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Init grid parallelisations #173

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66 changes: 59 additions & 7 deletions src/init_grid.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -110,18 +110,54 @@ int CSimulation::nInitGridAndCalcStillWaterLevel(void)
m_dStartIterConsSandAllCells = 0;
m_dStartIterConsCoarseAllCells = 0;

// And go through all cells in the RasterGrid array
// Use OpenMP parallel loop with reduction clauses for thread-safe accumulation
// Cache-aligned accumulator structure to prevent false sharing between threads
// Each structure is padded to 64 bytes (typical cache line size) to ensure
// different threads' accumulators reside in separate cache lines
struct alignas(64) ThreadLocalAccumulators
{
int nZeroThickness;
double dConsFine;
double dConsSand;
double dConsCoarse;
double dSuspFine;
double dUnconsFine;
double dUnconsSand;
double dUnconsCoarse;

ThreadLocalAccumulators() :
nZeroThickness(0), dConsFine(0), dConsSand(0), dConsCoarse(0),
dSuspFine(0), dUnconsFine(0), dUnconsSand(0), dUnconsCoarse(0) {}
};

// Determine maximum number of threads for pre-allocation
// This scales efficiently from dev machine (8 cores) to production (32+ cores)
#ifdef _OPENMP
#pragma omp parallel for collapse(2) reduction(+ : nZeroThickness) \
reduction(+ : m_dStartIterConsFineAllCells, m_dStartIterConsSandAllCells, m_dStartIterConsCoarseAllCells) \
reduction(+ : m_dStartIterSuspFineAllCells, m_dStartIterUnconsFineAllCells, m_dStartIterUnconsSandAllCells, m_dStartIterUnconsCoarseAllCells)
int const nMaxThreads = omp_get_max_threads();
#else
int const nMaxThreads = 1;
#endif

// Pre-allocate cache-aligned accumulators for each thread
// This eliminates false sharing and allows lock-free parallel accumulation
std::vector<ThreadLocalAccumulators> threadAccum(nMaxThreads);

// Parallel loop without reduction clauses - each thread accumulates into its own cache line
// This approach eliminates the OpenMP reduction overhead that caused thread synchronization bottlenecks
#ifdef _OPENMP
#pragma omp parallel for collapse(2) schedule(static)
#endif
for (int nX = 0; nX < m_nXGridSize; nX++)
{
for (int nY = 0; nY < m_nYGridSize; nY++)
{
// Get thread ID to access this thread's private accumulator
#ifdef _OPENMP
int const tid = omp_get_thread_num();
#else
int const tid = 0;
#endif
ThreadLocalAccumulators& acc = threadAccum[tid];

// Re-initialise values for this cell
m_pRasterGrid->m_Cell[nX][nY].InitCell();

Expand All @@ -132,7 +168,7 @@ int CSimulation::nInitGridAndCalcStillWaterLevel(void)

if (dSedThickness <= 0)
{
nZeroThickness++;
acc.nZeroThickness++;

// Note: Logging from parallel regions can cause race conditions, but this is for debugging only
// In production, consider collecting problematic cells and logging after the parallel region
Expand All @@ -149,6 +185,7 @@ int CSimulation::nInitGridAndCalcStillWaterLevel(void)
m_pRasterGrid->m_Cell[nX][nY].CalcAllLayerElevsAndD50();
}

// Accumulate into thread-local variables (no synchronization required!)
// Note that these totals include sediment which is both within and outside the polygons (because we have not yet defined polygons for this iteration, duh!)
m_dStartIterConsFineAllCells += m_pRasterGrid->m_Cell[nX][nY].dGetConsFineDepthAllLayers();
m_dStartIterConsSandAllCells += m_pRasterGrid->m_Cell[nX][nY].dGetConsSandDepthAllLayers();
Expand All @@ -169,6 +206,21 @@ int CSimulation::nInitGridAndCalcStillWaterLevel(void)
}
}

// After parallel region: combine thread-local results sequentially
// This is fast (O(nThreads), typically 8-32 operations) compared to the parallel work (O(nCells))
// and eliminates all the false sharing and synchronization overhead from the parallel loop
for (int t = 0; t < nMaxThreads; t++)
{
nZeroThickness += threadAccum[t].nZeroThickness;
m_dStartIterConsFineAllCells += threadAccum[t].dConsFine;
m_dStartIterConsSandAllCells += threadAccum[t].dConsSand;
m_dStartIterConsCoarseAllCells += threadAccum[t].dConsCoarse;
m_dStartIterSuspFineAllCells += threadAccum[t].dSuspFine;
m_dStartIterUnconsFineAllCells += threadAccum[t].dUnconsFine;
m_dStartIterUnconsSandAllCells += threadAccum[t].dUnconsSand;
m_dStartIterUnconsCoarseAllCells+= threadAccum[t].dUnconsCoarse;
}

if (m_bHaveWaveStationData && (! m_bSingleDeepWaterWaveValues))
{
// Each cell's value for deep water wave height and deep water wave orientation is interpolated from multiple user-supplied values
Expand Down Expand Up @@ -210,7 +262,7 @@ int CSimulation::nInitGridAndCalcStillWaterLevel(void)
// GDALDataset* pDataSet = pDriver->Create(strOutFile.c_str(), m_nXGridSize, m_nYGridSize, 1, GDT_Float64, m_papszGDALRasterOptions);
// pDataSet->SetProjection(m_strGDALBasementDEMProjection.c_str());
// pDataSet->SetGeoTransform(m_dGeoTransform);
// double* pdRaster = new double[m_ulNumCells];
// double* pdRaster = new double[m_ulNum_Cells];
// int nn = 0;
// for (int nY = 0; nY < m_nYGridSize; nY++)
// {
Expand Down