org.jctools.queues.IndexedQueueSizeUtil Maven / Gradle / Ivy
/*
* Licensed 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.jctools.queues;
import org.jctools.util.InternalAPI;
/**
* A note to maintainers on index assumptions: in a single threaded world it would seem intuitive to assume:
*
* producerIndex >= consumerIndex
*
* As an invariant, but in a concurrent, long running settings all of the following need to be considered:
*
* -
consumerIndex > producerIndex : due to counter overflow (unlikey with longs, but easy to reason)
* -
consumerIndex > producerIndex : due to consumer FastFlow like implementation discovering the
* element before the counter is updated.
* -
producerIndex - consumerIndex < 0 : due to above.
* -
producerIndex - consumerIndex > Integer.MAX_VALUE : as linked buffers allow constructing queues
* with more than Integer.MAX_VALUE elements.
*
*
*/
@InternalAPI
public final class IndexedQueueSizeUtil
{
public static final int PLAIN_DIVISOR = 1;
public static final int IGNORE_PARITY_DIVISOR = 2;
public static int size(IndexedQueue iq, int divisor) {
/*
* It is possible for a thread to be interrupted or reschedule between the reads of the producer and
* consumer indices. It is also for the indices to be updated in a `weakly` visible way. It follows that
* the size value needs to be sanitized to match a valid range.
*/
long after = iq.lvConsumerIndex();
long size;
while (true)
{
final long before = after;
// pIndex read is "sandwiched" between 2 cIndex reads
final long currentProducerIndex = iq.lvProducerIndex();
after = iq.lvConsumerIndex();
if (before == after)
{
size = (currentProducerIndex - after) / divisor;
break;
}
}
return sanitizedSize(iq.capacity(), size);
}
public static int sanitizedSize(int capacity, long size) {
// Concurrent updates to cIndex and pIndex may lag behind other progress enablers (e.g. FastFlow), so we need
// to check bounds [0,capacity]
if (size < 0)
{
return 0;
}
if (capacity != MessagePassingQueue.UNBOUNDED_CAPACITY && size > capacity)
{
return capacity;
}
// Integer overflow is possible for the unbounded indexed queues.
if (size > Integer.MAX_VALUE)
{
return Integer.MAX_VALUE;
}
return (int) size;
}
public static boolean isEmpty(IndexedQueue iq)
{
// Order matters!
// Loading consumer before producer allows for producer increments after consumer index is read.
// This ensures this method is conservative in it's estimate. Note that as this is an MPMC there is
// nothing we can do to make this an exact method.
return (iq.lvConsumerIndex() >= iq.lvProducerIndex());
}
@InternalAPI
public interface IndexedQueue
{
long lvConsumerIndex();
long lvProducerIndex();
int capacity();
}
}