Question

Can anyone help me out I am working on a presentation and would like to include a bit about – ‘The difference between multicore and concurrent programming’, I have googled a bit but not turning up many good descriptions, any help appreciated! 🙂

Thanks,

Eamonn

Answer 1

Concurrent (occurring or existing simultaneously) implies that different code MAY execute at the exact same cycle. It means that things can possibly happen in parallel if multiple processors or a processor with multiple cores is available and the program is crafted correctly. Just adding threads does not imply concurrent execution.

The reason I say MAY and possibly is that anytime the programs separate threads need to share volatile/mutable state, other threads that need access to that state can not continue executing and will have to wait their turn to access that state, and things start happening serially again.

Typically this is implemented in a single program as more than one thread executing code concurrently at the same exact cycle as another thread, given that there is no resource contentions as listed above. This requires multiple physical processors or cores. Other models run multiple heavyweight OS processes that can execute concurrently.

Concurrent programming is very hard to do correctly with mutable shared state.

You can write a concurrent program
that runs serially on a single single
core processor, but scales up to
execute more things at the same time
when more processors or cores, or even
multiple processors with multiple
cores is present.

You can also cause single threaded programs to appear concurrent on a multi-core / multi-processor system if they can operate on independent ranges of input data at the same time. Example: a single threaded 3D rendering program can on a dual core machine can run 2 separate instances the first rendering all the odd frames and the second rendering all the even frames. As long as they don’t try to share any mutable resources.

Multi-core means that a single CPU has multiple Processor cores that can execute threads or processes concurrently and typically appears as multiple processors to mainstream operating systems.

It does NOT imply that programs that are single threaded gain any concurrency behaviors or benefits from the additional processor cores available.