- Java Concurrency and Multithreading Tutorial
- Multithreading Benefits
- Multithreading Costs
- Concurrency Models
- Concurrency vs. Parallelism
- Creating and Starting Java Threads
- Race Conditions and Critical Sections
- Thread Safety and Shared Resources
- Thread Safety and Immutability
- Java Memory Model
- Java Happens Before Guarantee
- Java Synchronized Blocks
- Java Volatile Keyword
- Java ThreadLocal
- Thread Signaling
- Deadlock Prevention
- Starvation and Fairness
- Nested Monitor Lockout
- Slipped Conditions
- Locks in Java
- Read / Write Locks in Java
- Reentrance Lockout
- Blocking Queues
- Thread Pools
- Compare and Swap
- Anatomy of a Synchronizer
- Non-blocking Algorithms
- Amdahl's Law
- Java Concurrency References
Concurrency vs. Parallelism
The terms concurrency and parallelism are often used in relation to multithreaded programs. But what exactly does concurrency and parallelism mean, and how are these two terms different?
It took me some time to finally understand the difference between concurrency and parallelism. Therefore I decided to add a text about concurrency vs. parallelism to this Java concurrency tutorial.
Concurrency means that an application is making progress on more than one task at the same time (concurrently). Well, if the computer only has one CPU the application may not make progress on more than one task at exactly the same time, but more than one task is being processed at a time inside the application. It does not completely finish one task before it begins the next. Instead, the CPU switches between the different tasks until the tasks are complete.
It is possible to have a concurrent application even though it only has a single thread running inside it. This is one of the goals of our (Nanosai) Thread Ops toolkit, by the way.
Parallelism means that an application splits its tasks up into smaller subtasks which can be processed in parallel, for instance on multiple CPUs at the exact same time.
To achieve true parallelism your application must have more than one thread running, or at least be able to schedule tasks for execution in other threads, processes, CPUs, graphics cards etc.
Concurrency vs. Parallelism In Detail
As you can see, concurrency is related to how an application handles multiple tasks it works on. An application may process one task at at time (sequentially) or work on multiple tasks at the same time (concurrently).
Parallelism on the other hand, is related to how an application handles each individual task. An application may process the task serially from start to end, or split the task up into subtasks which can be completed in parallel.
As you can see, an application can be concurrent, but not parallel. This means that it processes more than one task at the same time, but the thread is only executing on one task at a time. There is no parallel execution of tasks going in parallel threads / CPUs.
An application can also be parallel but not concurrent. This means that the application only works on one task at a time, and this task is broken down into subtasks which can be processed in parallel. However, each task (+ subtask) is completed before the next task is split up and executed in parallel.
Additionally, an application can be neither concurrent nor parallel. This means that it works on only one task at a time, and the task is never broken down into subtasks for parallel execution.
Finally, an application can also be both concurrent and parallel, in that it both works on multiple tasks at the same time, and also breaks each task down into subtasks for parallel execution. However, some of the benefits of concurrency and parallelism may be lost in this scenario, as the CPUs in the computer are already kept reasonably busy with either concurrency or parallelism alone. Combining it may lead to only a small performance gain or even performance loss. Make sure you analyze and measure before you adopt a concurrent parallel model blindly.