What Use are Threads Outside of Parallel Problems on MultiCore Systems?_问答_开发者

Threads make the design, implementation and debugging of a program significantly more difficult.

Yet man开发者_JAVA百科y people seem to think that every task in a program that can be threaded should be threaded, even on a single core system.

I can understand threading something like an MPEG2 decoder that's going to run on a multicore cpu ( which I've done ), but what can justify the significant development costs threading entails when you're talking about a single core system or even a multicore system if your task doesn't gain significant performance from a parallel implementation?

Or more succinctly, what kinds of non-performance related problems justify threading?

Edit

Well I just ran across one instance that's not CPU limited but threads make a big difference:

TCP, HTTP and the Multi-Threading Sweet Spot

Multiple threads are pretty useful when trying to max out your bandwidth to another peer over a high latency network connection. Non-blocking I/O would use significantly less local CPU resources, but would be much more difficult to design and implement.

Performing a CPU intensive task without blocking the user interface, for example.

Any application in which you may be waiting around for a resource (for example, blocking I/O from network sockets or disk devices) can benefit from threading.

In that case the thread blocking on the slow operation can be put to sleep while other threads continue to run (including, under some operating systems, the GUI thread which, if the OS cannot contact it for a while, will offer the use the chance to destroy it, thinking it's deadlocked somehow).

So it's not just for multi-core machines at all.

An interesting example is a webserver - you need to be able to handle multiple incoming connections that have nothing to do with each other.

what kinds of non-performance related problems justify threading?

Web applications are the classic example. Each user request is conceptually a new thread. Nothing to do with performance, it's just a natural fit for the design.

Blocking code is usually much simpler to write and easier to read (and therefore maintain) than non-blocking code. Yet, using blocking code limits you to a single execution path and also locks out things like user interface (mentioned) and other IO ports. Threading is an elegant solution in these cases.

Another case when multithreading is to be considered is when you have several near-synchronous IO channels that should be managed: using multiple threads (and usually a local message queue) allows for much clearer code.

Here are a couple of specific and simple scenarios where I have launched threads...

A long running report request by the user. When the report is submitted, it is placed in a queue to be processed by a separate thread. The user can then go on within the application and check back later to see the status of their report, they aren't left with a "Processing..." page or icon.
A thread that iterates cache storage, removing data that has expired or no longer needed. The thread's job within the application is independent of any processing for a specific user, but part of the overall application run-time maintenance.
although, not specifically a threading scenario, logging within our web site is handed off to a parallel process, so the throughput of the web site isn't hindered by the time it takes to record log data.

I agree that threading just for threadings sake isn't a good idea and it can introduce problems within your application if isn't done properly, but it is an extremely useful tool for solving some problems.

Whenever you need to call some external component (be it a database query, a 3. party library, an operating system primitive etc.) that only provides a synchronous/blocking interface or using the asynchronous interface not worth the extra trouble and pain - and you also need some form of concurrency - e.g. serving multiple clients in a server or keep the GUI still responsive.

Well, how do you know if you're app is going to run on a multi-core system or not?

Beyond that, there are a lot of processes that take up time, but don't require the CPU. Such as writing to a disk or networking. Who wants to push a button in a GUI and then have to sit there and wait for a network connection. Even on a single core machine, having a separate IO thread greatly improves user experience. You always at least want a separate thread for the UI.

Yet many people seem to think that every task in a program that can be threaded should be threaded, even on a single core system.

"Many people"... Who?

Also from my experience many many programs that should be multithreaded aren't (especially games.. I have an i7 and yet most games still use only 1 of my cores), so I'm not sure what you're talking about. Definitely programs like calc.exe are not multithread (or, if they are, 1 thread does 99% of the work).

Performing a CPU intensive task without blocking the user interface, for example.

Yes, this is true but this is fairly easy to implement and it's not what the OP is referring to (since, in this case, 1 thread does almost all the work and you only need very few mutexes)