为什么OS线程被认为很昂贵? [英] Why are OS threads considered expensive?
问题描述
有许多解决方案旨在实现用户空间"线程.可能是golang.org goroutines，python的绿色线程，C#的异步，erlang的进程等.其想法是即使使用单个或有限数量的线程也允许并发编程.
There are many solutions geared toward implementing "user-space" threads. Be it golang.org goroutines, python's green threads, C#'s async, erlang's processes etc. The idea is to allow concurrent programming even with a single or limited number of threads.
我不明白的是，为什么操作系统线程如此昂贵?如我所见，无论哪种方式，您都必须保存任务堆栈(OS线程或userland线程)，该堆栈需要几十KB，并且需要一个调度程序才能在两个任务之间移动.
What I don't understand is, why are the OS threads so expensive? As I see it, either way you have to save the stack of the task (OS thread, or userland thread), which is a few tens of kilobytes, and you need a scheduler to move between two tasks.
操作系统免费提供这两个功能.为什么OS线程应该比绿色"线程更昂贵?由于每个任务"都有专用的操作系统线程，导致性能下降的原因是什么?
The OS provides both of this functions for free. Why should OS threads be more expensive than "green" threads? What's the reason for the assumed performance degradation caused by having a dedicated OS thread for each "task"?
推荐答案
我想修改Tudors的答案，这是一个很好的起点.线程有两个主要的开销:
I want to amend Tudors answer which is a good starting point. There are two main overheads of threads:
- 启动和停止它们.涉及创建堆栈和内核对象.涉及内核转换和全局内核锁定.
- 保持他们的堆叠状态.
(1)仅在始终创建和停止它们的情况下才是问题.通常使用线程池解决此问题.我认为这个问题实际上是可以解决的.在线程池上调度任务通常不涉及到内核的旅行，这使得它非常快.开销大约是一些互锁的内存操作和一些分配.
(1) is only a problem if you are creating and stopping them all the time. This is solved commonly using thread pools. I consider this problem to be practically solved. Scheduling a task on a thread pool usually does not involve a trip to the kernel which makes it very fast. The overhead is on the order of a few interlocked memory operations and a few allocations.
(2)仅当您有许多个线程(> 100个左右)时，这才很重要.在这种情况下，异步IO是摆脱线程的一种手段.我发现，如果您没有疯狂的线程数量，那么包括阻塞在内的同步IO会比异步IO快一点(您没看错:同步IO更快).
(2) This becomes important only if you have many threads (> 100 or so). In this case async IO is a means to get rid of the threads. I found that if you don't have insane amounts of threads synchronous IO including blocking is slightly faster than async IO (you read that right: sync IO is faster).
这篇关于为什么OS线程被认为很昂贵?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
