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libGPGPU

What is libGPGPU?

Some computers have multiple OpenCL-capable devices such as an integrated-GPU, a discrete-GPU and a CPU. But they do not automatically join their compute power to do some work quickly. There is a need of algorithm to separate a work into pieces and send them to all those devices. This library is written for unification of all(or only selected) devices within system, to run OpenCL kernels with load-balancing to minimize running-times of kernels as if they are part of a single GPU. See wiki for details. Examples here.

  • When CPU is included as a device, it is partitioned to dedicate some of threads for other devices' I/O management (copying buffers, synchronizing their threads, etc).
  • Each device is given a dedicated CPU thread that does independent scheduling/synchronization for high performance load-balancing.
  • RAM-sharing devices are given mapping ability instead of copying during computations. Integrated GPUs and CPUs get full RAM bandwidth when running kernels.
    • Only CPU or only iGPU can use this feature at the same time because OpenCL spec does undefined behavior if multiple devices use same host pointer during mapping/unmapping
    • Preferably (and by default) CPU is given the feature by constructor because non-gaming APUs have more core power than shader power. Gamers should have giveDirectRamAccessToCPU=falseon constructor
    • CPU RAM-sharing devices also benefit good from CPU L3 cache (especially if it is bigger than dataset)
  • Devices can be cloned for overlapping I/O/compute operations to decrease overall latency or increase throughput during load-balancing. CPU & iGPU are not cloned.

simplified load balancing

Dependencies

  • Visual Studio (2022 community edition, etc) with vcpkg (that auto-installs OpenCL for the project) vcpkg
    • Maybe works in Ubuntu without vcpkg too, just need explicitly linking of OpenCL libraries and headers
  • OpenCL 1.2 runtime (s) [Intel's runtime can find CPUs of AMD processors too & run AVX512 on Ryzen 7000 series CPU cores] (multiple platforms are scanned for all devices)
  • OpenCL device(s) like GTX 1050 ti graphics card, a new CPU that has teraflops of performance, integrated GPU, all at the same time can be used as a big unified GPU.
  • C++17

Hello World

// hello-world program that blends A and B vectors

#include <iostream>
#include <fstream>

// uncomment this if you use opencl v2.0 or v3.0 devices. By default, opencl v1.2 devices are queried. 
// must be defined before including "gpgpu.hpp"
//#define CL_HPP_MINIMUM_OPENCL_VERSION 200

#include "gpgpu.hpp"
int main()
{
    try
    {
        const int n = 16; // number of array elements to test

        GPGPU::Computer computer(GPGPU::Computer::DEVICE_ALL); // allocate all devices for computations
        for (auto& name : computer.deviceNames())
            std::cout << name << std::endl;

        // compile a kernel to do C=A*m+B for all elements
        computer.compile(R"(
            kernel void blendFunc(global float * multiplier, global float * A, global float * B, global float * C) 
            { 
                int id=get_global_id(0); 
                C[id] = A[id] * multiplier[0] + B[id];
             })", "blendFunc");

        // create host arrays that will be auto-copied-to/from GPUs/CPUs/Accelerators before/after kernel runs
        auto multiplier = computer.createScalarInput<float>("multiplier");

        // same as multiplier.access<float>(0) = 3.1415f;
        multiplier = 3.1415f;

        auto A = computer.createArrayInputLoadBalanced<float>("A", n);
        auto B = computer.createArrayInputLoadBalanced<float>("B", n);
        auto C = computer.createArrayOutput<float>("C", n);

        // initialize one element for testing
        for (int i = 0; i < 16; i++)
        {
            A.access<float>(i) = 2.0f;
            B.access<float>(i) = -3.1415f;
        }
        // initializing all elements at once
        C = 0.0f;


        // compute, uses all GPUs and other devices with load-balancing to give faster devices more job to minimize overall latency of kernel (including copy latency too)
        computer.compute(multiplier.next(A).next(B).next(C), "blendFunc", 0, n, 1);

        for (int i = 0; i < 16; i++)
        {
            std::cout << "PI = " << C.access<float>(i) << std::endl;
        }

        std::cout << " ---------------------- " << std::endl;

        multiplier = 2.0f * 3.1415f;

        // compute, uses all GPUs and other devices with load-balancing to give faster devices more job to minimize overall latency of kernel (including copy latency too)
        computer.compute(multiplier.next(A).next(B).next(C), "blendFunc", 0, n, 1); // normally workgroup-size should be like 64 or 256 instead of 1 and n=big multiple of it

        for (int i = 0; i < 16; i++)
        {
            std::cout << "3*PI = " << C.access<float>(i) << std::endl;
        }
    }
    catch (std::exception& ex)
    {
        std::cout << ex.what() << std::endl; // any error is handled here
    }
    return 0;
}

output:

