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// Copyright 2018 The Dawn Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include "dawn/native/RingBufferAllocator.h"
// Note: Current RingBufferAllocator implementation uses two indices (start and end) to implement a
// circular queue. However, this approach defines a full queue when one element is still unused.
// For example, [E,E,E,E] would be equivelent to [U,U,U,U].
// ^ ^
// S=E=1 S=E=1
// The latter case is eliminated by counting used bytes >= capacity. This definition prevents
// (the last) byte and requires an extra variable to count used bytes. Alternatively, we could use
// only two indices that keep increasing (unbounded) but can be still indexed using bit masks.
// However, this 1) requires the size to always be a power-of-two and 2) remove tests that check
// used bytes.
namespace dawn::native {
RingBufferAllocator::RingBufferAllocator(uint64_t maxSize) : mMaxBlockSize(maxSize) {
void RingBufferAllocator::Deallocate(ExecutionSerial lastCompletedSerial) {
// Reclaim memory from previously recorded blocks.
for (Request& request : mInflightRequests.IterateUpTo(lastCompletedSerial)) {
mUsedStartOffset = request.endOffset;
mUsedSize -= request.size;
// Dequeue previously recorded requests.
uint64_t RingBufferAllocator::GetSize() const {
return mMaxBlockSize;
uint64_t RingBufferAllocator::GetUsedSize() const {
return mUsedSize;
bool RingBufferAllocator::Empty() const {
return mInflightRequests.Empty();
// Sub-allocate the ring-buffer by requesting a chunk of the specified size.
// This is a serial-based resource scheme, the life-span of resources (and the allocations) get
// tracked by GPU progress via serials. Memory can be reused by determining if the GPU has
// completed up to a given serial. Each sub-allocation request is tracked in the serial offset
// queue, which identifies an existing (or new) frames-worth of resources. Internally, the
// ring-buffer maintains offsets of 3 "memory" states: Free, Reclaimed, and Used. This is done
// in FIFO order as older frames would free resources before newer ones.
uint64_t RingBufferAllocator::Allocate(uint64_t allocationSize, ExecutionSerial serial) {
// Check if the buffer is full by comparing the used size.
// If the buffer is not split where waste occurs (e.g. cannot fit new sub-alloc in front), a
// subsequent sub-alloc could fail where the used size was previously adjusted to include
// the wasted.
if (mUsedSize >= mMaxBlockSize) {
return kInvalidOffset;
// Ensure adding allocationSize does not overflow.
const uint64_t remainingSize = (mMaxBlockSize - mUsedSize);
if (allocationSize > remainingSize) {
return kInvalidOffset;
uint64_t startOffset = kInvalidOffset;
// Check if the buffer is NOT split (i.e sub-alloc on ends)
if (mUsedStartOffset <= mUsedEndOffset) {
// Order is important (try to sub-alloc at end first).
// This is due to FIFO order where sub-allocs are inserted from left-to-right (when not
// wrapped).
if (mUsedEndOffset + allocationSize <= mMaxBlockSize) {
startOffset = mUsedEndOffset;
mUsedEndOffset += allocationSize;
mUsedSize += allocationSize;
mCurrentRequestSize += allocationSize;
} else if (allocationSize <= mUsedStartOffset) { // Try to sub-alloc at front.
// Count the space at the end so that a subsequent
// sub-alloc cannot not succeed when the buffer is full.
const uint64_t requestSize = (mMaxBlockSize - mUsedEndOffset) + allocationSize;
startOffset = 0;
mUsedEndOffset = allocationSize;
mUsedSize += requestSize;
mCurrentRequestSize += requestSize;
} else if (mUsedEndOffset + allocationSize <=
mUsedStartOffset) { // Otherwise, buffer is split where sub-alloc must be
// in-between.
startOffset = mUsedEndOffset;
mUsedEndOffset += allocationSize;
mUsedSize += allocationSize;
mCurrentRequestSize += allocationSize;
if (startOffset != kInvalidOffset) {
Request request;
request.endOffset = mUsedEndOffset;
request.size = mCurrentRequestSize;
mInflightRequests.Enqueue(std::move(request), serial);
mCurrentRequestSize = 0; // reset
return startOffset;
} // namespace dawn::native