162 lines
6.6 KiB
Markdown
162 lines
6.6 KiB
Markdown
# Thread management
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This chapter describes how Apache ActiveMQ uses and pools threads and how you
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can manage them.
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First we'll discuss how threads are managed and used on the server side,
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then we'll look at the client side.
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## Server-Side Thread Management
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Each Apache ActiveMQ Server maintains a single thread pool for general use, and
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a scheduled thread pool for scheduled use. A Java scheduled thread pool
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cannot be configured to use a standard thread pool, otherwise we could
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use a single thread pool for both scheduled and non scheduled activity.
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A separate thread pool is also used to service connections. Apache ActiveMQ can
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use "old" (blocking) IO or "new" (non-blocking) IO also called NIO. Both
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of these options use a separate thread pool, but each of them behaves
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uniquely.
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Since old IO requires a thread per connection its thread pool is
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unbounded. The thread pool is created via `
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java.util.concurrent.Executors.newCachedThreadPool(ThreadFactory)`.
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As the JavaDoc for this method states: “Creates a thread pool that
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creates new threads as needed, but will reuse previously constructed
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threads when they are available, and uses the provided ThreadFactory to
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create new threads when needed.” Threads from this pool which are idle
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for more than 60 seconds will time out and be removed. If old IO
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connections were serviced from the standard pool the pool would easily
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get exhausted if too many connections were made, resulting in the server
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"hanging" since it has no remaining threads to do anything else.
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However, even an unbounded thread pool can run into trouble if it
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becomes too large. If you require the server to handle many concurrent
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connections you should use NIO, not old IO.
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When using new IO (NIO), Apache ActiveMQ will, by default, cap its thread pool
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at three times the number of cores (or hyper-threads) as reported by `
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Runtime.getRuntime().availableProcessors()` for processing
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incoming packets. To override this value, you can set the number of
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threads by specifying the parameter `nioRemotingThreads` in the
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transport configuration. See the [configuring transports](configuring-transports.md)
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for more information on this.
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There are also a small number of other places where threads are used
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directly, we'll discuss each in turn.
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### Server Scheduled Thread Pool
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The server scheduled thread pool is used for most activities on the
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server side that require running periodically or with delays. It maps
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internally to a `java.util.concurrent.ScheduledThreadPoolExecutor`
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instance.
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The maximum number of thread used by this pool is configure in
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`activemq-configuration.xml` with the `scheduled-thread-pool-max-size`
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parameter. The default value is `5` threads. A small number of threads
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is usually sufficient for this pool.
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### General Purpose Server Thread Pool
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This general purpose thread pool is used for most asynchronous actions
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on the server side. It maps internally to a
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`java.util.concurrent.ThreadPoolExecutor` instance.
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The maximum number of thread used by this pool is configure in
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`activemq-configuration.xml` with the `thread-pool-max-size` parameter.
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If a value of `-1` is used this signifies that the thread pool has no
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upper bound and new threads will be created on demand if there are not
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enough threads available to satisfy a request. If activity later
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subsides then threads are timed-out and closed.
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If a value of `n` where `n`is a positive integer greater than zero is
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used this signifies that the thread pool is bounded. If more requests
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come in and there are no free threads in the pool and the pool is full
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then requests will block until a thread becomes available. It is
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recommended that a bounded thread pool is used with caution since it can
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lead to dead-lock situations if the upper bound is chosen to be too low.
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The default value for `thread-pool-max-size` is `30`.
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See the [J2SE
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javadoc](http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/ThreadPoolExecutor.htm)
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for more information on unbounded (cached), and bounded (fixed) thread
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pools.
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### Expiry Reaper Thread
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A single thread is also used on the server side to scan for expired
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messages in queues. We cannot use either of the thread pools for this
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since this thread needs to run at its own configurable priority.
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For more information on configuring the reaper, please see ?.
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### Asynchronous IO
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Asynchronous IO has a thread pool for receiving and dispatching events
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out of the native layer. You will find it on a thread dump with the
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prefix ActiveMQ-AIO-poller-pool. Apache ActiveMQ uses one thread per opened
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file on the journal (there is usually one).
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There is also a single thread used to invoke writes on libaio. We do
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that to avoid context switching on libaio that would cause performance
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issues. You will find this thread on a thread dump with the prefix
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ActiveMQ-AIO-writer-pool.
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## Client-Side Thread Management
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On the client side, Apache ActiveMQ maintains a single static scheduled thread
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pool and a single static general thread pool for use by all clients
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using the same classloader in that JVM instance.
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The static scheduled thread pool has a maximum size of `5` threads, and
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the general purpose thread pool has an unbounded maximum size.
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If required Apache ActiveMQ can also be configured so that each
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`ClientSessionFactory` instance does not use these static pools but
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instead maintains its own scheduled and general purpose pool. Any
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sessions created from that `ClientSessionFactory` will use those pools
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instead.
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To configure a `ClientSessionFactory` instance to use its own pools,
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simply use the appropriate setter methods immediately after creation,
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for example:
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``` java
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ServerLocator locator = ActiveMQClient.createServerLocatorWithoutHA(...)
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ClientSessionFactory myFactory = locator.createClientSessionFactory();
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myFactory.setUseGlobalPools(false);
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myFactory.setScheduledThreadPoolMaxSize(10);
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myFactory.setThreadPoolMaxSize(-1);
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```
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If you're using the JMS API, you can set the same parameters on the
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ClientSessionFactory and use it to create the `ConnectionFactory`
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instance, for example:
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``` java
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ConnectionFactory myConnectionFactory = ActiveMQJMSClient.createConnectionFactory(myFactory);
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```
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If you're using JNDI to instantiate `ActiveMQConnectionFactory`
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instances, you can also set these parameters in the JNDI context
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environment, e.g. `jndi.properties`. Here's a simple example using the
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"ConnectionFactory" connection factory which is available in the context
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by default:
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java.naming.factory.initial=org.apache.activemq.jndi.ActiveMQInitialContextFactory
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java.naming.provider.url=tcp://localhost:61616
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connection.ConnectionFactory.useGlobalPools=false
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connection.ConnectionFactory.scheduledThreadPoolMaxSize=10
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connection.ConnectionFactory.threadPoolMaxSize=-1
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