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semaphores.go
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semaphores.go
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// 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()
}
}