CODEWITHSUNDEEP
javamultithreadingperformancetimetask
Continuity8
Continuity8

Reputation: 2591

Does multithreading cause each task to take longer?

I'm new to multithreading... multithreading is used to improve performance, however how can this be so if the processor is already working as fast as it can for a single thread?

To explain:

In a single threaded environment User A starts a task which takes 1 second - the task is completed in 1 second. User B starts the same task a fraction of a second later and has to wait for user A, therefore user Bs task completes in almost 2 seconds. Now, if this was a multithreaded environment would both tasks not be run similtaeously causing both to take 2 seconds? ...e.g. part of task A done, then part of task B, then part of task A again, then part of task B, ...till eventually both tasks complete in around two seconds?

Is it only faster if there is more than one processor? Or should multithreading be reserved for when there is a big task being dealt with and smaller tasks need to pop in and out of existence during this time?

Upvotes: 1

Views: 583

Answers (2)

Jim Garrison
Jim Garrison

Reputation: 86774

If the task is 100% CPU bound, and you have only one CPU core, then multithreading will make things slower. If you have more than one CPU core, clearly you can allocate as many threads as you have cores to improve performance. If the tasks interact with anything external (I/O) then multithreading can increase performance to a point. While one thread is waiting for I/O to complete, other threads can be performing CPU-based processing.

The classic example is a program that does computation and also displays a GUI. You update the GUI on one thread (the event thread) and do all processing on other "background" threads. The event thread does nothing but handle user interactions. If you don't do this then when a user-requested operation takes appreciable time the GUI stops responding. In this situation, you would run multithreaded even on a single-core system.

How to tune an application's use of threads depends on a lot of factors and could take up an entire textbook.

Upvotes: 4

iamyogish
iamyogish

Reputation: 2432

Ok now consider that Task A of your's needs a particular resource (ex a network file or a user input) to complete its work. Now say the resource needed by the Task A is not currently available so what happens next in a single threaded environment is that Task A has control of the CPU so it'll wait for the resource to be available and at this time the CPU will be idle which means we're wasting an important resource i.e, the CPU time while waiting for some other resource. But if the Task A has been implemented in multithreaded environment when Task A waits for the resource, it(Task A thread) gets suspended till the resource become available and the CPU time can be used efficiently to execute other tasks. Hope this helps :)

Upvotes: 0

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