Parallel Manual

Table of Contents

About

The static classes maxon::ParallelFor, maxon::ParallelImage and maxon::ParallelInvoke are used to execute parallelized code.

ParallelFor

The maxon::ParallelFor class is used to execute code defined in a worker method. This code is executed in a loop distributed over multiple threads. The workload is distributed either dynamically or statically.

// This example calculates an approximation of PI using random numbers.
// array that stores the inside/outside state for each generated point
// ParallelFor will write into this array from multiple threads simultaneously
maxon::BaseArray<maxon::Bool> pointStates;
pointStates.Resize(count) iferr_return;
auto worker = [&pointStates](maxon::Int i)
{
pointStates[i] = false;
// create random point
maxon::LinearCongruentialRandom<maxon::Float32> random;
random.Init(maxon::UInt32(i));
maxon::Vector2d64 point;
point.x = random.Get01();
point.y = random.Get01();
// check if inside circle
if (point.GetSquaredLength() < 1.0)
pointStates[i] = true;
};
// generate and check random points
maxon::ParallelFor::Dynamic(0, count, worker);
// get pi
maxon::Int64 insideCount = 0;
for (const maxon::Bool pointInside : pointStates)
{
if (pointInside)
insideCount++;
}
const maxon::Float pi = 4.0 * maxon::Float(insideCount) / maxon::Float(count);
DiagnosticOutput("PI approximation: @", pi);

maxon::ParallelFor::BreakContext is a base class to define a context for maxon::ParallelFor loops that support cancellation. A class based on maxon::ParallelFor::BreakContext stores the data handled in a thread.

maxon::ParallelFor can call additional "Init" and "Finalize" functions for each thread. The behaviour is defined with these flags:

// This example calculates an approximation of PI using random numbers.
// total number inside the unit-circle
maxon::Int insideCount = 0;
// data structure
struct LocalData : public maxon::ParallelFor::BreakContext
{
maxon::Int localCount; // number of points inside the unit-circle
};
// init thread (called threaded)
auto init = [](LocalData& context)
{
context.localCount = 0;
};
// worker
auto worker = [](maxon::Int i, LocalData& context)
{
maxon::LinearCongruentialRandom<maxon::Float32> random;
random.Init(maxon::UInt32(i));
maxon::Vector2d64 point;
point.x = random.Get01();
point.y = random.Get01();
if (point.GetSquaredLength() < 1.0)
context.localCount++;
};
// finalize thread (called synchronously)
auto finalize = [&insideCount](LocalData& context)
{
// get total number of points inside the unit-circle
insideCount += context.localCount;
};
// run parallel code
maxon::ParallelFor::Dynamic<LocalData, maxon::PARALLELFORFLAGS::INITTHREADED_FINALIZESYNC>(0, count, init, worker, finalize) iferr_return;
// get pi
const maxon::Float pi = 4.0 * maxon::Float(insideCount) / maxon::Float(count);
DiagnosticOutput("PI approximation: @", pi);

ParallelImage

maxon::ParallelImage is a special class to process image data of a given size per pixel.

// This example fills a BaseBitmap with an UV color gradient.
// allocate bitmap
AutoAlloc<BaseBitmap> bitmap;
if (bitmap == nullptr)
return maxon::OutOfMemoryError(MAXON_SOURCE_LOCATION);
// define bitmap size
const maxon::Int width = 1024;
const maxon::Int height = 1024;
const IMAGERESULT res = bitmap->Init(width, height, 24);
if (res != IMAGERESULT::OK)
return maxon::OutOfMemoryError(MAXON_SOURCE_LOCATION);
// dimensions as float values
const maxon::Float widthF = maxon::Float(width);
const maxon::Float heightF = maxon::Float(height);
auto worker = [widthF, heightF, &bitmap](maxon::Int x, maxon::Int y)
{
// get UV coords for the given pixel
const maxon::Float xf = maxon::Float(x);
const maxon::Float yf = maxon::Float(y);
const maxon::Float u = xf / widthF;
const maxon::Float v = yf / heightF;
// set colors based on UV coords
const maxon::Int32 red = maxon::SafeConvert<maxon::Int32>(u * 255.9);
const maxon::Int32 green = maxon::SafeConvert<maxon::Int32>(v * 255.9);
const maxon::Int32 blue = 0;
// store color in the bitmap
bitmap->SetPixel(Int32(x), Int32(y), red, green, blue);
};
// process every pixel of the given bitmap
maxon::ParallelImage::Process(width, height, 30, worker);
ShowBitmap(bitmap);

ParallelInvoke

The maxon::ParallelInvoke function can be used to execute multiple arbitrary functions in parallel.

// This example uses ParallelInvoke() to search for
// min/max values in two given arrays in parallel.
// min/max values
maxon::Float minA = 0.0, maxA = 0.0, minB = 0.0, maxB = 0.0;
// search for min/max values in the given arrays
// GetMinMax() is a custom function
maxon::ParallelInvoke(true,
[&arrayA, &minA, &maxA] { GetMinMax(arrayA, minA, maxA); },
[&araryB, &minB, &maxB] { GetMinMax(araryB, minB, maxB); }
) iferr_return;
// check min/max values
if ((minA > minB) && (maxA < maxB))
DiagnosticOutput("Min/max values of a array A are within the min/max values of array B"_s);

Further Reading