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Move priority queue to pq package; improve docs.

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This commit is contained in:
Sam Fredrickson 2023-03-02 01:53:12 -08:00
parent 5e23a92314
commit b3b491d9a9
8 changed files with 255 additions and 155 deletions
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8
README.md
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@ -31,10 +31,10 @@ queue that guarantees receipt of a high-priority items before low-priority
ones. This is primarily a fun exercise, I cannot recommend that anyone
actually use this in a real project.
Additionally, the root `priorityq` package implements a concurrent priority
queue, using a binary max-heap. This is more general than `mq`, because it
allows multiple levels of priority, instead of just "high" and "low". This, of
course, also makes operations slower.
Additionally, the `pq` package implements a concurrent priority queue, using a
binary max-heap. This is more general than `mq`, because it allows multiple
levels of priority, instead of just "high" and "low". This, of course, also
makes operations slower.
[reddit]: https://www.reddit.com/r/golang/comments/11drc17/worker_pool_reading_from_two_channels_one_chan/
[sol]: https://www.reddit.com/r/golang/comments/11drc17/worker_pool_reading_from_two_channels_one_chan/jabfvkh/

3
binheap/lib.go
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@ -1,8 +1,9 @@
// Package binheap implements a binary max-heap.
package binheap
import "golang.org/x/exp/constraints"
// H is a generic, non-concurrent binary max-heap.
// H is a binary max-heap.
//
// `I` is the type of the priority IDs, and `E` the type of the elements.
type H[I constraints.Ordered, E any] struct {

3
circ/lib.go
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@ -1,6 +1,7 @@
// Package circ implements a circular FIFO buffer.
package circ
// B is a generic, non-concurrent circular FIFO buffer.
// B is a circular FIFO buffer.
type B[T any] struct {
buf []T
len int

137
lib.go
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@ -1,117 +1,22 @@
// Package priorityq provides generic implementations of various concurrent,
// prioritized queues.
//
// # Behavior
//
// All types of queues in this module act similarly to buffered Go channels.
//
// - They are bounded to a fixed capacity, set at construction.
// - Closing and sending to an already-closed queue causes a panic.
// - Receivers can continue getting items after closure, and can use a final
// bool to determine when there are none remaining.
// - They are safe for multiple concurrent senders and receivers.
//
// # Implementation
//
// All data structures in this module use [generics], introduced in Go 1.18.
//
// All of the concurrent data structures in this package use a [sync.Mutex]
// and a few [sync.Cond] variables.
//
// [generics]: https://go.dev/blog/intro-generics
package priorityq
import (
"sync"
"gogs.humancabbage.net/sam/priorityq/binheap"
"golang.org/x/exp/constraints"
)
// Q is a generic, concurrent priority queue.
type Q[P constraints.Ordered, T any] struct {
*state[P, T]
}
// Make a new queue.
func Make[P constraints.Ordered, T any](cap int) Q[P, T] {
heap := binheap.Make[P, T](cap)
s := &state[P, T]{
heap: heap,
}
s.canRecv = sync.NewCond(&s.mu)
s.canSend = sync.NewCond(&s.mu)
return Q[P, T]{s}
}
type state[P constraints.Ordered, T any] struct {
mu sync.Mutex
heap binheap.H[P, T]
canSend *sync.Cond
canRecv *sync.Cond
closed bool
}
// Close marks the queue as closed.
//
// Subsequent attempts to send will panic. Subsequent calls to Recv will
// continue to return the remaining items in the queue.
func (s *state[P, T]) Close() {
s.mu.Lock()
s.closed = true
s.mu.Unlock()
s.canRecv.Broadcast()
}
// Recv returns an item from the prioritized buffers, blocking if empty.
//
// The returned bool will be true if the queue still has items or is open.
// It will be false if the queue is empty and closed.
func (s *state[P, T]) Recv() (P, T, bool) {
s.mu.Lock()
defer s.mu.Unlock()
for {
for !s.closed && !s.heap.CanExtract() {
s.canRecv.Wait()
}
if s.closed && !s.heap.CanExtract() {
var emptyP P
var emptyT T
return emptyP, emptyT, false
}
if s.heap.CanExtract() {
priority, value := s.heap.Extract()
s.canSend.Broadcast()
return priority, value, true
}
}
}
// Send adds an item to the queue, blocking if full.
func (s *state[P, T]) Send(priority P, value T) {
s.mu.Lock()
defer s.mu.Unlock()
for {
for !s.closed && !s.heap.CanInsert() {
s.canSend.Wait()
}
if s.closed {
panic("send on closed queue")
}
if s.heap.CanInsert() {
s.heap.Insert(priority, value)
s.canRecv.Broadcast()
return
}
}
}
// TryRecv attempts to return an item from the queue.
//
// This method does not block. If there is an item in the queue, it returns
// true. If the queue is empty, it returns false.
func (s *state[P, T]) TryRecv() (priority P, value T, ok bool) {
s.mu.Lock()
defer s.mu.Unlock()
if s.heap.CanExtract() {
priority, value = s.heap.Extract()
ok = true
s.canSend.Broadcast()
return
}
return
}
// TrySend attempts to add an item to the high priority buffer.
//
// This method does not block. If there is space in the buffer, it returns
// true. If the buffer is full, it returns false.
func (s *state[P, T]) TrySend(priority P, value T) bool {
s.mu.Lock()
defer s.mu.Unlock()
if !s.heap.CanInsert() {
return false
}
s.heap.Insert(priority, value)
s.canRecv.Broadcast()
return true
}

