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The Functional (or Task) Decomposition Design Pattern
Overall Problem
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A good example of this is the computer game SimPark.
Computer Graphics
mjb – March 10, 2022
Computer Graphics
Functional (Task) Decomposition
Mike Bailey
functional_decomposition.pptx
mjb – March 10, 2022
The Functional (or Task) Decomposition Design Pattern
Computer Graphics
Plants Money
Credit: Maxis (Sim Park)
mjb – March 10, 2022
How is this is different from Data Decomposition (such as the OpenMP for-loops)
• This is done less for performance and more for programming convenience.
• This is often done in simulations, where each quantity in the simulation needs to make decisions about what it does next based on what it and all the other quantities are doing right now.
• Each quantity takes all of the “Now” state data and computes its own “Next” state.
• The biggest trick is to synchronize the different quantities so that each of them is seeing only what the others’ data are right now. Nobody is allowed to switch their data states until they are all done consuming the current data and thus are ready to switch together.
• The synchronization is accomplished with barriers.
Quantity #1:
Complete Now State Compute
Quantity #2:
Complete Now State Compute
Quantity #3:
Complete Now State Compute
Computer G
Individual Next State raphics
Individual Next State
Individual Next State mjb – March 10,
The Functional Decomposition Design Pattern
main( int argc, char *argv[ ] ) {
omp_set_num_threads( 3 );
InitBarrier( 3 );
#pragma omp parallel sections
// don’t worry about this for now, we will get to this later
} // implied barrier — all functions must return to get past here
#pragma omp section
Watcher( );
#pragma omp section
Animals( );
#pragma omp section
Plants( );
Computer Graphics
mjb – March 10, 2022
Setup the Now global variables Calculate the current Environmental Parameters Spawn Threads using OpenMP Sections
Print results and increment time Calculate new Environmental Parameters
Computer Graphics
Using the entire Now state, compute A’s Next variables
Using the entire Now state, compute B’s Next variables
DoneComputing barrier
Copy A’s Next state into the Now state
Copy B’s Next state into the Now state
DoneAssigning barrier
nePrinting barrier
mjb – March 10, 2022
The Functional Decomposition Design Pattern
void Watcher( ) {
while( << You decide how to know when finished? >> ) {
// do nothing
WaitBarrier( ); // 1.
// do nothing
WaitBarrier( ); // 2.
<< write out the “Now” state of data >>
<< advance time and re-compute all environmental variables >>
WaitBarrier( ); // 3.
Computer Graphics
mjb – March 10, 2022
The Functional Decomposition Design Pattern
Computer Graphics
void Animals( ) {
while( << You decide how to know when finished? >> ) {
int nextXXX= << function of what all states are right Now >> …
WaitBarrier( );
NowXXX = nextXXX;
WaitBarrier( );
// do nothing
WaitBarrier( );
// copy the computed next state to the Now state
// 2. // 3.
mjb – March 10, 2022
Computer Graphics
Temperature
My Simulation Output
mjb – March 10, 2022
You Might Have to Make Your Own Barrier Function
Why can’t we just use #pragma omp barrier ?
The Functional Decomposition is a good example of when you sometimes can’t.
There are two ways to think about how to allow a program to use a barrier:
1. Let the barrier happen at a specific location in the code
2. Let the barrier work after a specific number of threads have gotten there
Computer Graphics
• g++ allows both #1 and #2
• Visual Studio requires #1
• The Functional Decomposition shown here wants to have #2,
because the barriers need to be in different functions
mjb – March 10, 2022
Computer Gr}aphics
Sometimes You Have to Make Your Own Barrier Function
omp_lock_t Lock;
int NumInThreadTeam; int NumAtBarrier;
int NumGone;
InitBarrier( int n ) {
NumInThreadTeam = n; NumAtBarrier = 0; omp_init_lock( &Lock );
void WaitBarrier( ) {
omp_set_lock( &Lock );
NumAtBarrier++;
if( NumAtBarrier == NumInThreadTeam ) {
NumGone = 0;
NumAtBarrier = 0;
// let all other threads return before this one unlocks: while( NumGone != NumInThreadTeam – 1 ); omp_unset_lock( &Lock );
omp_unset_lock( &Lock );
while( NumAtBarrier != 0 );
#pragma omp atomic
// all threads wait here until the last one arrives … // … and sets NumAtBarrier to 0
// number of threads you want to block at the barrier
// release the waiting threads
NumGone++;
mjb – March 10, 2022
The WaitAtBarrier( ) Logic
Computer Graphics
mjb – March 10, 2022
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