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Imperial College, HPCE 2013 CW4 - OpenCL

Overal Goals

The overall goals of this coursework are to:

  • Develop familiarity with building OpenCL programs

  • Follow a simple methodology for isolating parts of an existing program which can move to a kernel

  • Understand the OpenCL primitives needed to create and execute a kernel

  • Examine and remove some of the communications bottlenecks which can reduce performance

  • Look at some simple techniques for improving GPU performance

This coursework alone is not intended to make you a GPU expert. You should know how to create an OpenCL program from scratch by the end, but the performance you get may not be be as high as a TBB version.

Checking your OpenCL environment

Before writing any code, it is worth checking your OpenCL environment and SDK, just to check that you have the right devices and things installed. There is a file called src/test_opencl.cpp, which is simple, but complete, OpenCL program. It doesn't do a lot, but it does allocate GPU buffers, and try to execute them.

Compile the program, and make sure you can both build and execute it. You may need to fiddle with your include and link directories to make it build, and find/download an SDK. While an OpenCL run-time is installed in many systems, there is not necessarily always an SDK.

Windows users

There are a number of SDKs available that can be downloaded, from AMD, Intel, NVidia, and so on. These SDKs can be pretty big, so you may want to use the extremely minimal version included in the distribution. This just contains the headers and libraries, and is appropriate for use in machines you don't control, where OpenCL is installed, but the SDK is not.

A subset of lab machines on level 5 have OpenCL capable GPUs, plus the drivers installed. However, they do not have any SDKs.

Linux users

You should be able to get a working OpenCL driver via your GPU vendor - often that will come with an SDK as well. If you don't have a GPU, or it isn't supported, you can try using a software OpenCL implementation. Intel provide a multi-core version, and the AMD OpenCL run-time also contains a software version, though you'll need to be root to install it.

Many DoC machines have NVidia cards, with the OpenCL drivers installed.

Mac users

As far as I know all Macs come with OpenCL installed these days, but the SDK may not be installed. I would imagine you can bring the SDK over with whatever package managed OSX has, as Apple are very integrated into the whole OpenCL thing.

The Heat World code

To make this somewhat realistic, you are working with an existing proprietary application. This is a very simplified version of a research problem I once personally had to work with, where I was given a big lump of code and asked to make it go faster. The overall structure is a classic example of a finite-difference or stencil operation, where you are applying some sort of local smoothing operation over huge numbers of time-steps. Such stencil calculations are very common, but different enough that one often can't use an off-the-shelf piece of software. Application areas where they arise include:

  • Electric and magnetic field models

  • Percolation and diffusion of dopants

  • Temperature propagation within devices and heat-sinks

  • Computational finance

The core of the code is in the two files include/heat.hpp and src/heat.cpp, with the header giving the API for creating, storing, loading, and stepping a model of a heat system.

As well as the core src/heat.cpp, there are also a number of driver programs:

  • src/make_world.cpp : Creates a new two-dimensional model, and gives it an initial state.

  • src/step_world.cpp : Reads a world from a file, and advances it by a chosen number of time-steps.

  • src/render_world.cpp : Takes the current state of the world, renders it to a bitmap.

Each of these source files must be compiled together with heat.cpp to produce an executable. There are no other dependencies on external libraries.

Use your chosen development environment to compile the three files, resulting in make_world, step_world and render_world (you can have them build in whatever directory you like). If you now do:

make_world 10 0.1

You'll see it create a text description of a heat world. At the top is the header, describing the dimensions of the world, and the propagation constant (0.1 in this case). The propagation constant measures how fast heat migrates between conductive cells in this world, so a constant of 0.01 would result in slower movement of heat.

Below the header you'll see a grid of 0, 1, and 2, defining the shape of the world. Any 2s represent insulator, where heat cannot flow, while any 1s represents fixed temperature cells, such as heat-sinks and heat-sources. All the 0s are "normal" conductive cells.

Below the world grid is the current temperature or state of every cell in the grid, with values between 0 and 1. After creating the world it is cold, except for a row of 1s near the top of the state which are all hot. If you compare these cells with the world grid, you'll see these 1s are fixed, so they will keep their value of 1 as time progresses. However, surrounding cells will change temperature in response to the cells around them.

Now advance the world, but a single time-step of length 0.1 seconds:

make_world.exe 10 0.1 | step_world 0.1 1

You'll see that the the cells next to the heat source row has warmed up. The warming is not uniform, as at the far left and far right the cells are up against insulators, so according to the simple model using here, they end up slightly warmer.

You can now advance the world by a few seconds:

make_world.exe 10 0.1 | step_world 0.1 100

You should see that some of the heat is making it's way around the right hand side, but it can't come through the insulator across the middle-left. Step forward by a 1000 seconds:

make_world.exe 10 0.1 | step_world 0.1 10000

and you'll see the world has largely reached a steady-state, with high temperatures around the top, then slowly decreasing temperatures through to the single point heat-sink at the bottom.

In such problems we are usually interested in looking at very high spatial resolutions (i.e. lots of cells), and very fine temporal resolutions. This sort of discretisation is most accurate when both time and space are very fine. To visualise much finer-grain scenarios, it is conventient to view it as a bitmap:

make_world.exe 100 0.1 | step_world 0.1 100 | render_world dump.bmp

If you look at dump.bmp, you should see insulators rendered in green, and the temperature shown from blue (0) to red (1). However, over a total time of 0.1*100=10 seconds, very little will have happened. To see anything happen, you'll need to up the time significantly:

make_world.exe 100 0.1 | step_world 0.1 100000 | render_world dump.bmp

It is getting fairly slow, even at a resolution of 100x100, but in practise we'd like to increase resolution 10x in all three dimensions (both spatial axes + time), which increases the compute time by a factor of 1000.

This type of computation is ideal for a GPU, as it maps very well to the grid-structure provided by both the hardware and the progamming model.

Preparing the code for OpenCL

Create a new folder within src called src/your_login. This is where your additional code will live (as before, in principle I should be able to compile everyone in the classes's code together at the end).

We're going to convert the original C++ code to OpenCL quite incrementally, keeping a trail of working versions as we go. In the real-world you might choose to do this via source/revision control, but it is often still useful to have some intermediate versions around to show the process to anyone maintaining your code (which might be you in a years time, when you've forgotten how it all works). If all you have is the original sequential and the highly optimised parallel version, then it can be very difficult to see how the two are actually related.

