使用ifstream在两个线程中处理相同的文件 [英] Process same file in two threads using ifstream

问题描述

我的应用程序中有一个包含大量信息的输入文件。按顺序读取它，并且一次只读取一个文件，这对我的应用程序的使用是不够的。理想情况下，我希望有两个线程，它们具有独立且不同的 ifstream s读取同一文件的两个唯一文件偏移量。我不能只启动一个 ifstream ，然后使用它的复制构造函数复制它（因为它是不可复制的）。 那么，我该如何处理？

I have an input file in my application that contains a vast amount of information. Reading over it sequentially, and at only a single file offset at a time is not sufficient for my application's usage. Ideally, I'd like to have two threads, that have separate and distinct ifstreams reading from two unique file offsets of the same file. I can't just start one ifstream up, and then make a copy of it using its copy constructor (since its uncopyable). So, how do I handle this?

我可以立即想到两种方式，

Immediately I can think of two ways,

1. 为第二个帖子构建一个新的 ifstream ，在同一个文件中打开它。


2. 在两个线程中共享一个打开的 ifstream 的单个实例（例如使用 boost :: shared_ptr<> ）。当线程获得时间片时，寻找当前线程当前感兴趣的适当文件偏移量。

1. Construct a new ifstream for the second thread, open it on the same file.
2. Share a single instance of an open ifstream across both threads (using for instance boost::shared_ptr<>). Seek to the appropriate file offset that current thread is currently interested in, when the thread gets a time slice.

是一个这两种方法首选吗？

我还没有想到第三种（或第四种）选项吗？

显然我最终受到硬盘驱动器来回限制的限制，但我感兴趣的是利用（如果可能的话），是一些操作系统级别的磁盘缓存同时在两个文件偏移处。

Obviously I am ultimately limited by the hard drive having to spin back and forth, but what I am interested in taking advantage of (if possible), is some OS level disk caching at both file offsets simultaneously.

谢谢。

推荐答案

两个 std :: ifstream 实例可能是这里的最佳选择。现代硬盘驱动器针对大量I / O请求进行了优化，因此同时从两个 std :: ifstream 实例中读取应该可以提供相当不错的性能。

Two std::ifstream instances will probably be the best option here. Modern HDDs are optimized for a large queue of I/O requests, so reading from two std::ifstream instances concurrently should give quite nice performance.

如果你有一个 std :: ifstream 你将不得不担心同步对它的访问，加上它可能会破坏操作系统的自动化顺序访问预读缓存，导致性能较差。

If you have a single std::ifstream you'll have to worry about synchronizing access to it, plus it might defeat the operating system's automatic sequential access read-ahead caching, resulting in poorer performance.

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