Device 0: GeForce GT 1030 (OpenCL 1.2 CUDA ) [direct-RAM-access disabled]
Device 1: gfx1036 (OpenCL 2.0 AMD-APP (3444.0) )[has direct access to RAM] [direct-RAM-access disabled]
Device 2: AMD Ryzen 9 7900 12-Core Processor (OpenCL 1.2 (Build 37) )[has direct access to RAM]
PI = 3.1415
PI = 3.1415
PI = 3.1415
PI = 3.1415
PI = 3.1415
PI = 3.1415
PI = 3.1415
PI = 3.1415
PI = 3.1415
PI = 3.1415
PI = 3.1415
PI = 3.1415
PI = 3.1415
PI = 3.1415
PI = 3.1415
PI = 3.1415
 ----------------------
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245
3*PI = 9.4245

How to Select Parameters for a Kernel?

  • Explicitly setting parameters for only once, then calling kernel for multiple times
computer.setKernelParameter("kernelName", "a", 0);
computer.setKernelParameter("kernelName", "b", 1);
computer.run("kernelName", 0, n , 64); 
computer.run("kernelName", 0, n , 64); 
computer.run("kernelName", 0, n , 64);
  • Method-chaining to build a parameter-list in one-line:
computer.compute(a.next(b),"kernelName", 0, n, 64); 
computer.compute(a.next(b),"kernelName", 0, n, 64); 
computer.compute(a.next(b),"kernelName", 0, n, 64);

both versions are equivalent with a trivial amount of extra host latency on second version.

What Kind of Load Balancing is Implemented?

  • dynamic: a queue is filled with many small pieces of work, then all devices independently consume the queue until it is empty. this has good work-distribution quality but high latency due to multiple synchronizations
  • static: work is divided into bigger chunks and they are directly sent to their own devices. After each run, device performances are calculated and a new(and better) work-distribution ratio is found for next run.

Static load balancing: good for uniform work-loads over work-items / data elements (simple image-processing algorithms, nbody algorithm, string-searching, etc)

// sample system: iGPU with 128 shaders @ 2GHz, dGPU with 384 shaders @ 1.5 GHz, CPU with 192 pipelines @ 5.3 GHz
computer.run("kernel", 0, n, 256); // equal work for all (50 milliseconds)
computer.run("kernel", 0, n, 256); // iGPU=1x work-items, dGPU=1.2x work-items, CPU=1.4x work-items (45 milliseconds)
computer.run("kernel", 0, n, 256); // iGPU=1x work-items, dGPU=1.5x work-items, CPU=2.0x work-items (33 milliseconds)
computer.run("kernel", 0, n, 256); // iGPU=1x work-items, dGPU=2.2x work-items, CPU=3.4x work-items (20 milliseconds)
computer.run("kernel", 0, n, 256); // iGPU=1x work-items, dGPU=2.4x work-items, CPU=3.7x work-items (17 milliseconds)
computer.run("kernel", 0, n, 256); // 15 milliseconds
computer.run("kernel", 0, n, 256); // 15 milliseconds

Dynamic load balancing: good for non-uniform work-loads (mandelbrot-set generation, ray tracing, etc)

// sample system: iGPU with 128 shaders @ 2GHz, dGPU with 384 shaders @ 1.5 GHz, CPU with 192 pipelines @ 5.3 GHz
// grain size = 2048 work-items (or 8x work-groups), can be any multiple of work group size
// local threads = 256 (work group size)
computer.runFineGrainedLoadBalancing("kernel", 0, n, 256,2048); // 20 milliseconds iGPU=1x work-items, dGPU=2.4x work-items, CPU=3.7x work-items (17 milliseconds)
computer.runFineGrainedLoadBalancing("kernel", 0, n, 256,2048); // 20 milliseconds
computer.runFineGrainedLoadBalancing("kernel", 0, n, 256,2048); // 20 milliseconds
computer.runFineGrainedLoadBalancing("kernel", 0, n, 256,2048); // 20 milliseconds
computer.runFineGrainedLoadBalancing("kernel", 0, n, 256,2048); // 20 milliseconds
computer.runFineGrainedLoadBalancing("kernel", 0, n, 256,2048); // 20 milliseconds (with 5 milliseconds of extra sync-latency for queue-processing + 15 milliseconds of computation)

with this version, n work-items are divided into chunks of 2048 and are computed from a shared queue between all devices. Faster devices naturally take more chunks from queue and the work load is automatically balanced.