[GAP Forum] Multitasking with GAP

[Sergio Siccha]

Hi Alan,

unfortunately `RunTask` in GAP is more or less only a wrapper for
`CallFuncListWrap` and is indeed blocking. `RunTask` in GAP only is a
mock-up of the proper `RunTask` function from HPC-GAP. HPC-GAP didn't
make it out of alpha stage though and if I'm not mistaken nobody is
working on it ATM.

I'm not sure whether that's the best option as I have never used it, but
in principle you should be able to do parallel computations by spawning
several GAP processes via the SCSCP package, see chapter 8 "Parallel
computing with SCSCP" of the SCSCP manual. So I guess as long as your
computations don't need a lot of memory that should be fine.

Hope this helps! :-)

Best,
Sergio


On 18.05.21 07:35, Alan Hylton wrote:
> 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
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