mirror of
https://github.com/superseriousbusiness/gotosocial.git
synced 2024-11-23 20:26:39 +00:00
138 lines
3.3 KiB
Go
138 lines
3.3 KiB
Go
package xsync
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import (
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"runtime"
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"sync/atomic"
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"unsafe"
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)
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// A MPMCQueue is a bounded multi-producer multi-consumer concurrent
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// queue.
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//
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// MPMCQueue instances must be created with NewMPMCQueue function.
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// A MPMCQueue must not be copied after first use.
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//
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// Based on the data structure from the following C++ library:
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// https://github.com/rigtorp/MPMCQueue
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type MPMCQueue struct {
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cap uint64
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head uint64
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//lint:ignore U1000 prevents false sharing
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hpad [cacheLineSize - 8]byte
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tail uint64
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//lint:ignore U1000 prevents false sharing
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tpad [cacheLineSize - 8]byte
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slots []slotPadded
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}
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type slotPadded struct {
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slot
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//lint:ignore U1000 prevents false sharing
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pad [cacheLineSize - unsafe.Sizeof(slot{})]byte
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}
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type slot struct {
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turn uint64
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item interface{}
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}
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// NewMPMCQueue creates a new MPMCQueue instance with the given
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// capacity.
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func NewMPMCQueue(capacity int) *MPMCQueue {
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if capacity < 1 {
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panic("capacity must be positive number")
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}
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return &MPMCQueue{
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cap: uint64(capacity),
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slots: make([]slotPadded, capacity),
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}
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}
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// Enqueue inserts the given item into the queue.
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// Blocks, if the queue is full.
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func (q *MPMCQueue) Enqueue(item interface{}) {
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head := atomic.AddUint64(&q.head, 1) - 1
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slot := &q.slots[q.idx(head)]
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turn := q.turn(head) * 2
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for atomic.LoadUint64(&slot.turn) != turn {
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runtime.Gosched()
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}
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slot.item = item
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atomic.StoreUint64(&slot.turn, turn+1)
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}
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// Dequeue retrieves and removes the item from the head of the queue.
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// Blocks, if the queue is empty.
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func (q *MPMCQueue) Dequeue() interface{} {
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tail := atomic.AddUint64(&q.tail, 1) - 1
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slot := &q.slots[q.idx(tail)]
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turn := q.turn(tail)*2 + 1
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for atomic.LoadUint64(&slot.turn) != turn {
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runtime.Gosched()
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}
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item := slot.item
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slot.item = nil
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atomic.StoreUint64(&slot.turn, turn+1)
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return item
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}
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// TryEnqueue inserts the given item into the queue. Does not block
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// and returns immediately. The result indicates that the queue isn't
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// full and the item was inserted.
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func (q *MPMCQueue) TryEnqueue(item interface{}) bool {
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head := atomic.LoadUint64(&q.head)
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for {
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slot := &q.slots[q.idx(head)]
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turn := q.turn(head) * 2
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if atomic.LoadUint64(&slot.turn) == turn {
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if atomic.CompareAndSwapUint64(&q.head, head, head+1) {
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slot.item = item
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atomic.StoreUint64(&slot.turn, turn+1)
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return true
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}
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} else {
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prevHead := head
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head = atomic.LoadUint64(&q.head)
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if head == prevHead {
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return false
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}
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}
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runtime.Gosched()
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}
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}
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// TryDequeue retrieves and removes the item from the head of the
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// queue. Does not block and returns immediately. The ok result
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// indicates that the queue isn't empty and an item was retrieved.
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func (q *MPMCQueue) TryDequeue() (item interface{}, ok bool) {
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tail := atomic.LoadUint64(&q.tail)
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for {
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slot := &q.slots[q.idx(tail)]
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turn := q.turn(tail)*2 + 1
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if atomic.LoadUint64(&slot.turn) == turn {
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if atomic.CompareAndSwapUint64(&q.tail, tail, tail+1) {
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item = slot.item
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ok = true
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slot.item = nil
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atomic.StoreUint64(&slot.turn, turn+1)
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return
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}
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} else {
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prevTail := tail
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tail = atomic.LoadUint64(&q.tail)
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if tail == prevTail {
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return
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}
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}
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runtime.Gosched()
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}
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}
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func (q *MPMCQueue) idx(i uint64) uint64 {
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return i % q.cap
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}
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func (q *MPMCQueue) turn(i uint64) uint64 {
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return i / q.cap
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}
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