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To get an idea about my code, let's say that it is something like this: You can now choose to sort by Trending, which boosts votes that have happened recently, helping to surface more up-to-date answers. One alternative to multithread your code would be using pthreads ( provides more precise control than OpenMP ). If all of your processing is purely computational (as suggested by your example function) then you should do well to have only as many threads as you have logical processors. fairly well, as well as understanding when you will need to use them. C++11 introduced a standardized memory model. cs.utk.edu/~plank/plank/classes/cs360/360/notes/Setjmp/, https://computing.llnl.gov/tutorials/openMP/#SECTIONS, Multiple glibc libraries on a single host, https://en.cppreference.com/w/c/language/atomic, http://en.wikipedia.org/wiki/Thread_pool_pattern, http://docs.oracle.com/cd/E19253-01/816-5137/ggedn/index.html, http://www.advancedlinuxprogramming.com/alp-folder/alp-ch04-threads.pdf, How observability is redefining the roles of developers, Code completion isnt magic; it just feels that way (Ep. Say you have 2 cores and ntimes is 100, then 100/2=50, so create 2 versions of the program where the first iterates from 0 to 49, the other from 50 to 99. If you prefer not to use openMP you could use either pthreads or clone/wait directly. NB2: You can use iret[j] = to store the result of your thread, they will return 0 on success. Does database role permissions take precedence over schema/object level permissions? Those threads are treated as an resource. If you are hoping to provide concurrency for a single loop for some kind of scientific computing or similar, OpenMP as @Novikov says really is your best bet; this is what it was designed for. I'm not sure what your background might be with concurrency in other languages, but before going too deeply into that, you will want to know your synchronization primitives (mutexes, semaphores, etc.) Certainly easier to debug. Cilk Plus is a superset of standard C++. 464). Your code is not automatically multi-threaded by the compiler if that was your question. http://docs.oracle.com/cd/E19253-01/816-5137/ggedn/index.html, A good literature about pthreads to get going: How do I update the GUI from another thread? When the task is finished you can collect it's results. Find centralized, trusted content and collaborate around the technologies you use most. Usually you get the widest system support by using POSIX threads, more often called "pthreads". You have to be careful doing IO to the same file in threads, though. Also error handling can be a bit tricky (example: what to do when one task fails). Not quite what you have asked, but you might be interested in this: +1 Beat me to it. The two processes need to run for at least a minute or two to makeup for the cost of starting a new process via fork(). Note that auto-vectorization (done through -O3) always yield significant positive results, as there is no cost for using SIMD. Trending is based off of the highest score sort and falls back to it if no posts are trending. 
No other effort by the programmer is needed. It just contains a few extra keywords (_Cilk_spawn, _Cilk_sync, and _Cilk_for) that allow the programmer to tag parts of their program as parallelizable. I think all the answers lack a concrete example with implementation of threads across different function, passing parameters and some benchmarks: How comes that multithreading would be slower? http://www.advancedlinuxprogramming.com/alp-folder/alp-ch04-threads.pdf. To quote Wikipedia, the idea is that, "the programmer should be responsible Works great, I've used it on everything from apple macs, rabbit semiconductor, AVR, PC. The main idea could look similar to. Why is a "Correction" Required in Multiple Hypothesis Testing? The only major platform not supporting it is Windows and here you can use free 3rd party libraries like this one. Several other ports exists as well (Cygwin has one for sure). Also please note that this pattern does not say anything how you should "synchronize" your tasks with each other/outside surroundings. If the task is highly parallelizable and your compiler is modern, you could try OpenMP. it doesn't always succeed or resulting program is slower). 
The mechanism for collecting data from tasks and the amount of threads used in pool should be chosen to reflect your requirements and the capabilities of the hardware and runtime environment. You could instead implement stack-less multithreading using state machines. The more things that can be done completely independent from the rest, the better it lends itself to parallel execution. yes. An operating system can do multithreading almost exclusively in software, this will be awfully slow and there won't be memory protection, but it is possible, however even in that case you need at least programmable interrupts. Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, What OS as most of the threading functions are system dependant. Usually you will, but if each thread only ends up with 1 computation to do and the computations aren't that difficult to do then you may actually slow things down. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. It doesn't work as well as OpenMP though (ie. There is a really good book entitled "embedded multitasking" by Keith E. Curtis. identifying elements that can safely About thread pool: OpenMP is perfect for the kind of stuff that the OP describes. http://en.wikipedia.org/wiki/Thread_pool_pattern So how to write multi-threaded C code depends entirely on the operating system of your target platform. Human-like summarization of collection of texts, Short satire about a comically upscaled spaceship. for exposing the parallelism, this won't scale in a large app as you'll actually loose control of how many threads are running - one thread per task will result in depletion of OS resources. Without pthread but with optimization flag -O3: With pthread and optimization flag -O3 and without pthread_join: Without pthread but with auto-vectorization (-O3): With pthread but without optimization flags: With pthread and with auto-vectorization (-O3): With pthread, auto-vectorization and without pthread_join. The first run works fine, the second fails due to missing synchronization. There exists POSIX conform systems (OS X, FreeBSD, Linux, etc.) The programmer does not mandate that any code be run on a new thread, they just allow the lightweight runtime scheduler to spawn a new thread if and only if it is actually the right thing to do under the particular runtime conditions. But again, benchmark it! Code written in C can run on pretty much anything for that a C compiler exists for. scheduler, to decide during execution True state machines do not use context switches and are not "real" threads. here is a simple example based on pthreads. That said I do use fork() for batch processing when each concurrent task is long lived. Balancing needed bending strength of a wood railing post in concrete with the lifespan due to rot. how to actually divide the work If an iteration in loop is independent of the ones before it, then there's a very simple approach: try multi-processing, rather than multi-threading. 
