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#############################################################################
#
# Example 11. Parallel iterator
# Brute force computation of the sum of orders of elements of a permutation group
# (we deliberately not using representatives of conjugacy classes)
runtime:=function(t1,t2) return (t2-t1)/1000.; end;
n := 5;
S := SymmetricGroup(n);
map := Order;
# First do it sequentially
t1:=NanosecondsSinceEpoch();
Print("Sum of orders sequential : ", Sum( List(S, s -> CallFuncList( map, [s] ) ) ), "\n");
t2:=NanosecondsSinceEpoch();
Print("Iterator sequential : ", runtime(t1,t2), "\n");
# Now in parallel
S := SymmetricGroup(n);
nrworkers := 2; # number of workers
sizefactor := 1000; # input channel length "per worker"
jobsize := 1; # for workers in MultiReceiveChannel
chunksize := 1; # for MultiSendChannel
inch := CreateChannel( nrworkers*sizefactor ); # "shared" input channel
outch := CreateChannel( nrworkers ); # output channel
t1:=NanosecondsSinceEpoch();
worker := function()
local input, x, res;
res := 0;
while true do
input := MultiReceiveChannel( inch, jobsize );
for x in input do
if x = fail then
SendChannel(outch, res);
return;
else
res := res + CallFuncList( map, [x] );
fi;
od;
od;
end;
# master thread runs the iterator
master :=CreateThread(
function()
local s, chunk, i;
chunk := [];
i:=0;
for s in Iterator( S ) do
i:=i+1;
chunk[i]:=s;
if i=chunksize then
MultiSendChannel( inch, chunk );
chunk:=[];
i:=0;
fi;
od;
if Length(chunk) > 0 then
MultiSendChannel( inch, chunk );
fi;
Print("Iteration completed, waiting for workers ... \n");
for s in [1..nrworkers] do
SendChannel( inch, fail );
od;
end);
# worker threads start to wait
workers:=List([1..nrworkers], i -> CreateThread( worker ) );
# wait for all threads
WaitThread( master );
for i in [1..nrworkers] do
WaitThread( workers[i] );
od;
# now collect results
sum := 0;
for i in [1..nrworkers] do
sum := sum + ReceiveChannel( outch );
od;
t2:=NanosecondsSinceEpoch();
Print("Sum of orders parallel : ", sum, "\n");
Print("Iterator parallel : ", runtime(t1,t2), "\n");
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