Groups   11 of 99+      julia-users ›  threads all vs.  parallel ??? 7 posts by 4 authors      digxx    Aug 30   Sorry if there is already some information on this though I didnt find it... So: What is the difference between these? I have used  parallel so far for parallel loops but recently saw this  threads all in some video and I was wondering what the difference is? Could anyone elaborate or give me a link with some info? Thanks digxx     Yichao Yu    Aug 30     parallel uses worker processes  can also be on a different machine    threads uses threads. You shouldn't use  threads now unless you are working on threading support in julia since it is really experimental at the moment.      Andrew    Aug 30   I have also been wondering this. I tried  threads yesterday and it got me around a 4-fold speedup on a loop which applied a function to each element in an array, and I conveniently didn't need to bother using SharedArrays as I would with  parallel.      Chris Rackauckas    Aug 31   That's pretty much it. Threads are shared memory, which have less overhead  and are thus faster , and can share variables between them.  parallel is multiprocessing, i.e. each worker process has its own set of defined variables which do not overlap, and data has to be transferred between them.  parallel has the advantage that it does not have to be local: different processes can be on completely different computers nodes. But it has a higher startup cost and is thus better suited to larger parallelizable problems.  However, as Yichao states,  threads is still experimental. For one, since the memory is shared, you have have to make sure everything is  thread-safe  in order to be correct and not fail  example: two threads can't write to the same spot at once or else it can be non-deterministic as to what the result is . But also, the internals still have some rough edges. Just check out the repo for bug reports and you'll see that things can still go wrong, or that your performance can even decrease due to type-inference bugs. Thus it is something to play around with, but it definitely isn't something that you should put into production yet  though in many cases it is already looking pretty good! .      digxx    Sep 1   Do you have a  simple  example of how to write something  thread safe  if I plan to use  threads ?      Yichao Yu    Sep 1  Re:  julia-users  Re:  threads all vs.  parallel ???    On Thu, Sep 1, 2016 at 2:17 PM, digxx  diger... hotmail.com  wrote: Do you have a  simple  example of how to write something  thread safe  if I plan to use  threads ?  Some parts of the runtime are still not threasafe. I also don't think there's a meaningful  simple example  of writing something  thread safe . It strongly depend on what you want to do.       Chris Rackauckas    Sep 1   If you  threads and each time you're in the loop you're acting on a different part of the array, it'll be thread-safe.   I think the better examples are what's not thread-safe. If you're doing something like looping and tmp+ A i , then each thread can grab a different tmp and write into the same spot, ultimately not summing up the components of A and getting the wrong answer. Another problem comes up with globals. You might have a loop where you're using BigFloats, and in the loop it might change the precision if a certain condition is met. However, since BigFloat precision is a global, it will change the precision for all of the threads. This becomes a problem because this behavior is non-deterministic: which threads will have been run before this computation depends on random factors about why certain threads ran slightly faster slower.