Examining the source code

The first thing to do is to look at the source code, and to identify opportunities for parallelism. If you open src/heat.cpp, you'll see a number of functions, but the computationally intensive part is the function hpce::StepWorld. This is responsible for moving the world forwards by n steps which each increment time by dt.

Within StepWorld there is some initial setup which allocates buffers and calculates constants, followed three nested loops. These loops represent the bulk of the time in the algorithm, and it should be clear that the execution time complexity is O(nwh). So broadly speaking if we double any one of n, w, or h then the execution time will double, and if we double all of n, w, and h then the execution time will be eight times longer.

Recalling your experience with vectorisation in matlab, you should immediately see opportunities for parallelism here: all the iterations of the two inner loops are independent, and so can be parallelised. However, there is a loop carried dependency across the time loop, similar to the erode filter we looked at in lectures. However, unlike the erode example, where there were multiple frames which could be pipelined, here we have a single world to update, so pipelining will not help.

Extracting the inner loop to a lambda

First we want to isolate the part of the program that will be moved to the GPU. Because the GPU works in a different memory space, it is useful to emulate that hard boundary in software first, before starting to convert to kernels.

Writing the code

Take the file src/heat.cpp, and copy it to a new file called src/your_login/step_world_v1_lambda.cpp. The original file contains multiple support and IO functions which we aren't interested in (we'll still be linking with the original src/heat.cpp, so cut out all the functions which aren't StepWorld. Rename the remaining StepWorld to StepWorldV1Lambda, and put it in a namespace called your_login. So you should be left with a file which looks something like:

#include "heat.hpp"

... // More include files

namespace hpce{
namespace your_login{

void StepWorldV1Lambda(world_t &world, float dt, unsigned n)
{
	... // Existing implementation
}

}; // namespace your_login
}; // namespace hpce

Our first step is to pull the two inner loops out into a lambda, starting to isolate the core loops. Pull the body of the inner-most loop into a seperate lambda called kernel_xy, which should take an x and y parameter, but capture everything else by reference (i.e. use the [&] lambda form).

The resulting code should look something like:

std::vector<float> buffer(w*h);

auto kernel_xy = [&](unsigned x, unsigned y)
{
	... // Original loop body
};

for(unsigned t=0;t<n;t++){
    for(unsigned y=0;y<h;y++){
        for(unsigned x=0;x<w;x++){
            kernel_xy(x,y);
        }  // end of for(x...
    } // end of for(y...
	
	... // Rest of code

Note that the lambda has to be declared after buffer, as if you try to declare it before, then it is impossible to close over the variable - you can't refer to something that hasn't been named yet.

Building the test framework

To compile this, we need some sort of driver program to load the world, call our function, and save the world. Fortunately, we already have such a program in src/step_world.cpp. Take the main from that program and paste it in below your existing code. Note that it should appear outside the namespaces, as main goes in the global namespace. Modify the call to hpce::StepWorld to hpce::your_login::StepWorldV1Lambda, to ensure it uses your new function.

Modify your build environment so that src/heat.cpp is compiled together with src/your_login/step_world_v1_lambda.cpp to produce a single executable called step_world_v1_lambda. Again, it is up to you where you choose to have it built.

By making sure the executables have different names, and we can still run old versions, we retain the ability to select between different implementations. Often students - I'm sure not you - resort to #ifdef and commented out code as they develop new versions of existing code.

Test your program still does the same as the original implementation in whatever way you think best. Note that you have both step_world and step_world_v1_lambda available at the same time, and that there are programs like diff (posix) and FC (windows) which will tell you if two text files are different. In bash this is particularly easy to do, and it can be incorporated into your makefile if you wish.

Making the isolation clean

Your lambda version captures everything by reference, so within kernel_xy there are references to world, buffer, and so on. For the GPU version the kernel cannot make any references to things which are not explictly passed as parameters, and the best way of emulating this is to move the kernel outside the stepping function into a pure function. In this context, "pure function" means a function that is only able to interact with the world via its parameters, and accesses no global state or captured variables.

OpenCL also doesn't allow complex C++ data-types to cross the CPU-GPU boundary, so we'll also need to convert those to primitive types or pointers to primitive types. For example, the kernel currently closes over world which is of type world_t, and buffer, which has type std::vector<float>. These need to be turned into parameters with primitive types with known size, such as uint32_t, float, and/or pointers to those types.

Based on your step_world_v1_lambda.cpp code, create a new file called step_world_v2_function.cpp in the same directory, and rename the inner function to hpce::your_login::StepWorldV2Function. Make sure you can build and run the file before proceeding.

Currently it will have a lambda called kernel_xy; take the lambda outside the StepWorldV2Function, and make it a new function called kernel_xy:

// Note the change from unsigned -> uint32_t to get a known size type
void kernel_xy(uint32_t x, uint32_t y)
{
    ... // Original lambda body
}

void StepWorldV2Function(world_t &world, float dt, unsigned n)
{
    ... // Original code, now without lambda

If you try compiling this, you'll see that it doesn't work - you'll get undefined errors about w, world, and so on. This is telling you all the information which will eventually have to be manually copied to the GPU, and if necessary back again afterwards. When accelerating for real, this is a good point to think about communication costs - have you really chosen the best point to cut the program at? Are there any things being copied across which could more cheaply be recalculated within the kernel?

Resolve all these errors by adding parameters to the definition of kernel_xy, and by adding arguments to the point where kernel_xy is called. For example, to resolve the error with w, you would modify the declaration:

void kernel_xy(uint32_t x, uint32_t y, uint32_t w);

and the use:

for(unsigned x=0;x<w;x++){
	kernel_xy(x,y, w);
}  // end of for(x...

Note that some things will require conversion from C++ types to primitive types. For example to convert world.state, you'll need to pass a pointer to the data:

void kernel_xy(uint32_t x, uint32_t y, uint32_t w, const float *world_state);

and:

for(unsigned x=0;x<w;x++){
	kernel_xy(x,y, w, &world.state[0]);
}  // end of for(x...

