Question

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?

Immediately I can think of two ways,

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.

Is one of these two methods preferred?

Is there a third (or fourth) option that I have not yet thought of?

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.

Thanks.

Answer 1

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.

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.