Thread pools and context switching (tasks)?

Question

This is quite a general computer science question and not specific to any OS or framework.

So I am a little confused by the overhead associated with switching tasks on a thread pool. In many cases it doesn't make sense to give every job its own specific thread (we don't want to create too many hardware threads), so instead we put these jobs into tasks which can be scheduled to run on a thread. We setup up a pool of threads and then dynamically allocate the tasks to run on a thread taken from the thread pool.

I am just a little confused (can't find a in depth answer) on the overhead associated with switching tasks on a specific thread (in the thread pool). A DrDobbs article (sourced below) states it does but I need a more in depth answer to what is actually happening (a cite-able source would be fantastic :)).

By definition, SomeWork must be queued up in the pool and then run on a different thread than the original thread. This means we necessarily incur queuing overhead plus a context switch just to move the work to the pool. If we need to communicate an answer back to the original thread, such as through a message or Future or similar, we will incur another context switch for that.

Source: http://www.drdobbs.com/parallel/use-thread-pools-correctly-keep-tasks-sh/216500409?pgno=1

What components of the thread are actually switching? The thread itself isn't actually switching, just the data that is specific to the thread. What is the overhead associated with this (more, less or the same)?

Answer 1

let´s clarify first 5 key concepts here and then discuss how they correlates in a thread pool context:

• thread: In a brief resume it can be described as a program execution context, given by the code that is being run, the data in cpu registries and the stack. when a thread is created it is assigned the code that should be executed in that thread context. In each cpu cycle the thread has an instruction to execute and the data in cpu registries and stack in a given state.

• task: Represents a unit of work. It's the code that is assigned to a thread to be executed.

• context switch (from wikipedia): Is the process of storing and restoring the state (context) of a thread so that execution can be resumed from the same point at a later time. This enables multiple processes to share a single CPU and is an essential feature of a multitasking operating system. What constitutes the context is as explained above is the code that is being executed, the cpu registries and the stack.

What is context switched is the thread. A task represents only a peace of work that can be assigned to a thread to be executed. At given moment a thread can be executing a task.

• Thread Pool (from wikipedia): In computer programming, the thread pool is where a number of threads are created to perform a number of tasks, which are usually organized in a queue.

• Thread Pool Queue: Where tasks are placed to be executed by threads in the pool. This data structure is a shared peace of memory where threads may compete to queue/dequeue, may lead to contention in high load scenarios.

Illustrating a thread pool usage scenario:

• In your program (eventually running in the main thread), you create a task and schedules it to be executed in thread pool.
• The task is queued in the thread pool queue.
• When a thread from the pool executes it dequeues a task from the pool and starts to executed it.
• If there is no free cpus to execute the thread from the pool, the operating system at some point (depending on thread scheduler policy and thread priorities) will stop a thread from executing, context switching to other thread.

the operating system can stop the execution of a thread at any time, context switching to another thread, returning latter to continue where it stopped.

The overhead of the context switching is augmented when the number of active threads that competes for cpus grows. Thus, ideally, a thread pool tries to use the minimum necessary threads to occupy all available cpus in a machine.

If your tasks haven't code that blocks somewhere, context switching is minimized because it is used no more threads than the available cpus on machine.

Of course if you have only one core, your main thread and the thread pool will compete for the same cpu.

Answer 2

The article probably talks about the case in which work is posted to the pool and the result of it is being waited for. Running a task on the thread-pool in general does not incur any context switching overhead.

Imagine queueing 1000 work items. A thread-pool thread will executed them one after the other. All of that without a single context switch in between.

Switching happens doe to waiting/blocking.