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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.
/*
* 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.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import com.google.common.annotations.VisibleForTesting;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.apache.cassandra.metrics.ThreadPoolMetrics;
import org.apache.cassandra.utils.MBeanWrapper;
import org.apache.cassandra.utils.concurrent.SimpleCondition;
import static org.apache.cassandra.concurrent.SEPWorker.Work;
public class SEPExecutor extends AbstractLocalAwareExecutorService implements SEPExecutorMBean
{
private static final Logger logger = LoggerFactory.getLogger(SEPExecutor.class);
private final SharedExecutorPool pool;
private final AtomicInteger maximumPoolSize;
private final MaximumPoolSizeListener maximumPoolSizeListener;
public final String name;
private final String mbeanName;
@VisibleForTesting
public final ThreadPoolMetrics 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 [-resizeDelta..maximumPoolSize] (initially maximumPoolSize)
private final AtomicLong permits = new AtomicLong();
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> tasks = new ConcurrentLinkedQueue<>();
SEPExecutor(SharedExecutorPool pool, int maximumPoolSize, MaximumPoolSizeListener maximumPoolSizeListener, String jmxPath, String name)
{
this.pool = pool;
this.name = name;
this.mbeanName = "org.apache.cassandra." + jmxPath + ":type=" + name;
this.maximumPoolSize = new AtomicInteger(maximumPoolSize);
this.maximumPoolSizeListener = maximumPoolSizeListener;
this.permits.set(combine(0, maximumPoolSize));
this.metrics = new ThreadPoolMetrics(this, jmxPath, name).register();
MBeanWrapper.instance.registerMBean(this, mbeanName);
}
protected void onCompletion()
{
completedTasks.incrementAndGet();
}
@Override
public int getMaxTasksQueued()
{
return Integer.MAX_VALUE;
}
// 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();
}
}
public enum TakeTaskPermitResult
{
NONE_AVAILABLE, // No task permits available
TOOK_PERMIT, // Took a permit and reduced task permits
RETURNED_WORK_PERMIT // Detected pool shrinking and returned work permit ahead of SEPWorker exit.
};
// 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
TakeTaskPermitResult takeTaskPermit(boolean checkForWorkPermitOvercommit)
{
TakeTaskPermitResult result;
while (true)
{
long current = permits.get();
long updated;
int workPermits = workPermits(current);
int taskPermits = taskPermits(current);
if (workPermits < 0 && checkForWorkPermitOvercommit)
{
// Work permits are negative when the pool is reducing in size. Atomically
// adjust the number of work permits so there is no race of multiple SEPWorkers
// exiting. On conflicting update, recheck.
result = TakeTaskPermitResult.RETURNED_WORK_PERMIT;
updated = updateWorkPermits(current, workPermits + 1);
}
else
{
if (taskPermits == 0)
return TakeTaskPermitResult.NONE_AVAILABLE;
result = TakeTaskPermitResult.TOOK_PERMIT;
updated = updateTaskPermits(current, taskPermits - 1);
}
if (permits.compareAndSet(current, updated))
{
return result;
}
}
}
// 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)))
{
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)))
return;
}
}
@Override
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()
{
if (shuttingDown)
return;
shuttingDown = true;
pool.executors.remove(this);
if (getActiveTaskCount() == 0)
shutdown.signalAll();
// release metrics
metrics.release();
MBeanWrapper.instance.unregisterMBean(mbeanName);
}
public synchronized List shutdownNow()
{
shutdown();
List aborted = new ArrayList<>();
while (takeTaskPermit(false) == TakeTaskPermitResult.TOOK_PERMIT)
aborted.add(tasks.poll());
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();
}
@Override
public int getPendingTaskCount()
{
return taskPermits(permits.get());
}
@Override
public long getCompletedTaskCount()
{
return completedTasks.get();
}
public int getActiveTaskCount()
{
return maximumPoolSize.get() - workPermits(permits.get());
}
public int getCorePoolSize()
{
return 0;
}
public void setCorePoolSize(int newCorePoolSize)
{
throw new IllegalArgumentException("Cannot resize core pool size of SEPExecutor");
}
@Override
public int getMaximumPoolSize()
{
return maximumPoolSize.get();
}
@Override
public synchronized void setMaximumPoolSize(int newMaximumPoolSize)
{
final int oldMaximumPoolSize = maximumPoolSize.get();
if (newMaximumPoolSize < 0)
{
throw new IllegalArgumentException("Maximum number of workers must not be negative");
}
int deltaWorkPermits = newMaximumPoolSize - oldMaximumPoolSize;
if (!maximumPoolSize.compareAndSet(oldMaximumPoolSize, newMaximumPoolSize))
{
throw new IllegalStateException("Maximum pool size has been changed while resizing");
}
if (deltaWorkPermits == 0)
return;
permits.updateAndGet(cur -> updateWorkPermits(cur, workPermits(cur) + deltaWorkPermits));
logger.info("Resized {} maximum pool size from {} to {}", name, oldMaximumPoolSize, newMaximumPoolSize);
// If we we have more work permits than before we should spin up a worker now rather than waiting
// until either a new task is enqueued (if all workers are descheduled) or a spinning worker calls
// maybeSchedule().
pool.maybeStartSpinningWorker();
maximumPoolSizeListener.onUpdateMaximumPoolSize(newMaximumPoolSize);
}
private static int taskPermits(long both)
{
return (int) both;
}
private static int workPermits(long both) // may be negative if resizing
{
return (int) (both >> 32); // sign extending right shift
}
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;
}
}