(SharedExecutorPool pool, int maxWorkers, int maxTasksQueued, String jmxPath, String name)
     {
         this.pool = pool;
         this.name = name;
         this.maxWorkers = maxWorkers;
         this.maxTasksQueued = maxTasksQueued;
         this.permits.set(combine(0, maxWorkers));
         this.metrics = new SEPMetrics(this, jmxPath, name);
     }

     protected void onCompletion()
     {
         completedTasks.incrementAndGet();
     }

     // schedules another worker for this pool if there is work outstanding and there are no spinning threads that
     // will self-assign to it in the immediate future
     boolean maybeSchedule()
     {
         if (pool.spinningCount.get() > 0 || !takeWorkPermit(true))
             return false;

         pool.schedule(new Work(this));
         return true;
     }

     protected void addTask(FutureTask<?> task)
     {
         // we add to the queue first, so that when a worker takes a task permit it can be certain there is a task available
         // this permits us to schedule threads non-spuriously; it also means work is serviced fairly
         tasks.add(task);
         int taskPermits;
         while (true)
         {
             long current = permits.get();
             taskPermits = taskPermits(current);
             // because there is no difference in practical terms between the work permit being added or not (the work is already in existence)
             // we always add our permit, but block after the fact if we breached the queue limit
             if (permits.compareAndSet(current, updateTaskPermits(current, taskPermits + 1)))
                 break;
         }

         if (taskPermits == 0)
         {
             // we only need to schedule a thread if there are no tasks already waiting to be processed, as
             // the original enqueue will have started a thread to service its work which will have itself
             // spawned helper workers that would have either exhausted the available tasks or are still being spawned.
             // to avoid incurring any unnecessary signalling penalties we also do not take any work to hand to the new
             // worker, we simply start a worker in a spinning state
             pool.maybeStartSpinningWorker();
         }
         else if (taskPermits >= maxTasksQueued)
         {
             // register to receive a signal once a task is processed bringing the queue below its threshold
             WaitQueue.Signal s = hasRoom.register();

             // we will only be signalled once the queue drops below full, so this creates equivalent external behaviour
             // however the advantage is that we never wake-up spuriously;
             // we choose to always sleep, even if in the intervening time the queue has dropped below limit,
             // so long as we _will_ eventually receive a signal
             if (taskPermits(permits.get()) > maxTasksQueued)
             {
                 // if we're blocking, we might as well directly schedule a worker if we aren't already at max
                 if (takeWorkPermit(true))
                     pool.schedule(new Work(this));

                 metrics.totalBlocked.inc();
                 metrics.currentBlocked.inc();
                 s.awaitUninterruptibly();
                 metrics.currentBlocked.dec();
             }
             else // don't propagate our signal when we cancel, just cancel
                 s.cancel();
         }
     }

     // takes permission to perform a task, if any are available; once taken it is guaranteed
     // that a proceeding call to tasks.poll() will return some work
     boolean takeTaskPermit()
     {
         while (true)
         {
             long current = permits.get();
             int taskPermits = taskPermits(current);
             if (taskPermits == 0)
                 return false;
             if (permits.compareAndSet(current, updateTaskPermits(current, taskPermits - 1)))
             {
                 if (taskPermits == maxTasksQueued && hasRoom.hasWaiters())
                     hasRoom.signalAll();
                 return true;
             }
         }
     }

     // takes a worker permit and (optionally) a task permit simultaneously; if one of the two is unavailable, returns false
     boolean takeWorkPermit(boolean takeTaskPermit)
     {
         int taskDelta = takeTaskPermit ? 1 : 0;
         while (true)
         {
             long current = permits.get();
             int workPermits = workPermits(current);
             int taskPermits = taskPermits(current);
             if (workPermits == 0 || taskPermits == 0)
                 return false;
             if (permits.compareAndSet(current, combine(taskPermits - taskDelta, workPermits - 1)))
             {
                 if (takeTaskPermit && taskPermits == maxTasksQueued && hasRoom.hasWaiters())
                     hasRoom.signalAll();
                 return true;
             }
         }
     }

     // gives up a work permit
     void returnWorkPermit()
     {
         while (true)
         {
             long current = permits.get();
             int workPermits = workPermits(current);
             if (permits.compareAndSet(current, updateWorkPermits(current, workPermits + 1)))
             {
                 if (shuttingDown && workPermits + 1 == maxWorkers)
                     shutdown.signalAll();
                 break;
             }
         }
     }

     public void maybeExecuteImmediately(Runnable command)
     {
         FutureTask<?> ft = newTaskFor(command, null);
         if (!takeWorkPermit(false))
         {
             addTask(ft);
         }
         else
         {
             try
             {
                 ft.run();
             }
             finally
             {
                 returnWorkPermit();
                 // we have to maintain our invariant of always scheduling after any work is performed
                 // in this case in particular we are not processing the rest of the queue anyway, and so
                 // the work permit may go wasted if we don't immediately attempt to spawn another worker
                 maybeSchedule();
             }
         }
     }

     public synchronized void shutdown()
     {
         shuttingDown = true;
         pool.executors.remove(this);
         if (getActiveCount() == 0 && getPendingTasks() == 0)
             shutdown.signalAll();

