Channel buffering in Go

Go provides channels as a core concurrency primitive. Channels can be used to pass objects between goroutines safely and can be used to construct quite complex concurrent data structures, however they are still quite a low-level construct and it is not always clear how to use them best.

I was reminded of this when I saw Evan Huus’s excellent talk on Complex Concurrency Patterns in Go at the Golang UK conference. One of the points he made was that on his team infinite buffering was considered a “code smell”. This is something I find interesting, particularly as someone who has written a bit of Erlang code. In the Erlang concurrency model there are no channels but every process (processes in Erlang are lightweight, like goroutines) has an input message queue, which is effectively unbounded in size. In Go channels are first class objects and must provide a finite buffer size on construction, the default value of which is zero.

On the face of it the Go approach is appealing. Nobody wants runaway resource allocation in their program and Go provides a useful way of, for example, throttling excessively fast producers in a producer-consumer system to prevent them filling up memory with unprocessed data. But how large should you make channel buffers?

Unbuffered channels are the default in Go but can have some undesirable side effects. For example a producer and consumer connected by an unbuffered channel can cause reduced parallelism as the producer blocks waiting for the consumer to finish working and the consumer blocks waiting for the producer to produce output, for example in the following code increasing the channel buffer from zero to one will cause a speedup, at least on my machine:

package main

import (
	"fmt"
	"math/rand"
	"time"
)

func producer(ch chan bool) {
	for i := 0; i < 100000; i++ {
		time.Sleep(time.Duration(10 * rand.Float64() * float64(time.Microsecond)))
		ch <- true
	}
	close(ch)
}

func consumer(ch chan bool) {
	for _ = range ch {
		time.Sleep(time.Duration(10 * rand.Float64() * float64(time.Microsecond)))
	}
}

func unbuffered() {
	ch := make(chan bool, 0)
	go producer(ch)
	consumer(ch)
}

func buffered() {
	ch := make(chan bool, 1)
	go producer(ch)
	consumer(ch)
}

func main() {
	startTime := time.Now()
	unbuffered()
	fmt.Printf("unbuffered: %vn", time.Since(startTime))
	startTime = time.Now()
	buffered()
	fmt.Printf("buffered: %vn", time.Since(startTime))
}

Unbuffered channels are also more prone to deadlock. It can be argued whether this is a good thing or bad thing – deadlocks that may not be apparent with buffered channels become visible with unbuffered channels which allows them to be fixed and the behaviour of an unbuffered system is generally simpler and easier to reason about.

So if we decide we need to create a buffered channel, how large should that buffer be? That question turns out to be pretty hard to answer. Channel buffers are allocated with malloc at when the channel is created, so the buffer size is not an upper bound on the size of the buffer but the actual allocated size – the larger the buffer size the more system memory it will consume and the worse cache locality it will have. This means we can’t just use a very large number and treat the channel as if it was infinitely buffered.

In the example below, which is inspired by an open source project, we have a logging API that internally has a channel that is used to pass log messages from the API entry point to the log backend which writes the log messages to a file or network based on the configuration:

type Logger struct {
	logChan chan string
}

func NewLogger() *Logger {
	return &Logger{logChan: make(chan string, 100)}
}

func (l *Logger) WriteString(str string) {
	l.logChan <- str
}

In this case the channel is providing two things. Firstly, it is providing synchronization so multiple goroutines can call into the API safely at the same time. This is a fairly uncontentious use of a channel. Secondly, it is providing a buffer to prevent callers of the API from blocking unduly when many logging calls are made around the same time – it won’t increase throughput but will help if logging is bursty. But is this a good use of a channel? And is 100 the right size to use?

The two questions are somewhat intertwined in my opinion. Channels make good buffers for small buffer sizes. As mentioned above, the storage for a channel is statically allocated so a large buffer can more space efficiently be implemented in other ways. The right size of a buffer depends on many things, in this example the number of goroutines logging, how bursty the logging is, the tolerance for blocking, the size of the machine memory and probably many others. For this reason the API should provide a way of setting the size of the channel buffer so users can adapt it to their needs rather than hardcoding the size.

So a good size for a buffer could be zero, one or some other small number – say five or less – or a variable size that can be set by the API but it is unlikely, in my opinion, to be a large constant value like 100. Infinite buffer sizes, that is a buffer only limited by available memory, are sometimes useful and not something we should dismiss altogether although they are obviously not going to be possible to implement directly with channels as provided by Go. It is possible to create a buffer goroutine with an input channel and an output channel that can implement a memory efficient variable-sized buffer, for example as Evan does here and this is the best choice if you expect your buffering requirements to be large.

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