HowToDoInJava

  • Java 8
  • Regex
  • Concurrency
  • Best Practices
  • Spring Boot
  • JUnit5
  • Interview Questions

Concurrency vs. Parallelism

By Lokesh Gupta | Filed Under: Multi Threading

Concurrency means multiple tasks which start, run, and complete in overlapping time periods, in no specific order. Parallelism is when multiple tasks OR several part of a unique task literally run at the same time, e.g. on a multi-core processor. Remember that Concurrency and parallelism are NOT the same thing.

Let’s understand more in detail that what I mean when I say Concurrency vs. Parallelism.

Concurrency

Concurrency is essentially applicable when we talk about minimum two tasks or more. When an application is capable of executing two tasks virtually at same time, we call it concurrent application. Though here tasks run looks like simultaneously, but essentially they MAY not. They take advantage of CPU time-slicing feature of operating system where each task run part of its task and then go to waiting state. When first task is in waiting state, CPU is assigned to second task to complete it’s part of task.

Operating system based on priority of tasks, thus, assigns CPU and other computing resources e.g. memory; turn by turn to all tasks and give them chance to complete. To end user, it seems that all tasks are running in parallel. This is called concurrency.

Parallelism

Parallelism does not require two tasks to exist. It literally physically run parts of tasks OR multiple tasks, at the same time using multi-core infrastructure of CPU, by assigning one core to each task or sub-task.

Parallelism requires hardware with multiple processing units, essentially. In single core CPU, you may get concurrency but NOT parallelism.

Differences between concurrency vs. parallelism

Now let’s list down remarkable differences between concurrency and parallelism.

Concurrency is when two tasks can start, run, and complete in overlapping time periods. Parallelism is when tasks literally run at the same time, eg. on a multi-core processor.

Concurrency is the composition of independently executing processes, while parallelism is the simultaneous execution of (possibly related) computations.

Concurrency is about dealing with lots of things at once. Parallelism is about doing lots of things at once.

An application can be concurrent – but not parallel, which means that it processes more than one task at the same time, but no two tasks are executing at same time instant.

An application can be parallel – but not concurrent, which means that it processes multiple sub-tasks of a task in multi-core CPU at same time.

An application can be neither parallel – nor concurrent, which means that it processes all tasks one at a time, sequentially.

An application can be both parallel – and concurrent, which means that it processes multiple tasks concurrently in multi-core CPU at same time .

That’s all about Concurrency vs. Parallelism, a very important concept in java multi-threading concepts.

Happy Learning !!

About Lokesh Gupta

Founded HowToDoInJava.com in late 2012. I love computers, programming and solving problems everyday. A family guy with fun loving nature. You can find me on Facebook, Twitter and Google Plus.

Feedback, Discussion and Comments

  1. saurabh

    January 22, 2018

    Hi Lokesh,
    Thanks for bringing this point to notice. Since Java5 if we are using CachedThreadPool and program is running on multi-core CPU then can we say we are running program parallel.

    Reply

Ask Questions & Share Feedback Cancel reply

Your email address will not be published. Required fields are marked *

*Want to Post Code Snippets or XML content? Please use [java] ... [/java] tags otherwise code may not appear partially or even fully. e.g.
[java] 
public static void main (String[] args) {
...
}
[/java]

Search Tutorials

  • Email
  • Facebook
  • RSS
  • Twitter

Java Concurrency Tutorial

  • Java Concurrency – Introduction
  • Concurrency Evolution
  • Thread Safety
  • Concurrency vs. Parallelism
  • Compare and Swap [CAS]
  • synchronized keyword
  • Object vs. Class Level Locking
  • Runnable vs. Thread
  • wait(), notify() and notifyAll()
  • Yield() vs. Join()
  • Sleep() vs. Wait()
  • Lock vs. Monitor
  • Callable + Future
  • UncaughtExceptionHandler
  • Throttling Task Submission
  • Executor Best Practices
  • Inter-thread Communication
  • Write and Resolve Deadlock

Java Concurrency Utilities

  • ThreadFactory
  • Lock
  • ThreadLocal
  • ThreadPoolExecutor
  • FixedSizeThreadPoolExecutor
  • ScheduledThreadPoolExecutor
  • Semaphore
  • Binary Semaphore
  • BlockingQueue
  • ConcurrentLinkedDeque
  • CountDownLatch
  • ForkJoinPool

Popular Tutorials

  • Java 8 Tutorial
  • Core Java Tutorial
  • Java Collections
  • Java Concurrency
  • Spring Boot Tutorial
  • Spring AOP Tutorial
  • Spring MVC Tutorial
  • Spring Security Tutorial
  • Hibernate Tutorial
  • Jersey Tutorial
  • Maven Tutorial
  • Log4j Tutorial
  • Regex Tutorial

Meta Links

  • Advertise
  • Contact Us
  • Privacy policy
  • About Me

Copyright © 2016 · HowToDoInjava.com · All Rights Reserved. | Sitemap