I used "const" on the world_state parameter to indicate that it is never modified inside the kernel. However, the converted buffer parameter cannot be const, as the kernel must write to the array to produce output. This process helps show which arrays must be copied both to and from the GPU, and which only need to be copied one way.

When it comes to passing the cell properties, note that cell_flags_t was originally defined to have the underlying type uint32_t, so you can safely cast to a (const uint32_t *) when converting parameters.

Keep adding parameters until you have removed all errors, making sure that you convert all of them to primitive numbers or pointers to arrays.

Test the function to make sure it still works. If so, we are now in a position to map it into a CPU kernel.

Intial conversion to OpenCL

We've now got a version of the code which has separated the computationally intensive part out into a function. This function has:

  • An iteration identifier (x,y), which identifies where the computation is occuring within the overall parallel iteration space. This will become our OpenCL global work id.

  • A number of scalar (non-pointer) parameters, such as w, which will be passed by value at the kernel invocation.

  • A number of non-scalar (pointer) parameters, which will need to be manually copied to the GPU before kernel execution, passed by reference when we invoke the kernel, and then manually copied back after execution finishes.

To convert this to OpenCL, first we'll handle the host code (the long boring bit), then look at the GPU kernel. We'll do all this in a file called src/your_login/step_world_v3_opencl.cpp, with a function called src::your_login::StepWorldV3OpenCL.

Create the new file based on the `src/your_login/step_world_v2_function.cpp' code, and check you can still build and run it. While we're setting up the OpenCL parts, don't worry too much about the data going in and out. You can simply test it using something equivalent to:

bin/make_world | src/your_login/step_world_v3_opencl > /dev/null

as for a while we'll just look at stderr.

Opening the context, compiling kernels

Because OpenCL supports many devices, there are a number of stages involved in starting things up and connecting to various devices. Before we even think about executing kernels, we need to:

  1. Choose an OpenCL platform, as there might be multiple platforms available in one machine. For example, there may be both Intel and NVidia platforms installed.

  2. Select one of the devices exposed by the platform. As an example, the AMD platform may expose both CPU and GPU OpenCL devices.

  3. Create an OpenCL context for the platform, which describes the environment within which we can launch kernels and manage memory buffers.

  4. Create and build our kernels. It is common to load and build the GPU kernel code at run-time, just before the kernel is first used.

These steps can often be simplified with library code, but here we'll rely on just the standard OpenCL C++ wrappers. These bindings come from "CL/cl.hpp", and are installed in most recent OpenCL SDKs, but just in case, there is a version in the include directory too. The functions provided by this header are all documented at the [Khronos site][1].

Add the include at the top of src/your_login/step_world_v3_opencl.cpp:

#define __CL_ENABLE_EXCEPTIONS 
#include "CL/cl.hpp"

The __CL_ENABLE_EXCEPTIONS definition tells the C++ wrapper that it should throw exceptions when things go wrong, rather than just returning an error code. This makes it much harder to accidentally ignore steps which went wrong.

At this point you may hit an error about CL/opencl.h not being found. Make sure that OpenCL is available, and that you have appropriately modified your include directories to point towards the include files in the OpenCL SDK. Assuming the initial test_opencl.cpp program compiled this should work fine.

Choosing a platform

We'll add the generic OpenCL initialisation at the start of StepWorldV3OpenCL, before doing anything with the world-specific code. First get a list of platforms from OpenCL:

std::vector<cl::Platform> platforms;
	
cl::Platform::get(&platforms);
if(platforms.size()==0)
	throw std::runtime_error("No OpenCL platforms found.");

After this code executes we'll have a list of installed platforms, which we can print the names of (in production code you would comment this out or make it optional):

std::cerr<<"Found "<<platforms.size()<<" platforms\n";
for(unsigned i=0;i<platforms.size();i++){
	std::string vendor=platforms[i].getInfo<CL_PLATFORM_VENDOR>();
	std::cerr<<"  Platform "<<i<<" : "<<vendor<<"\n";
}

At this point we could select just the first platform, but it is nice to be able to provide a way to select a specific platform. Often this would be by a configuration file, but we'll do it by an environment variable:

int selectedPlatform=0;
if(getenv("HPCE_SELECT_PLATFORM")){
	selectedPlatform=atoi(getenv("HPCE_SELECT_PLATFORM"));
}
std::cerr<<"Choosing platform "<<selectedPlatform<<"\n";
cl::Platform platform=platforms.at(selectedPlatform);

This means that by default it will select the first platform, but if we change an evironment variable (e.g. export HPCE_SELECTED_PLATFORM=1 in bash), then the executable will do something different. While not as good as a configuration file, this makes it possible to select what happens without recompiling. Again, I'm sure you don't use #ifdef and commenting to change functionality, but many less enlightened students do.

We now have a handle to our chosen platform in platform. This handle will automatically be deleted when the object goes out of scope, so there is no explicit handle management needed.

Choose a device

Within the platform there might be multiple devices, which we can enumerate:

std::vector<cl::Device> devices;
platform.getDevices(CL_DEVICE_TYPE_ALL, &devices);	
if(devices.size()==0){
	throw std::runtime_error("No opencl devices found.\n");
}
	
std::cerr<<"Found "<<devices.size()<<" devices\n";
for(unsigned i=0;i<devices.size();i++){
	std::string name=devices[i].getInfo<CL_DEVICE_NAME>();
	std::cerr<<"  Device "<<i<<" : "<<name<<"\n";
}

Depending on your system you may now see two devices. On my laptop, I get:

dt10@TOTO /cygdrive/e/_dt10_/documents/teaching/2013/hpce/cw/CW4-opencl
$ bin/make_world.exe 64 | bin/step_world_v3_opencl > /dev/null
Loaded world with w=64, h=64
Stepping by dt=0.1 for n=1
Found 1 platforms
  Platform 0 : Advanced Micro Devices, Inc.
Choosing platform 0
Found 2 devices
  Device 0 : Caicos
  Device 1 :         Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz

This shows I have one GPU device (Caicos), and one CPU device. Within each device there may be multiple CPUs, but here OpenCL is just listing the physical clusters available.

Despite one of these devices being a CPU, remember that it is just as valid OpenCL device as a GPU. It is probably not as fast or powerful, but it can still be programmed in the same way. On massively multi-core machines the CPU OpenCL device may in fact be very powerful, with the ability to exploit wide SIMD instructions.

