Reading a single file with Multiple Thread: should speed up?

Question

I\'m reading a file which conatins 500000 rows. I\'m testing to see how multiple thread speed up the process....

private void multiThreadRead(int num){

             


        

Answer 1

Since file reading is mainly waiting for disk I/O, you have the problem that the disk won't spin faster just because it's used by many threads :)

Answer 2

Reading from a file is an inherently serial process, assuming no caching, meaning there is a limit to how fast you can retrieve data from a file. Even without file locks (i.e. opening the file read-only) all the threads after the 1st will just block on the disk read so you make all the other threads wait and whichever one is active when the data becomes available is the one that processes the next block.

Answer 3

The reason you are seeing a slow down when reading in parallel is because the magnetic hard disk head needs to seek the next read position (taking about 5ms) for each thread. Thus, reading with multiple threads effectively bounces the disk between seeks, slowing it down. The only recommended way to read a file from a single disk is to read sequentially with one thread.