/
moderator-core.go
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/
moderator-core.go
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/*
© 2022–present Harald Rudell <harald.rudell@gmail.com> (https://haraldrudell.github.io/haraldrudell/)
ISC License
*/
package parl
import (
"fmt"
"math"
"sync"
"sync/atomic"
)
const (
// default is to allow 20 threads at a time
defaultParallelism = 20
)
// ModeratorCore invokes functions at a limited level of parallelism
// - ModeratorCore is a ticketing system
// - ModeratorCore does not have a cancel feature
// - during low contention atomic performance
// - during high-contention lock performance
//
// Usage:
//
// m := NewModeratorCore(20, ctx)
// defer m.Ticket()() // waiting here for a ticket
// // got a ticket!
// …
// return or panic // ticket automatically returned
// m.String() → waiting: 2(20)
type ModeratorCore struct {
// parallelism is the maximum number of outstanding tickets
parallelism uint64
// number of issued tickets
// - if less than parallelism:
// - — moderator is in atomic mode, ie.
// - — tickets obtained by atomic access only
// - if equal to parallelism:
// - — moderator is in lock mode. ie.
// - — tickets are transfered orderly using queue,
// waiting and transferBehindLock
active atomic.Uint64
// lock used when moderator in lock mode
// - treads use the cond with waiting and transferBehindLock
// - orderly first-come-first-served
queue sync.Cond
// number of threads waiting for a ticket
// - behind lock
// - atomic so Status can read
waiting atomic.Uint64
// transferBehindLock facilitates locked ticket transfer
// - behind lock
// - atomic so it can be inspected
transferBehindLock atomic.Uint64
}
// moderatorCore is a parl-private version of ModeratorCore
type moderatorCore struct {
*ModeratorCore
}
// NewModerator creates a new Moderator used to limit parallelism
func NewModeratorCore(parallelism uint64) (m *ModeratorCore) {
if parallelism < 1 {
parallelism = defaultParallelism
}
return &ModeratorCore{
parallelism: parallelism,
queue: *sync.NewCond(&sync.Mutex{}),
}
}
// Ticket returns a ticket possibly blocking until one is available
// - Ticket returns the function for returning the ticket
//
// Usage:
//
// defer moderator.Ticket()()
func (m *ModeratorCore) Ticket() (returnTicket func()) {
returnTicket = m.returnTicket
// try available ticket at atomic performance
for {
if tickets := m.active.Load(); tickets == m.parallelism {
break // it’s lock mode
} else if m.active.CompareAndSwap(tickets, tickets+1) {
return // got atomic ticket return
}
}
// enter lock mode
m.queue.L.Lock()
defer m.queue.L.Unlock()
defer m.lastWaitCheck()
// critial section: ticket loop
var isWaiting bool
for {
// attempt atomic ticket
for {
if tickets := m.active.Load(); tickets == m.parallelism {
break // still lock mode
} else if m.active.CompareAndSwap(tickets, tickets+1) {
return // got atomic ticket return
}
}
// attempt transfer-behind-lock ticket
if m.transferBehindLock.Load() > 0 {
m.transferBehindLock.Add(math.MaxUint64)
return // ticket transfer successful return
}
// wait for ticket to become available
if !isWaiting {
isWaiting = true
m.waiting.Add(1)
defer m.waiting.Add(math.MaxUint64)
}
// blocks here
m.queue.Wait()
}
}
// lastWaitCheck prevents tickets from getting stuck as transfers
// - invoked while holding lock
// - this can happen if 1 thread is waiting and multiple threads transfer tickets
func (m *ModeratorCore) lastWaitCheck() {
if m.waiting.Load() > 0 {
return // more threads are waiting
}
var transfers = m.transferBehindLock.Load()
if transfers == 0 {
return // no extra transfers available return
}
// put extra transfers in atomic tickets
m.transferBehindLock.Store(0)
m.active.Add(math.MaxUint64 - transfers + 1)
}
// returnTicket returns a ticket obtained by Ticket
func (m *ModeratorCore) returnTicket() {
// attempt ticket-return atomically
for {
if tickets := m.active.Load(); tickets == m.parallelism {
break // lock mode: use transfer-ticket
} else if m.active.CompareAndSwap(tickets, tickets-1) {
return // ticket returned atomically return
}
}
// return ticket using transfer behind lock
m.queue.L.Lock()
defer m.queue.L.Unlock()
// if no thread waiting, return atomically
if m.waiting.Load() == 0 {
m.active.Add(math.MaxUint64)
return // atomic transfer complete return
}
// if thread waiting, do ticket transfer
m.transferBehindLock.Add(1)
m.queue.Signal() // signal while holding lock
}
// Status: values may lack integrity
func (m *ModeratorCore) Status() (parallelism, active, waiting uint64) {
parallelism = m.parallelism
active = m.active.Load()
waiting = m.waiting.Load()
return
}
// when tickets available: “available: 2(10)”
// - 10 - 2 = 8 threads operating
// - when threads waiting “waiting 1(10)”
// - 10 threads operating, 1 thread waiting
func (m *ModeratorCore) String() (s string) {
var parallelism, active, waiting = m.Status()
if active < parallelism {
s = fmt.Sprintf("available: %d(%d)", parallelism-active, parallelism)
} else {
s = fmt.Sprintf("waiting: %d(%d)", waiting, parallelism)
}
return
}