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