线程与异步

我已经从这篇非常好的文章中读到了编程的线程模型和异步模型。http://krondo.com/blog/?p=1209

然而，这篇文章提到了以下几点。

  1. 只要有 I/O，异步程序就会通过在任务之间切换而胜过同步程序。
  2. 线程由操作系统管理。

我记得读到过操作系统通过在就绪队列和等待队列(以及其他队列)之间移动 TCB 来管理线程。在这种情况下，线程也不会在等待上浪费时间，不是吗？

根据上面提到的，异步程序相对于线程程序的优势是什么？

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First of all, note that a lot of the detail of how threads are implemented and scheduled are very OS-specific. In general, you shouldn't need to worry about threads waiting on each other, since the OS and the hardware will attempt to arrange for them to run efficiently, whether asynchronously on a single-processor system or in parallel on multi-processors.

Once a thread has finished waiting for something, say I/O, it can be thought of as runnable. Threads that are runnable will be scheduled for execution at some point soon. Whether this is implemented as a simple queue or something more sophisticated is, again, OS- and hardware-specific. You can think of the set of blocked threads as a set rather than as a strictly ordered queue.

Note that on a single-processor system, asynchronous programs as defined here are equivalent to threaded programs.

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最佳答案
  1. It is very difficult to write code that is thread safe. With asyncronous code, you know exactly where the code will shift from one task to the next and race conditions are therefore much harder to come by.
  2. Threads consume a fair amount of data since each thread needs to have its own stack. With async code, all the code shares the same stack and the stack is kept small due to continuously unwinding the stack between tasks.
  3. Threads are OS structures and are therefore more memory for the platform to support. There is no such problem with asynchronous tasks.
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Async I/O means there is already a thread in the driver that does the job, so you are duplicating functionality and incurring some overhead. On the other hand, often it is not documented how exactly the driver thread behaves, and in complex scenarios, when you want to control timeout/cancellation/start/stop behaviour, synchronization with other threads, it makes sense to implement your own thread. It is also sometimes easier to reason in sync terms.

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see http://en.wikipedia.org/wiki/Thread_(computing)#I.2FO_and_scheduling

However, the use of blocking system calls in user threads (as opposed to kernel threads) or fibers can be problematic. If a user thread or a fiber performs a system call that blocks, the other user threads and fibers in the process are unable to run until the system call returns. A typical example of this problem is when performing I/O: most programs are written to perform I/O synchronously. When an I/O operation is initiated, a system call is made, and does not return until the I/O operation has been completed. In the intervening period, the entire process is "blocked" by the kernel and cannot run, which starves other user threads and fibers in the same process from executing.

According to this, your whole process might be blocked, and no thread will be scheduled when one thread is blocked in IO. I think this is os-specific, and will not always be hold.

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There are two ways to create threads:

synchronous threading - the parent creates one (or more) child threads and then must wait for each child to terminate. Synchronous threading is often referred to as the fork-join model.

asynchronous threading - the parent and child run concurrently/independently of one another. Multithreaded servers typically follow this model.

resource - http://www.amazon.com/Operating-System-Concepts-Abraham-Silberschatz/dp/0470128720

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  1. Assume you have 2 tasks, which does not involve any IO (on multiprocessor machine). In this case threads outperform Async. Because Async like a single threaded program executes your tasks in order. But threads can execute both the tasks simultaneously.

  2. Assume you have 2 tasks, which involve IO (on multiprocessor machine). In this case both Async and Threads performs more or less same (performance might vary based on number of cores, scheduling, how much process intensive the task etc.). Also Async takes less amount of resources, low overhead and less complex to program over multi threaded program.

How it works? Thread 1 executes Task 1, since it is waiting for IO, it is moved to IO waiting Queue. Similarly Thread 2 executes Task 2, since it is also involves IO, it is moved to IO waiting Queue. As soon as it's IO request is resolved it is moved to ready queue so the scheduler can schedule the thread for execution.

Async executes Task 1 and without waiting for it's IO to complete it continues with Task 2 then it waits for IO of both the task to complete. It completes the tasks in the order of IO completion.

Async best suited for tasks which involve Web service calls, Database query calls etc., Threads for process intensive tasks.

The below video explains aboutAsync vs Threaded model and also when to use etc., https://www.youtube.com/watch?v=kdzL3r-yJZY

Hope this is helpful.


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