add Semaphore and ServerSemaphores

irc/client.go

@@ -675,6 +675,11 @@ func (client *Client) destroy(beingResumed bool) {
 		return
 	}
 
+	// see #235: deduplicating the list of PART recipients uses (comparatively speaking)
+	// a lot of RAM, so limit concurrency to avoid thrashing
+	client.server.semaphores.ClientDestroy.Acquire()
+	defer client.server.semaphores.ClientDestroy.Release()
+
 	if beingResumed {
 		client.server.logger.Debug("quit", fmt.Sprintf("%s is being resumed", client.nick))
 	} else {

irc/semaphores.go

@@ -0,0 +1,81 @@
+// Copyright (c) 2018 Shivaram Lingamneni
+
+package irc
+
+import (
+	"log"
+	"runtime"
+	"runtime/debug"
+)
+
+// See #237 for context. Operations that might allocate large amounts of temporary
+// garbage, or temporarily tie up some other resource, may cause thrashing unless
+// their concurrency is artificially restricted. We use `chan bool` as a
+// (regrettably, unary-encoded) counting semaphore to enforce these restrictions.
+
+const (
+	// this is a tradeoff between exploiting CPU-level parallelism (higher values better)
+	// and not thrashing the allocator (lower values better). really this is all just
+	// guesswork. oragono *can* make use of cores beyond this limit --- just not for
+	// the protected operations.
+	MaxServerSemaphoreCapacity = 32
+)
+
+// Semaphore is a counting semaphore. Note that a capacity of n requires O(n) storage.
+type Semaphore (chan bool)
+
+// ServerSemaphores includes a named Semaphore corresponding to each concurrency-limited
+// sever operation.
+type ServerSemaphores struct {
+	// each distinct operation MUST have its own semaphore;
+	// methods that acquire a semaphore MUST NOT call methods that acquire another
+	ClientDestroy Semaphore
+}
+
+// NewServerSemaphores creates a new ServerSemaphores.
+func NewServerSemaphores() (result *ServerSemaphores) {
+	capacity := runtime.NumCPU()
+	if capacity > MaxServerSemaphoreCapacity {
+		capacity = MaxServerSemaphoreCapacity
+	}
+	result = new(ServerSemaphores)
+	result.ClientDestroy.Initialize(capacity)
+	return
+}
+
+// Initialize initializes a semaphore to a given capacity.
+func (semaphore *Semaphore) Initialize(capacity int) {
+	*semaphore = make(chan bool, capacity)
+	for i := 0; i < capacity; i++ {
+		(*semaphore) <- true
+	}
+}
+
+// Acquire acquires a semaphore, blocking if necessary.
+func (semaphore *Semaphore) Acquire() {
+	<-(*semaphore)
+}
+
+// TryAcquire tries to acquire a semaphore, returning whether the acquire was
+// successful. It never blocks.
+func (semaphore *Semaphore) TryAcquire() (acquired bool) {
+	select {
+	case <-(*semaphore):
+		return true
+	default:
+		return false
+	}
+}
+
+// Release releases a semaphore. It never blocks. (This is not a license
+// to program spurious releases.)
+func (semaphore *Semaphore) Release() {
+	select {
+	case (*semaphore) <- true:
+		// good
+	default:
+		// spurious release
+		log.Printf("spurious semaphore release (full to capacity %d)", cap(*semaphore))
+		debug.PrintStack()
+	}
+}

irc/server.go

@@ -131,6 +131,7 @@ type Server struct {
 	webirc                     []webircConfig
 	whoWas                     *WhoWasList
 	stats                      *Stats
+	semaphores                 *ServerSemaphores
 }
 
 var (
@@ -165,6 +166,7 @@ func NewServer(config *Config, logger *logger.Manager) (*Server, error) {
 		snomasks:            NewSnoManager(),
 		whoWas:              NewWhoWasList(config.Limits.WhowasEntries),
 		stats:               NewStats(),
+		semaphores:          NewServerSemaphores(),
 	}
 
 	if err := server.applyConfig(config, true); err != nil {

irc/socket.go

@@ -32,7 +32,7 @@ type Socket struct {
 	maxSendQBytes int
 
 	// this is a trylock enforcing that only one goroutine can write to `conn` at a time
-	writerSlotOpen chan bool
+	writerSemaphore Semaphore
 
 	buffer        []byte
 	closed        bool
@@ -47,9 +47,8 @@ func NewSocket(conn net.Conn, maxReadQBytes int, maxSendQBytes int) *Socket {
 		conn:          conn,
 		reader:        bufio.NewReaderSize(conn, maxReadQBytes),
 		maxSendQBytes: maxSendQBytes,
-		writerSlotOpen: make(chan bool, 1),
 	}
-	result.writerSlotOpen <- true
+	result.writerSemaphore.Initialize(1)
 	return &result
 }
 
@@ -140,14 +139,11 @@ func (socket *Socket) Write(data string) (err error) {
 
 // wakeWriter starts the goroutine that actually performs the write, without blocking
 func (socket *Socket) wakeWriter() {
-	// attempt to acquire the trylock
+	if socket.writerSemaphore.TryAcquire() {
-	select {
-	case <-socket.writerSlotOpen:
 		// acquired the trylock; send() will release it
 		go socket.send()
-	default:
-		// failed to acquire; the holder will check for more data after releasing it
 	}
+	// else: do nothing, the holder will check for more data after releasing it
 }
 
 // SetFinalData sets the final data to send when the SocketWriter closes.
@@ -179,19 +175,17 @@ func (socket *Socket) send() {
 		socket.performWrite()
 		// surrender the trylock, avoiding a race where a write comes in after we've
 		// checked readyToWrite() and it returned false, but while we still hold the trylock:
-		socket.writerSlotOpen <- true
+		socket.writerSemaphore.Release()
 		// check if more data came in while we held the trylock:
 		if !socket.readyToWrite() {
 			return
 		}
-		select {
+		if !socket.writerSemaphore.TryAcquire() {
-		case <-socket.writerSlotOpen:
-			// got the trylock, loop back around and write
-		default:
 			// failed to acquire; exit and wait for the holder to observe readyToWrite()
 			// after releasing it
 			return
 		}
+		// got the lock again, loop back around and write
 	}
 }