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.