How pthreads does cross-threading and scheduling

Question

Hi everyone

I was wondering, how does pthreads-win32 (windows implementation of pthreads) implement cross-threading? Is it written exclusively with windows API? I checked some of the sources and it seems that most is indeed written with windows API, tho i was wondering if it uses windows scheduler to switch between threads (and cores) as well or does it implement its own? Specifically, most processors these days implement their own scheduler (i've read about itanium arch for example, the hardwired logic supports two threads per core and it even automatically switches between them with hw logic, so evidently OS support for multiple cores is not necessarily needed), so if i have an obsolete OS like windows 32-bit or something, which doesn't support multi-core processors, would a program written with pthreads-win32 still run on more than one processor core or would only one core be used?

How about pthreads implementations (untainted posix threads)? Do they support multi-core processors even if the OS on which they are running doesn't?

I am guessing the answer is no, for both windows and posix versions, only one core is in use if the OS doesn't support for multiple cores. Tho this is just an educated guess and i would like to confirm it, so pls leave a comment.

On a side request, can you pls recommend a lib that DOES support for muli-core thread execution, even if the OS on which the program is running DOESN'T. If any exist ofc.

Also, is there a way to ensure two threads written with pthreads are being executed on different cores, or does the OS (or the processor, or pthreads lib) do the assignment automatically? Does pthreads guarantee execution on different cores if they are available?

Cheers, Val

EDIT: I know most of these questions are implementation specific, so i was referring to this implementation of pthreads for windows http://sourceware.org/pthreads-win32/. I didn't specifically mention it before, because as far as i know, this is the most popular and widely used implementation of pthreads for windows.

Answer 1

Generally each modern OS supports Threads by itself and schedules them to the different (virtual) Cores of a System. The OS provides some general synchronization techniques (like Mutexes or Semaphores or Barriers) which are used by pthread to implement the pthreads API.

With two threads per Core (I think you mean Hyper Threading) on some Intel Processors (like Itanium) the OS sees two "virtual" Cores. The processor indeed schedules the two threads onto one physical core. (See Wikipedia)

However, there are examples where Runtime-Plattforms implement their own Thread-Conceptepts and do the scheduling: I think of (at least older) implementations of Java having their own scheduling routines.

Answer 2

Hi again

So from what i'm getting, the most important thing to note in all of this is that threading has very little to do with parallelism (like UMA with multi-core processors). So while threading might be a technique to implement concurrency it is not a way of ensuring ACTUAL parallel execution, which is what i was looking for in the first place, since i am studying parallel and distributed systems and algorithms.

So to answer one question at a time. Yes, pthreads, and probably most (if not all) other threading APIs out there are based on the underlying OS API. Which ofc gives them the same limits that the OS has. Meaning, yes, if the OS (concretely in this case, some windows running for example pthreads-win32) doesn't support multiple cores, only one core is in use at all times. As is pointed out on the wiki page nob provided, to cite: "Hyper-threading requires not only that the operating system support multiple processors, but also that it be specifically optimised for HTT, and Intel recommends disabling HTT when using operating systems that have not been so optimized." http://en.wikipedia.org/wiki/Hyper-threading Meaning in most cases, just hardwired processors (basic) scheduler is not enough to take advantage of multiple cores, it has to be supported/used by SW (OS support).

While this might not be a definitive proof, i believe enough evidence points in the same direction to confirm this to be the case.

I did not sift through pthreads (for posix compliant OSes) sources, i am guessing the same goes for this API, since it is more than likely to use the underlying OS API. You will have to confirm this on your own. :)

Also, any potential libs out there that might support execution on multiple cores even if the OS on which they're running on doesn't support multiple cores, you will have to find them on your own (if they exist), please leave a comment.

To ensure parallelism (execution on different cores) manually, linux does provide a way to pin a thread to a specific virtual processor (under certain conditions). To pin an entire process to a specific (virtual) processor/core, sched_setaffinity() (from sched.h) can be used. As nos pointed out, pthreads provides pthread_setaffinity_np() to pin a particular thread to a specific core. Windows supports a similar functionality with SetThreadAffinityMask(), so clearly, assigning threads manually to run in parallel on different cores is possible (if the OS supports multi-cores).

From my experience coding with pthreads, if you write for code that uses multiple threads (more than 2), they SHOULD be executed on more than one physical core, if available (which is probably an OS feature used by pthreads).

My questions were quite general to begin with, since most of these things are implementation specific, it's hard to give one answer. I hope this answer is detailed enough to help you clarify a few things.

Cheers, Val