As before we'll select device 0 by default, but give the option of changing that via an environment variable:

int selectedDevice=0;
if(getenv("HPCE_SELECT_DEVICE")){
	selectedDevice=atoi(getenv("HPCE_SELECT_DEVICE"));
}
std::cerr<<"Choosing device "<<selectedDevice<<"\n";
cl::Device device=devices.at(selectedDevice);

So for example I can switch between device 0 and 1 without re-compiling:

dt10@TOTO /cygdrive/e/_dt10_/documents/teaching/2013/hpce/cw/CW4-opencl
$ bin/make_world.exe 64 | bin/step_world_v3_opencl > /dev/null
Loaded world with w=64, h=64
Stepping by dt=0.1 for n=1
Found 1 platforms
  Platform 0 : Advanced Micro Devices, Inc.
Choosing platform 0
Found 2 devices
  Device 0 : Caicos
  Device 1 :         Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz
Choosing device 1

dt10@TOTO /cygdrive/e/_dt10_/documents/teaching/2013/hpce/cw/CW4-opencl
$ export HPCE_SELECT_DEVICE=1

dt10@TOTO /cygdrive/e/_dt10_/documents/teaching/2013/hpce/cw/CW4-opencl
$ bin/make_world.exe 64 | bin/step_world_v3_opencl > /dev/null
Loaded world with w=64, h=64
Stepping by dt=0.1 for n=1
Found 1 platforms
  Platform 0 : Advanced Micro Devices, Inc.
Choosing platform 0
Found 2 devices
  Device 0 : Caicos
  Device 1 :         Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz
Choosing device 1

Creating a context

To actually make use of the device, we need to create an OpenCL context, which represents a domain within which we can create and use kernels and memory buffers. Each object can only be used within the context it was created for, and it is unusual to have more than one context in existence at a time (though it is legal).

Contexts can actually cover multiple devices (though we won't use that ability here), so to create a context we pass a vector of devices. Conveniently, we still have such a vector in devices from when we enumerated the devices, which we can use to construct the context:

cl::Context context(devices);

That's about it for creating the context - it doesn't do a lot, but we need it as a parameter to all the kernel and memory buffer functions coming up.

Loading and building a kernel

The system is now pretty much initialised, and we can start doing application specific stuff. The first thing we want to do is create and copmpile our kernels. Each kernel is a piece of code compiled for the GPU, which can then be launched over a particular iteration space. But before writing the kernel, we'll write the host code to load and compile it.

Kernel code often exists as text files outside the program, and it is largely up to the user to get the text of the source code - OpenCL doesn't know or care where it comes from. Finding and locating this code at run-time can be a pain, and it is not uncommon to embed them as resources within the executable. For the purposes of this coursework we will assume that all cl code is in the src/your_login directory. So for example, the opencl code for this particular executable will eventually be placed at src/your_login/step_world_v3_kernel.cl. However, as a get-out, we will again define an environment variable called HPCE_CL_SRC_DIR which can be used to override that at run-time.

Having define where the code will be found, the first thing to do is load the file contents. This is trivial, but worth knowing how to do simply, so briefly read this, then integrate it into your file:

// To go at the top of the file
#include <fstream>
#include <streambuf>

// To go in hpce::your_login, just above StepWorldV3OpenCL
std::string LoadSource(const char *fileName)
{
	// Don't forget to change your_login here
	std::string baseDir="src/your_login";
	if(getenv("HPCE_CL_SRC_DIR")){
		baseDir=getenv("HPCE_CL_SRC_DIR");
	}
	
	std::string fullName=baseDir+"/"+fileName;
	
	std::ifstream src(fullName, std::ios::in | std::ios::binary);
	if(!src.is_open())
		throw std::runtime_error("LoadSource : Couldn't load cl file from '"+fullName+"'.");
	
	return std::string(
		(std::istreambuf_iterator<char>(src)), // Node the extra brackets.
        std::istreambuf_iterator<char>()
	);
}

We need to collect together sources into a program, which is the unit of compilation in OpenCL. In our case it will only contain one kernel, but in principle there could be many kernels, spread across multiple sources. So, returning to where we left off, just after creating the context:

std::string kernelSource=LoadSource("step_world_v3_kernel.cl");

cl::Program::Sources sources;	// A vector of (data,length) pairs
sources.push_back(std::make_pair(kernelSource.c_str(), kernelSource.size()+1));	// push on our single string

cl::Program program(context, sources);
program.build(devices);

At this point you should be able to compile and run the overall program, but it should crash because there is no such kernel.

Writing a kernel

As OpenCL host code (what we were just doing in C++), and OpenCL kernel code is written in a different language, we need to create a new file to represent it. Create a file called src/your_login/step_world_v3_kernel.cl, and copy the entire kernel_xy function into it. You can leave the original kernel_xy function where it is, so that the software path still works.

Your file src_cl/your_login/step_world_v3_kernel.cl should now look like:

void kernel_xy(unsigned x, unsigned y,
	unsigned w,
	float inner, float outer,
	const uint32_t *properties,
	const float *world_state,
	float *buffer
	)
{
	unsigned index=y*w + x;
	
	... // Rest of code
}

Kernel functions are prefixed with __kernel, so as a minimum step towards creating a working kernel, change the function type in the cl file to:

__kernel void kernel_xy(unsigned x, unsigned y, // Rest of kernel...

Try running your C++ program again. It should now be able to find the file, but then OpenCL will throw an exception when it tries to compile it. The underlying cause is syntax errors, just as with a malformed C program, but here we can't see them.

To expose the specific error messages, go back to the C++ file and modify the build statement to the following:

cl::Program program(context, sources);
try{
	program.build(devices);
}catch(...){
	for(unsigned i=0;i<devices.size();i++){
		std::cerr<<"Log for device "<<devices[i].getInfo<CL_DEVICE_NAME>()<<":\n\n";
		std::cerr<<program.getBuildInfo<CL_PROGRAM_BUILD_LOG>(devices[i])<<"\n\n";
	}
	throw;
}

This code is catching the error being thrown, and then extracting the build log for each device. If you run it again, you should see a whole bunch of complaints.

