有两类程序员几乎不出 多线程bug:一类是啥也不懂,只要涉及到多 线程 就直接上锁;另一类熟读内存模型、体系结构、缓存一致性协议、内存屏障、竞态条件、指令重排……然后决定只要涉及到 多线程 就直接上锁。

“Concurrency is about dealing with lots of things at once. Parallelism is about doing lots of things at once” – Rob Pike

Concurrency

  • To overlap disk O requests
  • To reduce latency by prefetching results to expected queries
  • To take advantage of multiple processors.
// A concurrent prime sieve
 
package main
 
// Send the sequence 2, 3, 4, ... to channel 'ch'.
func Generate(ch chan<- int) {
	for i := 2; ; i++ {
		ch <- i // Send 'i' to channel 'ch'.
	}
}
 
// Copy the values from channel 'in' to channel 'out',
// removing those divisible by 'prime'.
func Filter(in <-chan int, out chan<- int, prime int) {
	for {
		i := <-in // Receive value from 'in'.
		if i%prime != 0 {
			out <- i // Send 'i' to 'out'.
		}
	}
}
 
// The prime sieve: Daisy-chain Filter processes.
func main() {
	ch := make(chan int) // Create a new channel.
	go Generate(ch)      // Launch Generate goroutine.
	for i := 0; i < 100; i++ {
		prime := <-ch
		print(prime, "\n")
		ch1 := make(chan int)
		go Filter(ch, ch1, prime)
		ch = ch1
	}
}
 

Parallelism

Multi process.

Asynchronous

Concurrency is having two tasks run in parallel on separate threads. However, asynchronous run in parallel but on the same thread.