Before going further, let's understand the difference between a CPU and a GPU. For reminder the CPU is what executes your Rust program, while the GPU is what we are trying to interface with.
Both the CPU and the GPU execute instructions one by one. The instructions available for regular programs that run on the CPU include for example modifying a value in memory or performing some mathematical operation.
The instructions that a GPU can execute are the same, except that they can operate on a lot of data at once. You can for example instruct the GPU to multiply thirty-two values by a constant, in approximately the same time that a CPU would take to multiply a single value by that constant.
This is what makes GPUs very good at parallel computations which require executing the same sequence of operation on multiple values. While a CPU would perform this sequence on each value one by one, a GPU can perform it on multiple values at once.
Note: See also SIMD.
Note: In a previous section we talked about queues. These queues are usually foremost software queues, and not actual hardware constructs.
Vulkan (or any other API) doesn't let you directly control the threading aspect of the GPU. In order to perform an operation with multiple values at once, you will only need to indicate the list of operations to perform on one value. The Vulkan implementation will automatically make the necessary adjustments to make your operation run on multiple values at once.
This makes using a GPU much easier than if you had to manually control everything. However you still need to be aware that your program will run multiple times in parallel, because it has consequences on what you can do without causing data races.
For the purpose of this guide, we are going to do something very simple: we are going to multiply 65536 values by the constant 12. Even though this doesn't serve any purpose, it is a good starting point example. Most real-world usages of the GPU involve complex mathematical algorithms, and thus are not really appropriate for a tutorial.
As explained above, you don't need to use any
for loop or anything similar of that sort. All we
have to do is write the operation that is performed on one value, and ask the GPU to execute
it 65536 times. Our operation here is therefore simply (in pseudo-code):
// `index` will range from 0 to 65536 buffer_content[index] *= 12;
While it may look like this code multiplies a single value by 12, in reality the Vulkan implementation will automatically handle all the details that make it possible to run this in parallel multiple times in the most optimized way.
As a preliminary action we are going to create the buffer that will contain the values. This is similar to what we already did twice:
let data_iter = 0 .. 65536; let data_buffer = CpuAccessibleBuffer::from_iter(device.clone(), BufferUsage::all(), data_iter).expect("failed to create buffer");
data_buffer buffer now contains the data before the transformation, and we are going to
perform the calculation on each element.
The next section of the guide will indicate how to actually code this operation.