[GAP Forum] Multitasking with GAP

[Alan Hylton]

Howdy,

Suppose I have a list of lists, and I wish to run some time-consuming
process on each of these sub-list (each sub-list is independent, so I am
not worried about race conditions).

I think the easiest way to demonstrate my thought process is with code:

I have a list of lists and the number of cores I wish to use -
master_list := [ [...], [...], ..., [...]];
n := Length(master_list);
cores:=15;

I have a time consuming function whose arguments are ranges into
master_list -
time_consuming_func := function(start_index, stop_index)
...
end;

I portion out master_list, creating a list a tasks -
task_list:=[];
start:=1;
for i in [1..cores] do
 flag:=0;
 if i <= n mod cores then
  flag:=1;
 fi;
 if i > n then
  break;
 fi;
 len:=Int(n/cores)+flag;

 Add(task_list, RunTask( time_consuming_func , start, start+len-1));

 start:=start+len;
od;

I had several hopes:
1: I could get a list of tasks, and then use something like TaskFinished to
see if each are done
2: Store the result of each time_consuming_func in the global master_list

But I ran into one problem: RunTask seems to be blocking. Instead of
spawning a process and continuing with my loop, it waits until each task
finishes. I considered DelayTask instead of RunTask so that I could just
use WaitTask, but DelayTask does not seem to exist (similarly for
asynchronous tasks). Is there an alternative? To follow the documentation,
I'd like to avoid the lower-level CreateThread if I can.

Also, number 2 makes some assumptions on how memory works. Is it actually
valid to have a thread working on element i of master_list modify the
global master_list[i]?

I'd greatly appreciate any insight!

Thanks,
Alan