You'll see a number of errors, most likely including:

  • uint32_t is undefined : In the c/c++ world, unsigneds and integers have a platform specific width, so we chose uint32_t to give it a width. However, in the OpenCL kernel, the uint type is defined a 32-bit integer on all platforms, so we can substitute uint for uint32_t.

  • Pointers must have an address space : the compiler needs to know where pointers can possibly point, and how they might be shared. We'll use global memory to start with, so add the __global anotation to the pointer parameters.

  • Cell_Fixed is not defined. These definitions were not brought over from the C++, so simply add enum definitions to the cl file:

      enum cell_flags_t{
  	Cell_Fixed		=0x1,
  	Cell_Insulator	=0x2
  };

  • std::min is not defined. OpenCL contains a number of equivalent built-ins such as min, max, sin, and cos, for use in kernels. You can simply delete the std:: part to get the builtin function.

After dealing with the errors, the code should now happily build. However, we are not done yet, as unlike tbb::parallel_for, the iteration space for OpenCL is embedded directly in the language. So rather than passing x and y as parameters, we will need to get the indices out of special indexing registers.

There are two types of builtin indices:

  • size_t get_local_id(uint dimindx) : This returns the position within the local work-group. Threads within a local work-group can communicate via local memory, and the size of the local group is limited by hardware resources.

  • size_t get_global_id(uint dimindx) : This returns the position within the entire work-group. Threads in different local work-groups cannot communicate via local memory, but can communicate via (slower) global memory, and the size of the work-group is almost unlimited.

The dimensionality of each id is determined by the dimension of the iteration space used at launch-time (which is coming up soon). In this specific case we are using a 2D iteration space, and our memory pointers are global, so we can extract the x and y co-ordinates from the global work-group. As well as the co-ordinates, the run-time and hardware also know the sizes of the groups via get_local_size and get_global_size, so we can also remove the w parameter:

__kernel void kernel_xy(
	float inner, float outer,
	__global const uint *properties,
	__global const float *world_state,
	__global float *buffer
	)
{
	uint x=get_global_id(0);
	uint y=get_global_id(1);
	uint w=get_global_size(0);
	
	unsigned index=y*w + x;
	
	... // Original code

In principle, this should now be a fully working OpenCL kernel!

Creating and executing kernels

We now have all the static infrastructure in place, with our devices and context open, and our OpenCL code compiled. At this point we have the choice of starting up multiple tasks which make use of the program we compiled, as the OpenCL interface is inherently multi-threaded, and there is nothing to stop multiple threads or tasks sharing a device.

However, within each thread we would have to make sure that all OpenCL tasks are synchronised and co-ordinated with each other (regardless of what other threads are doing). The mechanism for doing that is called a command queue, and as it suggests, it is a queueing or scheduling structure for issuing commands to the device. These commands can relate both to data movement between CPU and GPU, and to the actual execution of kernels on the GPU. It is essentially a way of building dependency graphs at run-time, then letting the GPU execute those graphs in the most efficient way possible.

To actually execute our kernel, we'll need to follow these steps:

  1. Allocate on-GPU memory buffers for the non-scalar parameters.

  2. Bind any fixed input parameters to the kernel.

  3. Create a command queue associated with the context.

  4. Copy and fixed input data to the GPU memory buffers.

  5. Bind any dynamic input parameters to the kernel.

  6. Copy any input data to the GPU memory buffers.

  7. Schedule the kernel onto the command queue. This (finally!) is where the actual execution happens.

  8. Copy any results back from GPU memory buffers.

Allocating buffers

We have three arrays in use while stepping the world:

  • properties : A read-only array describing the static structure of the world. This does not change across all iterations within the world (i.e. for multiple iterations of the outer t loop).

  • state : A read-only array containing the current state of the world. However, this array does change between each iteration.

  • buffer : A write-only array where we build up the output of each iteration.

Each of these arrays uses one float or uint32_t per cell, so the total bytes for each buffer is 4*world.w*world.h. We can allocate space for those arrays on the GPU's local memory using cl::Buffer. First we'll create the properties array:

size_t cbBuffer=4*world.w*world.h;
cl::Buffer buffProperties(context, CL_MEM_READ_ONLY, cbBuffer);
cl::Buffer buffState(context, CL_MEM_READ_ONLY, cbBuffer);
cl::Buffer buffBuffer(context, CL_MEM_WRITE_ONLY, cbBuffer);

So we have now allocated the buffers on the GPU, but don't have any idea about what data is on them. The flags about READ and WRITE are hints to the GPU about how the buffers will be used from the kernel - you can always pass CL_MEM_READ_WRITE and not think about it, but if you can determine how memory will be accessed, it is possible the OpenCL runtime can optimise memory transfers.

Setting kernel parameters

It is often the case that some kernel parameters are constant through multiple uses of the kernel, in which case we can create the kernel and bind those parameters while setting things up. In our specific case, all our parameters are actually fixed, so we can set them all up before entering the loop (in a later optimisation we will have to modify some of them as we execute).

To set the kernel parameters, we first have to create an instance of our kernel which we can set the parameters on and schedule for execution. Just after the buffers, add:

cl::Kernel kernel(program, "kernel_xy");

This will find the compiled kernel called kernel_xy in the program, and create a new instance of it. We may wish to have multiple instances of the same kernel in order to schedule parallel copies with different parameters, but here we need just one.

Parameters are bound to the kernel using the setArg member function, and are set by index, not name. So we can quickly go and look in our kernel definition to count off the indices:

__kernel void kernel_xy(
	float inner,	// 0
	float outer,	// 1
	__global const uint *properties,	// 2
	__global const float *world_state,	// 3
	__global float *buffer	// 4
);

Don't worry if your kernel parameters are in a different order, just note which order they occur in.

Given the order, you can then bind the arguments in the C++ code:

kernel.setArg(0, inner);
kernel.setArg(1, outer);
kernel.setArg(2, buffProperties);
kernel.setArg(3, buffState);
kernel.setArg(4, buffBuffer);

You'll need to make sure that this occurs after the inner and outer variables are declared, but before the loop is entered.