68
mq/lib.go
View File

@ -1,3 +1,30 @@
// Package mq implements a concurrent, dual-priority message queue.
//
// [Q] is similar to a buffered channel, except that senders can assign one of
// two priority levels to each item, "high" or "low." Receivers will always
// get a high-priority item ahead of any low-priority ones.
//
// For example:
//
// q := mq.Make[string](8)
// mq.SendLow("world")
// mq.SendHigh("hello")
// word1, _ := mq.Recv()
// word2, _ := mq.Recv()
// fmt.Println(word1, word2)
// pq.Close()
// // Output: hello world
//
// # Implementation
//
// Each queue has two circular buffers, one for each priority level.
// Currently, the capacities for these are fixed and equal. If one buffer is
// full, attempts to send further items with its priority level will block
// ([Q.Send]) or fail ([Q.TrySend]).
//
// Compared the pq package, the limitation on priority levels increases
// performance, as its circular buffers are much less expensive than the heap
// operations of a traditional priority queue.
package mq
import (
@ -6,14 +33,7 @@ import (
"gogs.humancabbage.net/sam/priorityq/circ"
)
// Q is a precise, concurrent, prioritized message queue.
//
// Each queue has two internal buffers, high and low. This implementation
// guarantees that when there are items in both buffers, consumers receive
// ones from the high priority buffer first.
//
// Each buffer has the same capacity, set on initial construction. Sending to
// a buffer will block if it is full, even if the other buffer has space.
// Q is a concurrent, dual-priority message queue.
type Q[T any] struct {
*state[T]
}
@ -44,16 +64,18 @@ type state[T any] struct {
// Close marks the queue as closed.
//
// Subsequent attempts to send will panic. Subsequent calls to Recv will
// continue to return the remaining items in the queue.
// Attempting to close an already-closed queue results in a panic.
func (s *state[T]) Close() {
s.mu.Lock()
if s.closed {
panic("close of closed queue")
}
s.closed = true
s.mu.Unlock()
s.canRecv.Broadcast()
}
// Recv returns an item from the prioritized buffers, blocking if empty.
// Recv gets an item, blocking when empty until one is available.
//
// The returned bool will be true if the queue still has items or is open.
// It will be false if the queue is empty and closed.
@ -86,20 +108,19 @@ func (s *state[T]) Send(value T) {
s.SendLow(value)
}
// SendHigh adds an item to the high priority buffer, blocking if full.
// SendHigh adds an item with high priority, blocking if full.
func (s *state[T]) SendHigh(value T) {
s.send(value, &s.high, s.canSendHigh)
}
// SendLow adds an item to the low priority buffer, blocking if full.
// SendLow adds an item with low buffer, blocking if full.
func (s *state[T]) SendLow(value T) {
s.send(value, &s.low, s.canSendLow)
}
// TryRecv attempts to return an item from the prioritized buffers.
// TryRecv attempts to get an item from the queue, without blocking.
//
// This method does not block. If there is an item in a buffer, it returns
// true. If the buffer is empty, it returns false.
// If the attempt succeeds, the returned bool is true. Otherwise, it is false.
func (s *state[T]) TryRecv() (value T, ok bool) {
s.mu.Lock()
defer s.mu.Unlock()
@ -118,18 +139,21 @@ func (s *state[T]) TryRecv() (value T, ok bool) {
return
}
// TrySendHigh attempts to add an item to the high priority buffer.
// TrySend is an alias for TrySendLow.
func (s *state[T]) TrySend(value T) bool {
return s.trySend(value, &s.low)
}
// TrySendHigh attempts to add an item with high priority, without blocking.
//
// This method does not block. If there is space in the buffer, it returns
// true. If the buffer is full, it returns false.
// If the attempt succeeds, the returned bool is true. Otherwise, it is false.
func (s *state[T]) TrySendHigh(value T) bool {
return s.trySend(value, &s.high)
}
// TrySendLow attempts to add an item to the low priority buffer.
// TrySendLow attempts to add an item with low priority, without blocking.
//
// This method does not block. If there is space in the buffer, it returns
// true. If the buffer is full, it returns false.
// If the attempt succeeds, the returned bool is true. Otherwise, it is false.
func (s *state[T]) TrySendLow(value T) bool {
return s.trySend(value, &s.low)
}

