## Enqueueing a Kernel
#### The queue
* In SYCL all work is submitted via commands to a `queue`.
* The `queue` has an associated device that any commands enqueued to it will target.
* There are several different ways to construct a `queue`.
* The most straight forward is to default construct one.
* This will have the SYCL runtime choose a device for you.
#### Precursor
* In SYCL there are two models for managing data:
* The buffer/accessor model.
* The USM (unified shared memory) model.
* Which model you choose can have an effect on how you enqueue kernel functions.
* For now we are going to focus on the buffer/accessor model.
#### Command groups
* In the buffer/accessor model commands must be enqueued via command groups.
* A command group represents a series of commands to be executed by a device.
* These commands include:
* Invoking kernel functions on a device.
* Copying data to and from a device.
* Waiting on other commands to complete.
#### Composing command groups
* Command groups are composed by calling the `submit` member function on a `queue`.
* The `submit` function takes a command group function which acts as a factory for composing the command group.
* The `submit` function creates a `handler` and passes it into the command group function.
* The `handler` then composes the command group.
#### Composing command groups
gpuQueue.submit([&](handler &cgh){
/* Command group function */
});
* The `submit` member function takes a C++ function object, which takes a reference to a `handler`.
* The function object can be a lambda expression or a class with a function call operator.
* The body of the function object represents the command group function.
#### Composing command groups
gpuQueue.submit([&](handler &cgh){
/* Command group function */
});
* The command group function is processed exactly once when `submit` is called.
* At this point all the commands and requirements declared inside the command group function are processed to produce a command group.
* The command group is then submitted asynchronously to the scheduler.
#### Composing command groups
gpuQueue.submit([&](handler &cgh){
/* Command group function */
}).wait();
* The `queue` will not wait for commands to complete on destruction.
* However `submit` returns an `event` to allow you to synchronize with the completion of the commands.
* Here we call `wait` on the `event` to immediately wait for it to complete.
* There are other ways to do this, that will be covered in later lectures.
#### Scheduling
* Once `submit` has created a command group it will submit it to the scheduler.
* The scheduler will then execute the commands on the target device
once all dependencies and requirements are satisfied.
#### Scheduling
* The same scheduler is used for all queues.
* This allows sharing dependency information.
#### Enqueueing SYCL Kernel Functions
class my_kernel;
gpuQueue.submit([&](handler &cgh){
cgh.single_task<my_kernel>([=]() {
/* kernel code */
});
}).wait();
* SYCL kernel functions are defined using one of the kernel function invoke APIs provided by the `handler`.
* These add a SYCL kernel function command to the command group.
* There can only be one SYCL kernel function command in a command group.
* Here we use `single_task`.
class my_kernel;
gpuQueue.submit([&](handler &cgh){
cgh.single_task<my_kernel>([=]() {
/* kernel code */
});
}).wait();
* The kernel function invoke APIs take a function object representing the kernel function.
* This can be a lambda expression or a class with a function call operator.
* This is the entry point to the code that is compiled to execute on the device.
class my_kernel;
gpuQueue.submit([&](handler &cgh){
cgh.single_task<my_kernel>([=]() {
/* kernel code */
});
}).wait();
* Different kernel invoke APIs take different parameters describing the iteration space to be invoked in.
* Different kernel invoke APIs can also expect different arguments to be passed to the function object.
* The `single_task` function describes a kernel function that is invoked exactly once, so there are no additional parameters or arguments.
class my_kernel;
gpuQueue.submit([&](handler &cgh){
cgh.single_task<my_kernel>([=]() {
/* kernel code */
});
}).wait();
* The template parameter passed to `single_task` is used to name the kernel function.
* This is necessary when defining kernel functions with lambdas to allow the host and device compilers to communicate.
* SYCL 2020 allows kernel lambdas to be unnamed, but not all implementations support that yet.
#### SYCL kernel function rules
* Must be defined using a C++ lambda or function object, they cannot be a function pointer or std::function.
* Must always capture or store members by-value.
* SYCL kernel functions declared with a lambda ~~must be named using a forward declarable C++ type, declared in global scope~~ can be anonymous since SYCL 2020!
* SYCL kernel function names follow C++ ODR rules, which means you cannot have two kernels with the same name.
#### SYCL kernel function restrictions
* No dynamic allocation
* No dynamic polymorphism
* No function pointers
* No recursion
#### Kernels as function objects
class my_kernel;
queue gpuQueue;
gpuQueue.submit([&](handler &cgh){
cgh.single_task<my_kernel>([=]() {
/* kernel code */
});
}).wait();
* All the examples of SYCL kernel functions up until now have been defined using lambda expressions.
#### Kernels as function objects
struct my_kernel {
void operator()(){
/* kernel function */
}
};
* As well as defining SYCL kernels using lambda expressions,
You can also define a SYCL kernel using a regular C++ function object.
#### Kernels as function objects
struct my_kernel {
void operator()(){
/* kernel function */
}
};
queue gpuQueue;
gpuQueue.submit([&](handler &cgh){
cgh.single_task(my_kernel{});
}).wait();
* To use a C++ function object you simply construct an instance of the type and pass it to `single_task`.
* Notice you no longer need to name the SYCL kernel.
#### Streams
* A `stream` can be used in a kernel function to print text to the console from the device, similarly to how you would with `std::cout`.
* The `stream` is a buffered output stream so the output may not appear until the kernel function is complete.
* The `stream` is useful for debugging, but should not be relied on in performance critical code.
#### Streams
sycl::stream(size_t bufferSize, size_t workItemBufferSize, handler &cgh);
* A `stream` must be constructed in the command group function, as a `handler` is required.
* The constructor also takes a `size_t` parameter specifying the total size of the buffer that will store the text.
* It also takes a second `size_t` parameter specifying the work-item buffer size.
* The work-item buffer size represents the cache that each invocation of the kernel function (in the case of `single_task` 1) has for composing a stream of text.
#### Streams
class my_kernel;
queue gpuQueue;
gpuQueue.submit([&](handler &cgh){
auto os = sycl::stream(1024, 1024, cgh);
cgh.single_task<my_kernel>([=]() {
/* kernel code */
});
}).wait();
* Here we construct a `stream` in our command group function with a buffer size of `1024` and a work-item size of also `1024`.
* This means that the total text that the stream can receive is 1024 bytes.
#### Streams
class my_kernel;
queue gpuQueue;
gpuQueue.submit([&](handler &cgh){
auto os = sycl::stream(1024, 1024, cgh);
cgh.single_task<my_kernel>([=]() {
os << "Hello world!\n";
});
}).wait();
* Next we capture the `stream` in the kernel function's lambda expression.
* Then we can print `"Hello World!"` to the console using the `<<` operator.
* This is where the work-item size comes in, this is the cache available to store text on the right-hand-size of the `<<` operator.
## Questions
#### Exercise
Code_Exercises/Enqueueing_a_Kernel/source
Implement a SYCL application which enqueues a kernel function to a device and streams "Hello world!" to the console.