The setArg function is overloaded, so it knows how to deal with scalar parameters such as inner as well as non-scalar buffers like buffState. However, you cannot bind a raw c++ pointer, such as a float *: at best you will get a compile-time error, and at worst it will crash unpredictably at run-time.

Creating a command queue

This is our final part of our overall application setup before moving inside the execution loop. As with the context, this one is quite simple. After setting the kernel arguments, simply create a command queue:

cl::CommandQueue queue(context, device);

Note that the command queue is bound to a single specific device, even though there may be multiple devices in our context. If we want to use multiple devices, we need multiple command queues.

Copying over fixed data

The properties array is constant across all iterations, so we may as well copy it over to the GPU just once, before starting the iterations. Copies are scheduled to the command queue using enqueueWriteBuffer, which copies a specified area from host (CPU) memory to device (GPU) memory:

queue.enqueueWriteBuffer(buffProperties, CL_TRUE, 0, cbBuffer, &world.properties[0]);

The parameters to the function are:

  • buffProperties : The GPU buffer we're writing too.

  • CL_TRUE : A flag to indicate we want synchronous operation, so the function will not complete until the copy has finished.

  • 0 : The starting offset within the GPU buffer.

  • cbBuffer : The number of bytes to copy.

  • &world.properties[0] : Pointer to the data in host memory we want to copy.

Check that your program still compiles and runs. At this point it should still perform its normal function using software, but we are about to rip that out.

So assuming everything compiles and runs, we will now move into the loop over t. The part we are going to replace is the double loop over y and x, so:

  1. Check you know what the two loops are doing, and their loop bounds.

  2. Delete both inner loops.

You should be left with just a loop over t, with a call to std::swap and the time increment at the end. At this point the program is now broken, until we get the kernel working.

Copying memory buffers

The first thing to do within the loop is to copy the current state over to the GPU:

cl::Event evCopiedState;
queue.enqueueWriteBuffer(buffState, CL_FALSE, 0, cbBuffer, &world.state[0], NULL, &evCopiedState);

Unlike the previous copy, we are doing this one asynchronously, shown by the second parameter being CL_FALSE rather than CL_TRUE. That means that the call to enqueueWriteBuffer can return even before the copy has completed. We will have other operations which need the copy to complete before they can start, so we pass a pointer to evCopiedState to the function. This event can then be passed to any future function which must wait for this copy, explicitly indicating the dependency to the OpenCL run-time. Once this write has finished, the evCopiedState even will be signalled, releasing any waiting tasks.

Executing the kernel

Finally! A lot of GPU programming involves setting up a whole load of infrastructure before actually executing a kernel. Most of it is boiler-plate, and can be hidden by libraries, but it is all leading up to this:

cl::NDRange offset(0, 0);				// Always start iterations at x=0, y=0
cl::NDRange globalSize(w, h);	// Global size must match the original loops
cl::NDRange localSize=cl::NullRange;	// We don't care about local size

std::vector<cl::Event> kernelDependencies(1, evCopiedState);
cl::Event evExecutedKernel;
queue.enqueueNDRangeKernel(kernel, offset, globalSize, localSize, &kernelDependencies, &evExecutedKernel);

The first part is setting up the iteration space, with the offset specifying that both x and y start and zero, and globalSize saying that we should loop up to x<w and y<h. The localSize parameter determines clustering of threads into local work-groups, which we don't yet care about here (as we're only using global work-groups).

The second part establishes the dependencies of the kernel, by creating a vector of all the things that must complete before the kernel can run. For this case, there is only one dependency: the copy must have completed, so the event evCopiedState must have completed. We also produce an event called evExecutedKernel, so that we can tell when this kernel finishes.

After enqueueing the kernel, we don't know whether the copy has finished, or the kernel has finished. Maybe both have, maybe neither has. We cannot assume anything until we synchronise.

Copying the results back

The final job is to copy the results back after the kernel finishes:

std::vector<cl::Event> copyBackDependencies(1, evExecutedKernel);
queue.enqueueReadBuffer(buffBuffer, CL_TRUE, 0, cbBuffer, &buffer[0], &copyBackDependencies);

The copy cannot start until the kernel has finished, so we have to indicate the depdendency on evExecutedKernel. However, after this completes we need to interact with buffer, so we make this call synchronous using CL_TRUE.

And that's it. You should now have (laboriously) accelerated and existing C++ program. If all has gone well, the program should correctly spawn the calculation on the CPU, and perform all steps correctly. But as always: check to be sure.

Is it fast?

The whole point of this exercise was to make things faster, and if you time things as they stand, you may not be that impresssed:

dt10@TOTO /cygdrive/e/_dt10_/documents/teaching/2013/hpce/cw/CW4-opencl
$ time (bin/make_world.exe | bin/step_world 0.1 1 > /dev/null)
Loaded world with w=128, h=128
Stepping by dt=0.1 for n=1

real    0m0.287s
user    0m0.372s
sys     0m0.015s

dt10@TOTO /cygdrive/e/_dt10_/documents/teaching/2013/hpce/cw/CW4-opencl
$ time (bin/make_world.exe | bin/step_world_v3_opencl 0.1 1 > /dev/null)
Loaded world with w=128, h=128
Stepping by dt=0.1 for n=1
Found 1 platforms
  Platform 0 : Advanced Micro Devices, Inc.
Choosing platform 0
Found 2 devices
  Device 0 : Caicos
  Device 1 :         Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz
Choosing device 1
Exception : clEnqueueNDRangeKernel

real    0m0.545s
user    0m0.498s
sys     0m0.046s

So... half the speed. But first, lets see how much is due to actual processing time:

dt10@TOTO /cygdrive/e/_dt10_/documents/teaching/2013/hpce/cw/CW4-opencl
$ time (bin/make_world.exe | bin/step_world 0.1 0 > /dev/null)
Loaded world with w=128, h=128
Stepping by dt=0.1 for n=0

real    0m0.283s
user    0m0.342s
sys     0m0.015s

So at the moment the CPU version is essentially taking no CPU time, and all the cost is in std::cin and std::cout (remember the comment about fscanf and fprintf being major bottlenecks). Also, observe that the user time is greater than the real time, so there is some overlapped processing going on here due to pipes - as make_world generates data, step_world is already reading and parsing it.

