Question

I’m currently doing a simulation of a hard disk drive IOs in C++, and I’m using pthread threads and a mutex to do the reading on the disk.

However I’m trying to optimize the reading time by ordering my threads. The problem is that is my disk is currently reading a sector, and a bunch of requests to read arrive, any of them will be executed. What I want is ordering them so that the request with the closest sector is executed next.

This way, the head of the virtual hard disk drive won’t move excessively.

My question is : Is using Linux process priority system a good way to make sure that the closest reading request will be executed before the others? If not, what could I rely on to do this?

PS: Sorry for my english.

Thanks for your help.

Answer 1

It is very rarely a good idea to rely on the exact behaviour of process priority schemes, especially on a general purpose operating system like Linux, because they don’t really guarantee you any particular behaviour. Making something the very highest priority won’t help if it references some address in memory or some I/O call that causes it to held up for an instant – the operating system will then run some lower priority process instead, and you will be unpleasantly surprised.

If you want to be sure of the order in which disk I/O requests are completed, or to simulate this, you could create a thread that keeps a list of pending I/O and asks for the requests to be executed one at a time, in an order it controls.