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// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
use std::sync::Arc;
const NUM_TILE_BUCKETS: usize = 6;
/// A pool of blob tile buffers to mitigate the overhead of
/// allocating and deallocating blob tiles.
///
/// The pool keeps a strong reference to each allocated buffers and
/// reuses the ones with a strong count of 1.
pub struct BlobTilePool {
largest_size_class: usize,
buckets: [Vec<Arc<Vec<u8>>>; NUM_TILE_BUCKETS],
}
impl BlobTilePool {
pub fn new() -> Self {
// The default max tile size is actually 256, using 512 here
// so that this still works when experimenting with larger
// tile sizes. If we ever make larger adjustments, the buckets
// should be changed accordingly.
let max_tile_size = 512;
BlobTilePool {
largest_size_class: max_tile_size * max_tile_size * 4,
buckets: [
Vec::with_capacity(32),
Vec::with_capacity(32),
Vec::with_capacity(32),
Vec::with_capacity(32),
Vec::with_capacity(32),
Vec::with_capacity(32),
],
}
}
/// Get or allocate a tile buffer of the requested size.
///
/// The returned buffer is zero-inizitalized.
/// The length of the returned buffer is equal to the requested size,
/// however the buffer may be allocated with a larger capacity to
/// conform to the pool's corresponding bucket tile size.
pub fn get_buffer(&mut self, requested_size: usize) -> MutableTileBuffer {
if requested_size > self.largest_size_class {
// If the requested size is larger than the largest size class,
// simply return a MutableBuffer that isn't tracked/recycled by
// the pool.
// In Firefox this should only happen in pathological cases
// where the blob visible area ends up so large that the tile
// size is increased to avoid producing too many tiles.
// See wr_resource_updates_add_blob_image.
let mut buf = vec![0; requested_size];
return MutableTileBuffer {
ptr: buf.as_mut_ptr(),
strong_ref: Arc::new(buf),
};
}
let (bucket_idx, cap) = self.bucket_and_size(requested_size);
let bucket = &mut self.buckets[bucket_idx];
let mut selected_idx = None;
for (buf_idx, buffer) in bucket.iter().enumerate() {
if Arc::strong_count(buffer) == 1 {
selected_idx = Some(buf_idx);
break;
}
}
let ptr;
let strong_ref;
if let Some(idx) = selected_idx {
{
// This works because we just ensured the pool has the only strong
// ref to the buffer.
let buffer = Arc::get_mut(&mut bucket[idx]).unwrap();
debug_assert!(buffer.capacity() >= requested_size);
// Ensure the length is equal to the requested size. It's not
// strictly necessay for the tile pool but the texture upload
// code relies on it.
unsafe { buffer.set_len(requested_size); }
// zero-initialize
buffer.fill(0);
ptr = buffer.as_mut_ptr();
}
strong_ref = Arc::clone(&bucket[idx]);
} else {
// Allocate a buffer with the adequate capacity for the requested
// size's bucket.
let mut buf = vec![0; cap];
// Force the length to be the requested size.
unsafe { buf.set_len(requested_size) };
ptr = buf.as_mut_ptr();
strong_ref = Arc::new(buf);
// Track the new buffer.
bucket.push(Arc::clone(&strong_ref));
};
MutableTileBuffer {
ptr,
strong_ref,
}
}
fn bucket_and_size(&self, size: usize) -> (usize, usize) {
let mut next_size_class = self.largest_size_class / 4;
let mut idx = 0;
while size < next_size_class && idx < NUM_TILE_BUCKETS - 1 {
next_size_class /= 4;
idx += 1;
}
(idx, next_size_class * 4)
}
/// Go over all allocated tile buffers. For each bucket, deallocate some buffers
/// until the number of unused buffer is more than half of the buffers for that
/// bucket.
///
/// In practice, if called regularly, this gradually lets go of blob tiles when
/// they are not used.
pub fn cleanup(&mut self) {
for bucket in &mut self.buckets {
let threshold = bucket.len() / 2;
let mut num_available = 0;
bucket.retain(&mut |buffer: &Arc<Vec<u8>>| {
if Arc::strong_count(buffer) > 1 {
return true;
}
num_available += 1;
num_available < threshold
});
}
}
}
// The role of tile buffer is to encapsulate an Arc to the underlying buffer
// with a reference count of at most 2 and a way to view the buffer's content
// as a mutable slice, even though the reference count may be more than 1.
// The safety of this relies on the other strong reference being held by the
// tile pool which never accesses the buffer's content, so the only reference
// that can access it is the `TileBuffer` itself.
pub struct MutableTileBuffer {
strong_ref: Arc<Vec<u8>>,
ptr: *mut u8,
}
impl MutableTileBuffer {
pub fn as_mut_slice(&mut self) -> &mut[u8] {
unsafe { std::slice::from_raw_parts_mut(self.ptr, self.strong_ref.len()) }
}
pub fn into_arc(self) -> Arc<Vec<u8>> {
self.strong_ref
}
}
unsafe impl Send for MutableTileBuffer {}