However, the problem we want to solve is:

  1. Large worlds. The greater the resolution of the world, the more precise the model, and the less the effect of spatial discretisaion.

  2. Small timesteps and large time periods. Smaller values of dt will give less discretisation error, resulting in a more accurate model.

Let's eliminate the generation, and isolate just the execution time a bit more. We'll still have parsing and output time, but it should be the same for both:

dt10@TOTO /cygdrive/e/_dt10_/documents/teaching/2013/hpce/cw/CW4-opencl
$ time (cat world.txt | bin/step_world_v3_opencl 0.1 1000 > /dev/null)
Loaded world with w=1024, h=1024
Stepping by dt=0.1 for n=1000
Found 1 platforms
  Platform 0 : Advanced Micro Devices, Inc.
Choosing platform 0
Found 2 devices
  Device 0 : Caicos
  Device 1 :         Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz
Choosing device 0

real    0m22.679s
user    0m15.990s
sys     0m0.373s

dt10@TOTO /cygdrive/e/_dt10_/documents/teaching/2013/hpce/cw/CW4-opencl
$ time (cat world.txt | bin/step_world 0.1 1000 > /dev/null)
Loaded world with w=1024, h=1024
Stepping by dt=0.1 for n=1000

real    0m23.690s
user    0m23.634s
sys     0m0.045s

dt10@TOTO /cygdrive/e/_dt10_/documents/teaching/2013/hpce/cw/CW4-opencl
$ time (cat world.txt | bin/step_world 0.1 0 > /dev/null)
Loaded world with w=1024, h=1024
Stepping by dt=0.1 for n=0

real    0m15.432s
user    0m15.381s
sys     0m0.107s

Pulling the times apart, we have:

  • No processing: 15.432
  • CPU processing: 23.69, effectively 8.26 secs ignoring IO
  • GPU processing: 22.7, effectively 7.27 secs ignoring IO

So a speedup of... 1.13 times! Amazing! If you care to make the minor modifications needed to use TBB, you'll see that it actually does much better on this problem and you should see something close to linear scaling (e.g. 3x faster on 4 CPUs).

So what is wrong? We have a number of problems here:

  1. Unecessary copying : we are copying the new state from GPU buffer back to CPU buffer, doing nothing with it, then copying the same data back again.

  2. Too little work per thread : there is a fixed scheduling cost in order to spin a GPU thread up. If there is not much work to do, then the scheduling overhead outweighs the benefits.

  3. No memory optimisations : the current approach places all data in global memory, which is the slowest memory type in the GPU. We should be taking advantage of private and shared memory as well.

Optimising memory accesses

The first thing to try is to remove the redundant memory copies: if you look at StepWorldV3OpenCL the enqueue read at the bottom of the time loop will usually end up getting copied straight back over to the GPU at the top of the next iteration. We only look at the data right at the end, so there is no point bringing it back to the CPU in-between. So the only time the data needs to be transferred is:

  1. Before the very first execution of the kernel

  2. After the very last execution of the kernel.

We can achieve this by moving the write of buffState off the top of the loop, and the read of buffBuffer off the bottom. Note that as you move the read over the call to std::swap, the target of the read should change: instead of reading into &buffer[0], it should read directly into &world.state[0]. To keep things simple, make both the copies synchronous, so pass CL_TRUE instead of CL_FALSE, and remove any input and output events.

The only thing that will be left in the time loop is the enqueue of the kernel, and the swap to flip the world state with the buffer. First let's fix the buffer, by observing that swapping the host buffers now has no effect, and we need to swap the GPU buffers being passed to the kernel instead. One possibility would be to dynamically call setArg within the loop, but it is simpler to just swap the device buffers instead of swapping the host buffers:

std::swap(buffState, buffBuffer);	// Used to be std::swap(world.state, buffer)

Now we need to fix the kernel, as currently it has dependencies on events that no longer exist, so remove the input dependency list and the output event. You'll now have a loop which just enqueues kernels, without knowing when they have executed. The problem is that there is no fixed ordering between the kernels: the run-time could queue up 10 or 20 calls to enqueue kernel, then issue them all in parallel.

However, we need one kernel to finish before the next one starts, in order to make sure the output buffer is completely written before the next kernel call uses it as an input buffer. There are multiple solutions to this, but the easiest is to add a call to enqueueBarrier just after the kernel is enqueue'd:

queue.enqueueNDRangeKernel(kernel, offset, globalSize, localSize);
	
queue.enqueueBarrier();	// <- new barrier here

std::swap(buffState, buffBuffer);

This creates a synchronisation point within the command queue, so anything before the barrier must complete before anything after the barrier can start. However, it doesn't say anything about the relationship with the C++ program. It is entirely possible that the host program could queue up hundreds of kernel calls and barriers before the OpenCL run-time gets round to executing them. Indeed, it may only be the synchronised memory copy after the loop that forces the chain of kernels and barriers to run at all.

The kernel code itself is identical to that for the previous implementation, so you don't need to create a new .cl file. Sometimes you can optimise execution purely by modifying the host code, and other times tweaks to the kernel code are all that is needed.

A final problem is that our memory buffers were originally declared with read-only and write-only flags, but now we are violating that requirements. Go back to your buffer declarations, and modify them to:

cl::Buffer buffState(context, CL_MEM_READ_WRITE, cbBuffer);
cl::Buffer buffBuffer(context, CL_MEM_READ_WRITE, cbBuffer);

Depending on your platform, you should now see a reasonable speed-up over software (though probably still not over TBB). On my computer, I get:

dt10@TOTO /cygdrive/e/_dt10_/documents/teaching/2013/hpce/cw/CW4-opencl
$ time (cat world.txt | bin/step_world_v4_double_buffered 0.1 1000 > /dev/null)
Loaded world with w=1024, h=1024
Stepping by dt=0.1 for n=1000
Found 1 platforms
  Platform 0 : Advanced Micro Devices, Inc.
Choosing platform 0
Found 2 devices
  Device 0 : Caicos
  Device 1 :         Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz
Choosing device 0

real    0m19.922s
user    0m18.297s
sys     0m0.919s

This reduces the non-IO execution time down to 19.9-15.4 = 4.5 seconds, for a 1.84x speedup.

