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use crate::{
util::align_to, Buffer, BufferAddress, BufferDescriptor, BufferSize, BufferUsages,
BufferViewMut, CommandEncoder, Device, MapMode,
};
use std::fmt;
use std::sync::{mpsc, Arc};
struct Chunk {
buffer: Arc<Buffer>,
size: BufferAddress,
offset: BufferAddress,
}
/// `Sync` wrapper that works by providing only exclusive access.
///
/// See https://doc.rust-lang.org/nightly/std/sync/struct.Exclusive.html
struct Exclusive<T>(T);
unsafe impl<T> Sync for Exclusive<T> {}
impl<T> Exclusive<T> {
fn new(value: T) -> Self {
Self(value)
}
fn get_mut(&mut self) -> &mut T {
&mut self.0
}
}
/// Efficiently performs many buffer writes by sharing and reusing temporary buffers.
///
/// Internally it uses a ring-buffer of staging buffers that are sub-allocated.
/// It has an advantage over [`Queue::write_buffer()`] in a way that it returns a mutable slice,
/// which you can fill to avoid an extra data copy.
///
/// Using a staging belt is slightly complicated, and generally goes as follows:
/// 1. Write to buffers that need writing to using [`StagingBelt::write_buffer()`].
/// 2. Call [`StagingBelt::finish()`].
/// 3. Submit all command encoders that were used in step 1.
/// 4. Call [`StagingBelt::recall()`].
///
/// [`Queue::write_buffer()`]: crate::Queue::write_buffer
pub struct StagingBelt {
chunk_size: BufferAddress,
/// Chunks into which we are accumulating data to be transferred.
active_chunks: Vec<Chunk>,
/// Chunks that have scheduled transfers already; they are unmapped and some
/// command encoder has one or more `copy_buffer_to_buffer` commands with them
/// as source.
closed_chunks: Vec<Chunk>,
/// Chunks that are back from the GPU and ready to be mapped for write and put
/// into `active_chunks`.
free_chunks: Vec<Chunk>,
/// When closed chunks are mapped again, the map callback sends them here.
sender: Exclusive<mpsc::Sender<Chunk>>,
/// Free chunks are received here to be put on `self.free_chunks`.
receiver: Exclusive<mpsc::Receiver<Chunk>>,
}
impl StagingBelt {
/// Create a new staging belt.
///
/// The `chunk_size` is the unit of internal buffer allocation; writes will be
/// sub-allocated within each chunk. Therefore, for optimal use of memory, the
/// chunk size should be:
///
/// * larger than the largest single [`StagingBelt::write_buffer()`] operation;
/// * 1-4 times less than the total amount of data uploaded per submission
/// (per [`StagingBelt::finish()`]); and
/// * bigger is better, within these bounds.
pub fn new(chunk_size: BufferAddress) -> Self {
let (sender, receiver) = std::sync::mpsc::channel();
StagingBelt {
chunk_size,
active_chunks: Vec::new(),
closed_chunks: Vec::new(),
free_chunks: Vec::new(),
sender: Exclusive::new(sender),
receiver: Exclusive::new(receiver),
}
}
/// Allocate the staging belt slice of `size` to be uploaded into the `target` buffer
/// at the specified offset.
///
/// The upload will be placed into the provided command encoder. This encoder
/// must be submitted after [`StagingBelt::finish()`] is called and before
/// [`StagingBelt::recall()`] is called.
///
/// If the `size` is greater than the size of any free internal buffer, a new buffer
/// will be allocated for it. Therefore, the `chunk_size` passed to [`StagingBelt::new()`]
/// should ideally be larger than every such size.
pub fn write_buffer(
&mut self,
encoder: &mut CommandEncoder,
target: &Buffer,
offset: BufferAddress,
size: BufferSize,
device: &Device,
) -> BufferViewMut<'_> {
let mut chunk = if let Some(index) = self
.active_chunks
.iter()
.position(|chunk| chunk.offset + size.get() <= chunk.size)
{
self.active_chunks.swap_remove(index)
} else {
self.receive_chunks(); // ensure self.free_chunks is up to date
if let Some(index) = self
.free_chunks
.iter()
.position(|chunk| size.get() <= chunk.size)
{
self.free_chunks.swap_remove(index)
} else {
let size = self.chunk_size.max(size.get());
Chunk {
#[allow(clippy::arc_with_non_send_sync)] // False positive on emscripten
buffer: Arc::new(device.create_buffer(&BufferDescriptor {
label: Some("(wgpu internal) StagingBelt staging buffer"),
size,
usage: BufferUsages::MAP_WRITE | BufferUsages::COPY_SRC,
mapped_at_creation: true,
})),
size,
offset: 0,
}
}
};
encoder.copy_buffer_to_buffer(&chunk.buffer, chunk.offset, target, offset, size.get());
let old_offset = chunk.offset;
chunk.offset = align_to(chunk.offset + size.get(), crate::MAP_ALIGNMENT);
self.active_chunks.push(chunk);
self.active_chunks
.last()
.unwrap()
.buffer
.slice(old_offset..old_offset + size.get())
.get_mapped_range_mut()
}
/// Prepare currently mapped buffers for use in a submission.
///
/// This must be called before the command encoder(s) provided to
/// [`StagingBelt::write_buffer()`] are submitted.
///
/// At this point, all the partially used staging buffers are closed (cannot be used for
/// further writes) until after [`StagingBelt::recall()`] is called *and* the GPU is done
/// copying the data from them.
pub fn finish(&mut self) {
for chunk in self.active_chunks.drain(..) {
chunk.buffer.unmap();
self.closed_chunks.push(chunk);
}
}
/// Recall all of the closed buffers back to be reused.
///
/// This must only be called after the command encoder(s) provided to
/// [`StagingBelt::write_buffer()`] are submitted. Additional calls are harmless.
/// Not calling this as soon as possible may result in increased buffer memory usage.
pub fn recall(&mut self) {
self.receive_chunks();
for chunk in self.closed_chunks.drain(..) {
let sender = self.sender.get_mut().clone();
chunk
.buffer
.clone()
.slice(..)
.map_async(MapMode::Write, move |_| {
let _ = sender.send(chunk);
});
}
}
/// Move all chunks that the GPU is done with (and are now mapped again)
/// from `self.receiver` to `self.free_chunks`.
fn receive_chunks(&mut self) {
while let Ok(mut chunk) = self.receiver.get_mut().try_recv() {
chunk.offset = 0;
self.free_chunks.push(chunk);
}
}
}
impl fmt::Debug for StagingBelt {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("StagingBelt")
.field("chunk_size", &self.chunk_size)
.field("active_chunks", &self.active_chunks.len())
.field("closed_chunks", &self.closed_chunks.len())
.field("free_chunks", &self.free_chunks.len())
.finish_non_exhaustive()
}
}