Now that we are familiar with devices, queues, and buffers, we are going to see how to ask the GPU to actually do something.
What we are going to ask in this example is very simple: we will ask it to copy data from one buffer to another.
The first step is to create two
CpuAccessibleBuffers: the source and the destination. This
was covered in the previous section.
let source_content = 0 .. 64; let source = CpuAccessibleBuffer::from_iter(device.clone(), BufferUsage::all(), source_content).expect("failed to create buffer"); let dest_content = (0 .. 64).map(|_| 0); let dest = CpuAccessibleBuffer::from_iter(device.clone(), BufferUsage::all(), dest_content).expect("failed to create buffer");
The iterators might look a bit tricky. The
source_content iterator produces 64 values ranging
from 0 to 63. The
dest_content iterator produces 64 values that are all equal to 0.
In other words, once created the source buffer contains sixty-four values ranging from 0 to 63
while the destination buffer contains sixty-four 0s.
In order to ask the GPU to perform an operation, we need to create a type of object that we haven't covered yet: command buffer.
With Vulkan and vulkano you can't just execute commands one by one, as it would be too inefficient. Instead, we need to build a command buffer that contains a list of commands that we want to execute.
Note: Submitting a command to the GPU can take up to several hundred microseconds, which is why we submit as many things as we can at once.
Note: OpenGL (Vulkan's predecessor) allows you to execute commands one by one, but in reality implementations buffer commands internally into command buffers. In other words, OpenGL automatically does what Vulkan requires us to do manually. In practice OpenGL's automatic buffering often causes more harm than good in performance-critical applications.
Here is how you create a command buffer:
use vulkano::command_buffer::AutoCommandBufferBuilder; let command_buffer = AutoCommandBufferBuilder::new(device.clone(), queue.family()).unwrap() .copy_buffer(source.clone(), dest.clone()).unwrap() .build().unwrap();
As you can see, it is very straight-forward. We create a builder, add a copy command to it with
copy_buffers, then turn that builder into an actual command buffer with
build. Like we saw in
the buffers creation section, we call
clone() multiple times but in
reality we only clone
One thing to notice is that the
AutoCommandBufferBuilder::new() method takes as
parameter a queue family. This must be the queue family that the command buffer is going to run on.
In this example we don't have much choice anyway (as we only use one queue and thus one queue
family), but when you design a real program you have to be aware of this specificity.
And now we submit the command buffer so that it gets executed:
use vulkano::command_buffer::CommandBuffer; let finished = command_buffer.execute(queue.clone()).unwrap();
execute function returns an object that represents the execution of the command buffer.
After submitting the command buffer, we might be tempted to try read the content of the
destination buffer as demonstrated in the previous section. However
destination.read() now would return an error, because the buffer is maybe currently being
written by the GPU.
Submitting an operation doesn't wait for the operation to be complete. Instead it just sends some kind of signal to the GPU to instruct it that it must start processing the command buffer, and the actual processing is performed asynchronously.
In order to read the content of
destination and make sure that our copy succeeded, we need to
wait until the operation is complete. This is done by making use of the
finished object that
was returned by
use vulkano::sync::GpuFuture; finished.then_signal_fence_and_flush().unwrap() .wait(None).unwrap();
This may look a bit complicated, but we will cover what a fence is in a later section of the
guide and what signalling it means. The
wait() function blocks the current thread until the GPU
has finished execution.
Only after this is done can we call
destination.read() and check that our copy succeeded.
let src_content = source.read().unwrap(); let dest_content = dest.read().unwrap(); assert_eq!(&*src_content, &*dest_content);