Groups   31 of 99+      julia-users › Parallel  sync  async 5 posts by 2 authors      digxx    11 22 15   As you could see, the reduction operator can be omitted if it is not needed. In that case, the loop executes asynchronously, i.e. it spawns independent tasks on all available workers and returns an array of RemoteRef  page 399  immediately without waiting for completion. The caller can wait for the RemoteRef  page 399  completions at a later point by calling fetch    page 398  on them, or wait for completion at the end of the loop by prefixing it with  sync  page 399 , like  sync  parallel for.   From the manual   When I try: a randn 1000  r  parallel for i 1:100000        a rand 1:end          end I get an array of remoteref   When calling fetch r 1   for example I get an empty line?  What does completion mean in this sense? Each worker runs its own own loop and completion at the end or after each iteration? e.g. 2 workers run from i  1 to 100. Now completion means that after each i the first finished worker waits for the other so they can proceed to i+1 ??  For example the following test is much faster when I include an  async infront of the execution part and leave the  sync out Does this parallel program even run parallel if I leave both  sync and  sync out, since its seems to be slower than the serial alternative?   everywhere function myrange q::SharedArray   idx   indexpids q   if idx    0   return 1:0  else  nchunks   length procs q    splits    round Int, s  for s in linspace 0,size q,1 ,nchunks+1    splits idx +1:splits idx+1   end end   everywhere function g q::SharedArray   q myrange q  . 2 end   function h q::SharedArray     sync begin  res Array Float64,size q,1  ;  for p in procs q     async res remotecall_fetch p,myrange,q   remotecall_fetch p,g,q   end   end  res end     digxx    12 2 15   Any idea anyone? :-      Ismael Venegas Castelló    12 2 15   You can use `vcat` to reduce it to an array:  julia  r    parallel vcat for i 1:100000            a rand 1:end          end;  julia  typeof r , size r   Array Float64,1 , 100000,    Is this what you are after?      digxx    12 5 15   Though why does fetch r 1   not work? Wouldnt one expect it to give the result a specific worker calculated? Do I always need a reduction operator? in this case vcat, right? The other question was maybe more fundamental  and simple? : Why is the calculation so much faster with a  async prefix? Does this prefix also work serial? I tried it and he does sth but the result is: Task  done   0x00000000828ec710 Is an accompanied  sync begin ... end mandatory or not? And what exactly does  completion  mean in this sense? Like the  sync waits for the other workers to complete ? Say each worker has a task and then  sync makes sure that every worker finished his task before the next one is given to each worker?     digxx    12 5 15   And yes, the vcat gave the wanted result, though why  as asked above  do I need this reduction operator?