What is a Fixed ThreadPool Executor?

It is a thread pool with

• fixed number of threads
• and unbounded queue for the tasks

Basically a thread pool with following attributes

• corePoolSize = n threads
• maximumPoolSize = n threads
• keepAliveTime = 0 milliseconds.

Behaviour of a Thread Pool with fixed number of threads

• At any point, at most n Threads threads will be active processing tasks.
• If additional tasks are submitted when all threads are active, they will wait in the queue until a thread is available.
• If any thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks.

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When to use a FixedThreadPool?

• When you don’t want your application to consume all the available threads in your system.
  
• Or when you want to run your application within limited concurrency (controlled by number of threads in fixed thread pool)

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What is the right number of threads for the thread pool?

• Technically if you have a n core CPU, then it means you can create n threads in the system.
  E.g if you have a 16 core CPU, then you can easily run 16 threads on it.
• In reality you can run many more threads
  • This is because in most scenarios the threads DON’T always do just computations.
  • They may be doing I/O (network, disk, etc), and hence the CPU may be free to process more threads.
  • E.g you if your application is I/O heavy, then you may even run 100s of threads, because most threads will be busy doing I/O and CPU will concurrently execute them
• So,
  • If each task does not perform any I/O operation, probably the correct number of parallel threads is the number of cores of your hardware.
  • If your tasks involve network or disk I/O, it is more difficult to determine the correct level of parallelism to maximize performance.
• Deciding on the number of threads
  • First find number of cores available = Runtime.getRuntime().availableProcessors()
  • A good rule of thumb is to start around 10 threads per core for a workload that involves disk and network I/O, and see if it’s sufficient to keep all CPUs busy.
  • Increase or decrease the number of maximum threads based on results obtained from load testing.
  • Multiply this factor by number of cores, to obtain the total size of the thread pool.

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When can a thread pool cause a problem

• Generally creating and destroying a thread and its associated resources can be expensive process in terms of time.
• So thread pool generally helps, like a fixed thread pool that has n number of threads always available.
• However, if we end up spawning too many threads in the Java application, then we can very quickly
  • run out of resources – as each thread needs memory for its thread stack (2MB per thread)
  • and the performance of your application will decrease rapidly – as among others, the CPU needs to do a lot of context switching.

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What happens if you have a small thread pool, but huge amounts of tasks to process

• Suppose you have a fixed thread Pool of size 50
• But you have a million of tasks to process
• If you submit these tasks to the thread pool executor – it will not block
  • The reason is that the FixedThreadPool Executor, by default uses a LinkedBlockingQueue with capacity as Integer.MAX_VALUE
  • This means for all practical purposes it behaves like an “unbounded” queue

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Code Examples

Lets look at some of the code examples to understand how to use a fixedThreadPool.

Consider the code example below where we are creating a Fixed Thread Pool with 5 threads and handle 20 tasks concurrently.

As you can see from the output, at max only 5 threads are processed at any given time.

A newFixedThreadPool(1) , i.e., a fixed pool with size 1, is equivalent to a SingleThreadExecutor

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Code located at – https://github.com/chatterjeesunit/java-playground/blob/master/src/main/java/com/corejava/multithreading/executors/nonscheduled/FixedThreadPoolExecutorDemo.java

In our future blogs, we will see other different types of ThreadPoolExecutors like cached, scheduled, etc.

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