         // release metrics
         metrics.release();
     }

     public synchronized List<Runnable> shutdownNow()
     {
         shutdown();
         List<Runnable> aborted = new ArrayList<>();
         while (takeTaskPermit())
             aborted.add(tasks.poll());
         if (getActiveCount() == 0)
             shutdown.signalAll();
         return aborted;
     }

     public boolean isShutdown()
     {
         return shuttingDown;
     }

     public boolean isTerminated()
     {
         return shuttingDown && shutdown.isSignaled();
     }

     public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException
     {
         shutdown.await(timeout, unit);
         return isTerminated();
     }

     public long getPendingTasks()
     {
         return taskPermits(permits.get());
     }

     public long getCompletedTasks()
     {
         return completedTasks.get();
     }

     public int getActiveCount()
     {
         return maxWorkers - workPermits(permits.get());
     }

     private static int taskPermits(long both)
     {
         return (int) both;
     }

     private static int workPermits(long both)
     {
         return (int) (both >>> 32);
     }

     private static long updateTaskPermits(long prev, int taskPermits)
     {
         return (prev & (-1L << 32)) | taskPermits;
     }

     private static long updateWorkPermits(long prev, int workPermits)
     {
         return (((long) workPermits) << 32) | (prev & (-1L >>> 32));
     }

     private static long combine(int taskPermits, int workPermits)
     {
         return (((long) workPermits) << 32) | taskPermits;
     }
 }
	/*
	* 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.cassandra.concurrent;

	import java.util.ArrayList;
	import java.util.List;
	import java.util.concurrent.ConcurrentLinkedQueue;
	import java.util.concurrent.TimeUnit;
	import java.util.concurrent.atomic.AtomicLong;

	import org.apache.cassandra.metrics.SEPMetrics;
	import org.apache.cassandra.utils.concurrent.SimpleCondition;
	import org.apache.cassandra.utils.concurrent.WaitQueue;

	import static org.apache.cassandra.concurrent.SEPWorker.Work;

	public class SEPExecutor extends AbstractLocalAwareExecutorService
	{
	private final SharedExecutorPool pool;

	public final int maxWorkers;
	public final String name;
	private final int maxTasksQueued;
	private final SEPMetrics metrics;

	// stores both a set of work permits and task permits:
	// bottom 32 bits are number of queued tasks, in the range [0..maxTasksQueued] (initially 0)
	// top 32 bits are number of work permits available in the range [0..maxWorkers] (initially maxWorkers)
	private final AtomicLong permits = new AtomicLong();

	// producers wait on this when there is no room on the queue
	private final WaitQueue hasRoom = new WaitQueue();
	private final AtomicLong completedTasks = new AtomicLong();

	volatile boolean shuttingDown = false;
	final SimpleCondition shutdown = new SimpleCondition();

	// TODO: see if other queue implementations might improve throughput
	protected final ConcurrentLinkedQueue<FutureTask<?>> tasks = new ConcurrentLinkedQueue<>();

	SEPExecutor(SharedExecutorPool pool, int maxWorkers, int maxTasksQueued, String jmxPath, String name)
	{
	this.pool = pool;
	this.name = name;
	this.maxWorkers = maxWorkers;
	this.maxTasksQueued = maxTasksQueued;
	this.permits.set(combine(0, maxWorkers));
	this.metrics = new SEPMetrics(this, jmxPath, name);
	}

	protected void onCompletion()
	{
	completedTasks.incrementAndGet();
	}

	// schedules another worker for this pool if there is work outstanding and there are no spinning threads that
	// will self-assign to it in the immediate future
	boolean maybeSchedule()
	{
	if (pool.spinningCount.get() > 0 \|\| !takeWorkPermit(true))
	return false;

	pool.schedule(new Work(this));
	return true;
	}

	protected void addTask(FutureTask<?> task)
	{
	// we add to the queue first, so that when a worker takes a task permit it can be certain there is a task available
	// this permits us to schedule threads non-spuriously; it also means work is serviced fairly
	tasks.add(task);
	int taskPermits;
	while (true)
	{
	long current = permits.get();
	taskPermits = taskPermits(current);
	// because there is no difference in practical terms between the work permit being added or not (the work is already in existence)
	// we always add our permit, but block after the fact if we breached the queue limit
	if (permits.compareAndSet(current, updateTaskPermits(current, taskPermits + 1)))
	break;
	}

	if (taskPermits == 0)
	{
	// we only need to schedule a thread if there are no tasks already waiting to be processed, as
	// the original enqueue will have started a thread to service its work which will have itself
	// spawned helper workers that would have either exhausted the available tasks or are still being spawned.
	// to avoid incurring any unnecessary signalling penalties we also do not take any work to hand to the new
	// worker, we simply start a worker in a spinning state
	pool.maybeStartSpinningWorker();
	}
	else if (taskPermits >= maxTasksQueued)
	{
	// register to receive a signal once a task is processed bringing the queue below its threshold
	WaitQueue.Signal s = hasRoom.register();