Optimising memory accesses

One of the biggest problems in GPU programming is managing the different memory spaces. Slight differences in memory layout can cause large changes in execution time, while moving arrays and variables from global to private memory can have huge performance implications. In general, the best memory accesses are those which never happen, so it is worth-while trying to optimise them out. GPU compilers can be more conservative then CPU compilers, so it is a good idea to help them out.

Looking at our current kernel, we can see that there are five reads to the properties array for a normal cell (non insulator). However, four of those reads are getting very little information back, as we only depend on a single bit of information from the four neighbours. A good approach in such scenarios is to try to pack data into spare bit-fields, increasing the information density and reducing the number of memory reads. In this case, we already have one 32-bit integer describing the properties, of which only 2 bits are currently being used. So as well as the 2 bits describing the properties of the current cell, we could quite reasonably include four bits describing whether the four neigbouring cells are insulators or not, saving a memory access.

This requires two modifications: one in the host code to set up the more complex flags, and another in the kernel code to take advantage of the bits.

Kernel code

At the top of the code, read the properties for the current cell into a private uint variable. This value will be read once into fast private memory, and then from then on it can be accessed very cheaply. For each of the branches, rewrite it to check bit-flags in the variable. For example, if your private properties variable is called myProps, and you decided bit 3 of the properties indicated whether the cell above is an insulator, the first branch could be re-written to:

// Cell above
if(myProps & 0x4) {
	contrib += outer;
	acc += outer * world_state[index-w];
}

The other neighbours will need to depend on different bits in the properties.

Host code

In the host code, you need to make sure the the correct flags have been inserted into the properties buffer. This should only have local effect, so we cannot modify world.properties directly. Instead create a temporary array in host memory:

std::vector<uint32_t> packed(w*h, 0);

and fill it with the appropriate bits. This will involve looping over all the co-ordinates, following the following process at each (x,y) co-ordinate:

packed(x,y) = world.properties(x,y)
if cell(x,y) is normal:
	if packed(x,y-1)(
		packed(x,y) = packed(x,y) + 4
	if packed(x,y+1)(
		packed(x,y) = packed(x,y) + 8
	# Handle left and right cases

This process takes some time (though it could be paralellised), but we don't care too much, as it only happens once for multiple time loops. Once the array is prepared, it can be transferred to the GPU instead of the properties array.

This got my laptop up to about 2.5x speedup, so faster than the two cores in my device can go. On a machine with a research-level GPU, I was seeing 100x speed-up over the original sequential C++ code (compiled with optimisations turned on).

Further optimisations

In this context of this particular coursework, we won't go any further, but the next issue to tackle would be the small amount of work being done by each thread. At each co-ordinate there are only a handful of operations performed, do it would be good to batch up or agglomerate some of those operations, to reduce scheduling overhead. This agglomeration could be temporal, but partially unrolling multiple iterations, or spatial, but grouping together consecutive iterations into one larger iteration.

Another optimisation is to cache the compiled binary for the kernel code. Depending on the device, the run-time, and other system characteristics, compiling the binary may be very fast, or quite slow. For example, on my desktop it takes well under a second, but on my laptop it takes around 10 seconds, both using the AMD OpenCL implementation. For large numbers of time-steps this cost will shrink, but for short-lived programs it is a potentially significant cost.

Deliverables

Clean your directory of object files and executables, making sure not to delete the .cpp and .cl files. I'd prefer if you left the build artefacts (makefiles, projects) in, as it is useful to see how you have handled it, but this is not part of the marks.

So you should end up with the following set of files that you have created:

  • src/your_login/step_world_v1_lambda.cpp

  • src/your_login/step_world_v2_function.cpp

  • src/your_login/step_world_v3_opencl.cpp

  • src/your_login/step_world_v3_kernel.cl

  • src/your_login/step_world_v4_double_buffered.cpp

  • src/your_login/step_world_v5_packed_properties.cl

  • src/your_login/step_world_v5_kernel.cl

Each implementation should perform the same step as the original step_world. While there may be minor numerical differences due to the single-precision arithmetic, there should be no major differences in the output over time.

Zip up your directory, and submit it via blackboard.

Debugging tips

Debug output

In this particular exercise, you shouldn't encounter too many probles, but one useful feature of modern GPUs is that they allow debugging output, even from within kernels running in the GPU. In OpenCL 1.2 the printf support is enabled by default, but for other platforms you may need to use a #pragma to enable it. For example, in AMD platforms you can add the following to your kernel code:

#pragma OPENCL EXTENSION cl_amd_printf : enable

and after that it is possible to use printf. Note that the printf output from different GPU threads may become interleaved - they are executing truly in parallel, after all.

Software implementations

Another useful trick when debugging is to fall back on a software OpenCL implementation if possible. This can sometimes make it more obvious where a particular memory address is causing problems, as the exception occurs on the processor, rather than in the GPU.

Dynamically changing kernels

When OpenCL kernels are held as seperate files, it makes it very easy to play around with them without recompiling. For example, when trying to debug a particular crash or bug in a new or optimised kernel, you can easily comment out particular lines and re-run the program. Each kernel gets compiled anyway, so there is very little overhead to experimenting.

Locked up and crashing machines

Some systems are more or less stable than others when it comes to OpenCL. In principle it should be impossible to cause any problems using OpenCL code, and each process should still be isolated from each other. However, in practise the drivers can be a bit buggy, and it is not unheard of to see a blue-screen or kernel panic, The AMD drivers and hardware are particularly problematic in this regard, at least historically.

However, any such problems are usually due to incorrect kernel code, which is reading or writing where it shouldn't. A good starting point is to comment out suspicious memory writes, or to try working within a software OpenCL implementation as suggested above.

Even in a correct program, you may find that the OpenCL code can cause the machine to stutter or lag, in a way that is quite different to standard CPU load. This is usually caused due to time-slicing of the GPU with the graphics display. Older cards cannot run multiple kernels at the same time, so if the system is running your code, it cannot update the windows and display. It is generally a good idea to limit the execution time of any one kernel, so if possible tune things so that any given thread does not take more a few hundred milli-seconds. If you are on a server this is less of an issue, but even there you may find that the OS aborts kernel calls which take multiple seconds.

[1] - http://www.khronos.org/registry/cl/specs/opencl-cplusplus-1.2.pdf