GAP (HAP commands) - Chapter 11: Parallel computation

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11 Parallel computation
11.1 An embarassingly parallel computation
11.2 An non-embarassingly parallel computation

### 11 Parallel computation

#### 11.1 An embarassingly parallel computation

The following example creates five child processes and uses them simultaneously to compute the second integral homology of each of the 267 groups of order 64. The final command shows that

H_2(G, Z)= Z_2^15

for the 267-th group G in GAP's library of small groups.

    gap> Processes:=List([1..5],i->ChildProcess());;
    gap> fn:=function(i);return GroupHomology(SmallGroup(64,i),2);end;;
    gap> for p in Processes do
    > ChildPut(fn,"fn",p);
    > od;

    gap> NrSmallGroups(64);
    267

    gap> L:=ParallelList([1..267],"fn",Processes);;

    gap> L[267];
    [ 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2 ]

The function `ParallelList()` is built from HAP's six core functions for parallel computation.

#### 11.2 An non-embarassingly parallel computation

The following commands use core functions to compute the product A=M× N of two random matrices by distributing the work over two processors.

    gap> M:=RandomMat(2000,2000);;
    gap> N:=RandomMat(2000,2000);;

    gap> s:=ChildProcess();;

    gap> Mtop:=M{[1..1000]};;
    gap> Mbottom:=M{[1001..2000]};;

    gap> ChildPut(Mtop,"Mtop",s);
    gap> ChildPut(N,"N",s);
    gap> NextAvailableChild([s]);;

    gap> ChildCommand("Atop:=Mtop*N;;",s);;
    gap> Abottom:=Mbottom*N;;
    gap> A:=ChildGet("Atop",s);;
    gap> Append(A,Abottom);;

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