	// we will only be signalled once the queue drops below full, so this creates equivalent external behaviour
	// however the advantage is that we never wake-up spuriously;
	// we choose to always sleep, even if in the intervening time the queue has dropped below limit,
	// so long as we _will_ eventually receive a signal
	if (taskPermits(permits.get()) > maxTasksQueued)
	{
	// if we're blocking, we might as well directly schedule a worker if we aren't already at max
	if (takeWorkPermit(true))
	pool.schedule(new Work(this));

	metrics.totalBlocked.inc();
	metrics.currentBlocked.inc();
	s.awaitUninterruptibly();
	metrics.currentBlocked.dec();
	}
	else // don't propagate our signal when we cancel, just cancel
	s.cancel();
	}
	}

	// takes permission to perform a task, if any are available; once taken it is guaranteed
	// that a proceeding call to tasks.poll() will return some work
	boolean takeTaskPermit()
	{
	while (true)
	{
	long current = permits.get();
	int taskPermits = taskPermits(current);
	if (taskPermits == 0)
	return false;
	if (permits.compareAndSet(current, updateTaskPermits(current, taskPermits - 1)))
	{
	if (taskPermits == maxTasksQueued && hasRoom.hasWaiters())
	hasRoom.signalAll();
	return true;
	}
	}
	}

	// takes a worker permit and (optionally) a task permit simultaneously; if one of the two is unavailable, returns false
	boolean takeWorkPermit(boolean takeTaskPermit)
	{
	int taskDelta = takeTaskPermit ? 1 : 0;
	while (true)
	{
	long current = permits.get();
	int workPermits = workPermits(current);
	int taskPermits = taskPermits(current);
	if (workPermits == 0 \|\| taskPermits == 0)
	return false;
	if (permits.compareAndSet(current, combine(taskPermits - taskDelta, workPermits - 1)))
	{
	if (takeTaskPermit && taskPermits == maxTasksQueued && hasRoom.hasWaiters())
	hasRoom.signalAll();
	return true;
	}
	}
	}

	// gives up a work permit
	void returnWorkPermit()
	{
	while (true)
	{
	long current = permits.get();
	int workPermits = workPermits(current);
	if (permits.compareAndSet(current, updateWorkPermits(current, workPermits + 1)))
	{
	if (shuttingDown && workPermits + 1 == maxWorkers)
	shutdown.signalAll();
	break;
	}
	}
	}

	public void maybeExecuteImmediately(Runnable command)
	{
	FutureTask<?> ft = newTaskFor(command, null);
	if (!takeWorkPermit(false))
	{
	addTask(ft);
	}
	else
	{
	try
	{
	ft.run();
	}
	finally
	{
	returnWorkPermit();
	// we have to maintain our invariant of always scheduling after any work is performed
	// in this case in particular we are not processing the rest of the queue anyway, and so
	// the work permit may go wasted if we don't immediately attempt to spawn another worker
	maybeSchedule();
	}
	}
	}

	public synchronized void shutdown()
	{
	shuttingDown = true;
	pool.executors.remove(this);
	if (getActiveCount() == 0 && getPendingTasks() == 0)
	shutdown.signalAll();

	// release metrics
	metrics.release();
	}

	public synchronized List<Runnable> shutdownNow()
	{
	shutdown();
	List<Runnable> aborted = new ArrayList<>();
	while (takeTaskPermit())
	aborted.add(tasks.poll());
	if (getActiveCount() == 0)
	shutdown.signalAll();
	return aborted;
	}

	public boolean isShutdown()
	{
	return shuttingDown;
	}

	public boolean isTerminated()
	{
	return shuttingDown && shutdown.isSignaled();
	}

	public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException
	{
	shutdown.await(timeout, unit);
	return isTerminated();
	}

	public long getPendingTasks()
	{
	return taskPermits(permits.get());
	}

	public long getCompletedTasks()
	{
	return completedTasks.get();
	}

	public int getActiveCount()
	{
	return maxWorkers - workPermits(permits.get());
	}

	private static int taskPermits(long both)
	{
	return (int) both;
	}

	private static int workPermits(long both)
	{
	return (int) (both >>> 32);
	}

	private static long updateTaskPermits(long prev, int taskPermits)
	{
	return (prev & (-1L << 32)) \| taskPermits;
	}

	private static long updateWorkPermits(long prev, int workPermits)
	{
	return (((long) workPermits) << 32) \| (prev & (-1L >>> 32));
	}

	private static long combine(int taskPermits, int workPermits)
	{
	return (((long) workPermits) << 32) \| taskPermits;
	}
	}