cs4402 – parallel computing
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CS4402 – Parallel Computing
Lecture 9 – Sorting Algorithms (2)
Compare and Exchange Operation
Compare and Exchange Sorting
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Compare and Exchange Operation
Take place between processors rank1, rank2.
Each processor keeps the sub-array a=(a[i],i=0,1,…,n).
if(rank is rank1){MPI_Send(&a,n,MPI_INT,rank2, tag1,MPI_COMM_WORLD);MPI_Recv(&b,n,MPI_INT,rank2, tag2,MPI_COMM_WORLD,&status);c = merge(n,a,n,b);for(i=0;i<n;i++)a[i]=c[i];
} if(rank is rank2){MPI_Send(&a,n,MPI_INT,rank2, tag2,MPI_COMM_WORLD);MPI_Recv(&b,n,MPI_INT,rank2, tag1,MPI_COMM_WORLD,&status);c = merge(n,a,n,b);for(i=0;i<n;i++)a[i]=c[i+n];
}
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Compare and Exchange Operation
Complexity?
What amount of computation is being used?
What amount of communication takes place?
CAN YOU FIND ARGUMENTS TO PROVE
THAT THIS IS OPTIMAL OR EFFICIENT?
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Compare and Exchange Algorithms
Step 1. The array is scattered onto p sub-arrays.
Step 2. Processor rank sorts a sub-array.At any time the processors keep the sub-arrays sorted.
Step 3. While is not sorted / is needed compare and exchange between some processors
Step 4. Gather of arrays to restore a sorted array.
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Bubble Sort
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Bubble Sort
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Bubble Sort
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Odd-Even Sort1. Scatter the array onto processors.2. Sort each sub-array aa.3. Repeat for step=0,1,2,…, p-1
if (step is odd){if(rank is odd)exchange(aa,n/size,rank, rank+1); if(rank is even) exchange(aa,n/size,rank-1, rank);
} if (step is even){
if(rank is even)exchange(aa,n/size,rank, rank+1); if(rank is odd) exchange(aa,n/size,rank-1, rank);
}4. Gather the sub-arrays back to root.
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Odd-Even SortSimple Remarks:
- Odd-Even Sort uses size rounds of exchange.
- Odd-Even Sort keeps all processors busy … or almost all.
- The complexity is given by
- Scatter and Gather the array n/size elements
- Sorting the array n/size elements
- Compare and Exchange process size rounds involving n/size
elements
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if( rank == 0 ){
array = (double *) calloc( n, sizeof(double) ); srand( ((unsigned)time(NULL)+rank) );
for( x = 0; x < n; x++ ) array[x]=((double)rand()/RAND_MAX)*m;}
MPI_Scatter( array, n/size, MPI_DOUBLE, a, n/size, MPI_DOUBLE, 0, MPI_COMM_WORLD );
merge_sort(n/size,a);
for(i=0;i<size;i++){
if( (i+rank)%2 ==0 ){ if( rank < size-1 ) exchange(n/size,a,rank,rank+1,MPI_COMM_WORLD); } else { if( rank > 0 ) exchange(n/size,a,rank-1,rank,MPI_COMM_WORLD);
} MPI_Barrier(MPI_COMM_WORLD)
}
MPI_Gather( a, n/size, MPI_DOUBLE, array, n/size, MPI_DOUBLE, 0, MPI_COMM_WORLD );
if( rank == 0 ){ for( x = 0; x < n; x++ ) printf( "Output : %f\n", array[x] ); }
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Comments on Odd-EvenFeatures of the algorithm:
- Simple and quite efficient.
- In p steps of compare and exchange the array is sorted out
- Why???
- The number of steps can be reduced if test “array sorted” but still in O(p).
- C&E operations only between neighbors.
Can we do C&E operations between other processors?
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Odd-Even Sort Complexity
Stage 0. To sort out the scattered array
Stage 1. Odd-Even for p levels
Scatter and Gather
Total computation complexity
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isSorted(n, a, comm)The parallel routine int isSorted(int n, double *a, MPI_Comm comm)
1. Test if the processors have all the local arrays in order.
2. rank1 < rank2 elements of rank1 < rank2.
3. If the answer if yes then no exchange is needed.
How to do it?1. The test is done at the root.
2. The test is done collectively by all processors.
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isSorted(n,a,comm) – Strategy 1The test is done collectively by all processors
1. Send last to the right processor
2. Receive last from the left processor
3. Test if last > a[0] then answer = 0
4. All_Reduce answer by using MIN
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isSorted(n,a,comm) – Strategy 2The test is done at the root.
1. Gather the first elements to the root.
2. Gather the last elements to the root.
3. If rank == 0 then1. For size-1 times do
- test if last[i] > first[i+1]
• Broadcast the answer
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Shell Sort It is based on the notion of “shell/group” of consecutive processors.
- C&E take place between equally extreme procs. - The shell is then divided into 2.
(0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15) #(shell)=p
(0 1 2 3 4 5 6 7) (8 9 10 11 12 13 14 15) #(shell)=p/2 (0 1 2 3) (4 5 6 7) (8 9 10 11) (12 13 14 15) #(shell)=p/4 (0 1) (2 3) (4 5) (6 7) (8 9) (10 11) (12 13) (14 15) #(shell)=p/8
- There are log(p) levels of division.
For the level l we have- there are pow(2,l) shells each of size p/pow(2,l).
- The shell k contains the processors
1
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Shell SortShell Sort is based on two stages:
Stage 1. Divide the shells
for l=0,1,2, log(p)
- exchange in parallel between extreme processors in each shell.
Stage 2. Odd-Even
for l=0,1,2, …,p
- if rank and l are both even then exchange in parallel betw rank and rank+1
- if rank and l are both odd then exchange in parallel betw rank and rank+1
- test “array sorted”
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Shell Sort Complexity
Stage 0. To sort out the scattered array
Stage 1. Odd-Even for l levels
Catch the average complexity of l is in this case O(log^2(p)) so that in average the shell can be
Scatter and Gather
Total computation complexity
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Complexity Comparison for Parallel Sorting
Odd-Even Sort
Shell Sort
Merge Sort
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AssignmentDescription: Write a MPI program to sort out an array:
1. Use a MPI method to compare and exchange
2. Use a MPI method to test isSorted()
3. Use the odd-even sort.
4. Evaluate the performances of the program in a readme.doc
General Points:1. It is for 10% of the marks.
2. Deadline on Monday 2/12/2013 at 5 pm.
3. The following elements must be submitted by email to j.horan@4c.ucc.ie:1. The c program name with your name and student number e.g. SabinTabirca_111111111.c.
2. The Makefile file
3. Readme.doc in which you have 1) to give your student details and 2) to state the performances.
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