Multi-thread code with single-core processor and single-thread code with multi-core processor

2
votes

I'm new to multi-threaded programming. I have been reading some articles, but two main points I'm not completely sure about.

  1. If I have a single-thread code (sequential), and I run it on multi-core processor. Will the OS try to divide the thread into multiple threads (while taking care of dependencies) to take advantage of the muli-core processor?
  2. If I have a multi-thread code, and I run it on single-core processor. Will the OS make time-sharing between different threads (the same way it does with multiple processes)?
2
multithreadingparallel-processingoperating-systemcpumulticore
1. No. 2. Yes ........ – Mitch Wheat
I fail to see why this question has received 4 upvotes? 4 people with same homework? – Mitch Wheat
Trust me, pal. Not a homework. – KhaledWas

2 Answers

3
votes

1) No

If an application makes use of, for example, the Intel maths libraries and has been compiled with the right switches, routines like FFTs will at runtime be split out into separate threads matching the number of cores in the machine. Your source code remains 'single threaded', but the library is creating and destroying threads behind your back.

Similarly some compilers (e.h. Intel's icc, Sun's C compiler) may turn some loops into separate threads, each tackling a share of the iterations. Again the source code looks single threaded, but the compiler generates threaded code on your behalf. It's a bit like automatically applying some OpenMP to your source code.

OSes cannot second guess what an application is going to do, so they cannot intervene like this. Libraries and compilers know what is about to happen, so they can.

Libraries and compiler tricks like this have been developed so as to make it easy for programmers to extract higher performance from 'single' threaded code. Intel started adding features like that to their maths library around about the same time they started heading towards multi-core CPUs. The idea was to create (from the programmer's point of view) the impression of better 'single' thread performance, whilst the speed was actually being delivered by multiple cores. Similarly with Sun when they started doing multi-processor computers.

And with everyone more or less giving up on making significant improvements to the performance of a single core, this is the only way ahead.

2) Yes. How else would it do it?

1
votes

  1. No, the operating system has not enough information to do that. In parallelization you need to consider the dependencies between operations. Some compiler try to do that, they have more information about the intent of the code. But even they often fail to do that effectively.

  2. Yes, for example the Linux scheduler does not even distinguish between threads and processes.