C++ Boost Multithread is slower than single thread because of CPU type?

Question

I had posted some boost multithreads before. This time I just curious and disappointed because I thought multithreads suppose to be faster than single one.

Two threads are FILE I/O read/parse the CSV data. When I used multithreads, it took about 40 seconds average per machine PENTIUM D CPU from DELL DESKTOP OPTILEX 745.

With single thread, it took about 8-10 seconds average same PC.

I had tried to use completely different parameters name from these two threads, the result is no different. If someone had been used c++ boost multi-threads for reading big data file and parsing before, I would love to hear your opinions. Thanks. Andrew

Answer 1

Two threads are FILE I/O read/parse the CSV data.

If they're reading the same file with the same file handle, then they might be spending most of their time blocked waiting for the other thread to get done. If they're using different file handles to read the same file, they're forcing the disk to keep seeking back and forth, which isn't as efficient an operation as a straight sequential read.

Threading doesn't speed up big file reading and parsing. What it does is let you do something else entirely while the file is being read and parsed.

You've created an I/O bottleneck, which threading does not help with. Threading is intended for reducing CPU bottlenecks when the algorithm can be broken into independent threads of execution; algorithms that have a lot of dependency on previous output (file parsing is one case) generally don't thread well.

If you can split up the parsing problem and have each thread parse a different part of the file, you might get a little improvement, but probably not since the seeking will be wasting your time. If you can have one thread doing bulk reading and some preprocessing, then handing off chunks to a thread pool for the real heavy processing (is ther any?), then you might see some noticeable improvement over single threading.

This is all general and a bit stream-of-consciousness, but it's hard to do much more with what you're giving. I hope it helps.

Answer 2

Without seeing your code it's hard to say exactly what's going on, but in general, multiple threads don't necessarily get you better performance, and in fact can very often lead to obvious performance degradation.

In your situation, if you are having both threads read and parse, then they could be contending for I/O, and possibly the locks surrounding any shared read/write memory areas, both of which would introduce a slow-down where the single-threaded version would have no issue.

To do this properly, you would probably want a single thread reading from the file, and another thread parsing the data as it came in on a producer/consumer queue. This would minimize the lock contention (since it can be implemented with waiters only), and would ensure that you were acutally taking advantage of the parralellization available in your problem.

That being said, a single-threaded version might still be faster; it's often the case.