When should you use multithreading? And would multi threading be beneficial if the different threads execute mutually independent tasks?
This were the only two questions I couldn't answer in the interview I got rejected from last night.
Q: When should you use multithreading?
A: "Your question is very broad. There are few non-trivial systems where the functionality can be met simply, quickly and reliably with only one thread. For example: [pick out a typical system that the target company sells and pick out a couple aspects of its function that would be better threaded off - heavy CPU, comms, multi-user - just pick out something likely & explain].
Q: Would multithreading be beneficial if the different threads execute mutually independent tasks?
A: "Depends on what you mean by 'executing tasks'. Multithreading would surely be beneficial if the threads process mutually independent data in a concurrent fashion - it reduces requirements for locks and probabilty of deadlocks increases in a super-linear fashion with the number of locks. OTOH, there is no issue with threads executing the same code, this is safe and very common."
You should use multithreading when you want to perform heavy operations without "blocking" the flow.
Example in UIs where you do a heavy processing in a background thread but the UI is still active.
If the threads execute mutually exclusive tasks it is the best since there is no overhead for synchronization among threads needed
Multithreading is a way to introduce parallelness in your program. In any case if there can be parallel paths (parts which do not depend on result from a other part) in your program, use can make use of it.
Specially with all these multiple core machines now days, this is a feature which one should exploit.
Some examples would be processing of large data where you can divide it in chunks and get it done in multiple threads, file processing, long running I/O works like network data transfers etc.
To your second question, it would be best if the tasks are mutually independent - reasons
- no shared data means no contentions
- no need for any ordered processing (dependency), so each thread can work when have resources
- more easy to implement
You should definitely use multithreading in GUI applications when you invoke time consuming tasks from the main event loop. Same applies for server application that might block while doing the I/O.
For the second question, it is usually yes when you have machine with multiple CPU cores. In this case these independent tasks can be executed in parallel.