12
mq/lib_test.go
View File

@ -62,6 +62,18 @@ func TestRecvClosed(t *testing.T) {
}
}
func TestDoubleClose(t *testing.T) {
t.Parallel()
q := mq.Make[int](4)
defer func() {
if r := recover(); r == nil {
t.Errorf("closing a closed queue did not panic")
}
}()
q.Close()
q.Close()
}
func TestTrySendRecv(t *testing.T) {
t.Parallel()
q := mq.Make[int](4)

145
pq/lib.go Normal file
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@ -0,0 +1,145 @@
// Package pq implements a concurrent priority queue.
//
// [Q] is similar to a buffered channel, except that senders attach to each
// item a priority, and receivers always get the highest-priority item.
//
// For example:
//
// import "gogs.humancabbage.net/sam/priorityq/pq"
// q := pq.Make[int, string](8)
// q.Send(1, "world")
// q.Send(2, "hello")
// _, word1, _ := pq.Recv()
// _, word2, _ := pq.Recv()
// fmt.Println(word1, word2)
// pq.Close()
// // Output: hello world
//
// # Implementation
//
// Each queue has a [binary max-heap]. Sending and receiving items require
// heap-up and heap-down operations, respectively.
//
// [binary max-heap]: https://en.wikipedia.org/wiki/Binary_heap
package pq
import (
"sync"
"gogs.humancabbage.net/sam/priorityq/binheap"
"golang.org/x/exp/constraints"
)
// Q is a generic, concurrent priority queue.
type Q[P constraints.Ordered, T any] struct {
*state[P, T]
}
// Make a new queue.
func Make[P constraints.Ordered, T any](cap int) Q[P, T] {
heap := binheap.Make[P, T](cap)
s := &state[P, T]{
heap: heap,
}
s.canRecv = sync.NewCond(&s.mu)
s.canSend = sync.NewCond(&s.mu)
return Q[P, T]{s}
}
type state[P constraints.Ordered, T any] struct {
mu sync.Mutex
heap binheap.H[P, T]
canSend *sync.Cond
canRecv *sync.Cond
closed bool
}
// Close marks the queue as closed.
//
// Attempting to close an already-closed queue results in a panic.
func (s *state[P, T]) Close() {
s.mu.Lock()
if s.closed {
panic("close of closed queue")
}
s.closed = true
s.mu.Unlock()
s.canRecv.Broadcast()
}
// Recv gets an item, blocking when empty until one is available.
//
// This returns both the item itself and the its assigned priority.
//
// The returned bool will be true if the queue still has items or is open.
// It will be false if the queue is empty and closed.
func (s *state[P, T]) Recv() (P, T, bool) {
s.mu.Lock()
defer s.mu.Unlock()
for {
for !s.closed && !s.heap.CanExtract() {
s.canRecv.Wait()
}
if s.closed && !s.heap.CanExtract() {
var emptyP P
var emptyT T
return emptyP, emptyT, false
}
if s.heap.CanExtract() {
priority, value := s.heap.Extract()
s.canSend.Broadcast()
return priority, value, true
}
}
}
// Send adds an item with some priority, blocking if full.
func (s *state[P, T]) Send(priority P, value T) {
s.mu.Lock()
defer s.mu.Unlock()
for {
for !s.closed && !s.heap.CanInsert() {
s.canSend.Wait()
}
if s.closed {
panic("send on closed queue")
}
if s.heap.CanInsert() {
s.heap.Insert(priority, value)
s.canRecv.Broadcast()
return
}
}
}
// TryRecv attempts to get an item without blocking.
//
// This returns both the item itself and the its assigned priority.
//
// If the attempt succeeds, the returned bool is true. Otherwise, it is false.
func (s *state[P, T]) TryRecv() (priority P, value T, ok bool) {
s.mu.Lock()
defer s.mu.Unlock()
if s.heap.CanExtract() {
priority, value = s.heap.Extract()
ok = true
s.canSend.Broadcast()
return
}
return
}
// TrySend attempts to add an item with some priority, without blocking.
//
// This method does not block. If there is space in the buffer, it returns
// true. If the buffer is full, it returns false.
func (s *state[P, T]) TrySend(priority P, value T) bool {
s.mu.Lock()
defer s.mu.Unlock()
if !s.heap.CanInsert() {
return false
}
s.heap.Insert(priority, value)
s.canRecv.Broadcast()
return true
}

34
lib_test.go → pq/lib_test.go
View File

@ -1,4 +1,4 @@
package priorityq_test
package pq_test
import (
"math/rand"
@ -6,12 +6,12 @@ import (
"sync"
"testing"
"gogs.humancabbage.net/sam/priorityq"
"gogs.humancabbage.net/sam/priorityq/pq"
)
func TestRecvHighestFirst(t *testing.T) {
t.Parallel()
q := priorityq.Make[int, int](8)
q := pq.Make[int, int](8)
q.Send(4, 4)
q.Send(2, 2)
q.Send(1, 1)
@ -42,14 +42,14 @@ func TestSendClosedPanic(t *testing.T) {
t.Errorf("sending to closed queue did not panic")
}
}()
q := priorityq.Make[int, int](4)
q := pq.Make[int, int](4)
q.Close()
q.Send(1, 1)
}
func TestRecvClosed(t *testing.T) {
t.Parallel()
q := priorityq.Make[int, int](4)
q := pq.Make[int, int](4)
q.Send(1, 1)
q.Close()
_, _, ok := q.Recv()
@ -62,9 +62,21 @@ func TestRecvClosed(t *testing.T) {
}
}
func TestDoubleClose(t *testing.T) {
t.Parallel()
q := pq.Make[int, int](4)
defer func() {
if r := recover(); r == nil {
t.Errorf("closing a closed queue did not panic")
}
}()
q.Close()
q.Close()
}
func TestTrySendRecv(t *testing.T) {
t.Parallel()
q := priorityq.Make[int, int](4)
q := pq.Make[int, int](4)
assumeSendOk := func(n int) {
ok := q.TrySend(n, n)
if !ok {
@ -101,7 +113,7 @@ func TestTrySendRecv(t *testing.T) {
func TestConcProducerConsumer(t *testing.T) {
t.Parallel()
q := priorityq.Make[int, int](4)
q := pq.Make[int, int](4)
var wg sync.WaitGroup
produceDone := make(chan struct{})
wg.Add(2)
@ -126,7 +138,7 @@ func TestConcProducerConsumer(t *testing.T) {
}
func BenchmarkSend(b *testing.B) {
q := priorityq.Make[int, int](b.N)
q := pq.Make[int, int](b.N)
// randomize priorities to get amortized cost per op
ps := make([]int, b.N)
for i := 0; i < b.N; i++ {
@ -139,7 +151,7 @@ func BenchmarkSend(b *testing.B) {
}
func BenchmarkRecv(b *testing.B) {
q := priorityq.Make[int, int](b.N)
q := pq.Make[int, int](b.N)
// randomize priorities to get amortized cost per op
for i := 0; i < b.N; i++ {
q.Send(rand.Int(), i)
@ -151,7 +163,7 @@ func BenchmarkRecv(b *testing.B) {
}
func BenchmarkConcSendRecv(b *testing.B) {
q := priorityq.Make[int, int](b.N)
q := pq.Make[int, int](b.N)
// randomize priorities to get amortized cost per op
ps := make([]int, b.N)
for i := 0; i < b.N; i++ {
@ -180,7 +192,7 @@ func BenchmarkConcSendRecv(b *testing.B) {
}
func BenchmarkHighContention(b *testing.B) {
q := priorityq.Make[int, int](b.N)
q := pq.Make[int, int](b.N)
var wg sync.WaitGroup
start := make(chan struct{})
done := make(chan struct{})