C ++ / Linux中的线程排序

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.

Answer 2

The I/O schedulers in the Linux kernel can re-order and coalesce reads (and to some extent writes) so that their ordering is more favorable for the disk, just like you are describing. This affects the process scheduler (which takes care of threads too) in that the threads waiting for I/O also get "re-ordered" - their read or write requests complete in the order in which the disk served them, not in the order in which they made their request. (This is a very simplified view of what really happens.)

But if you're simulating disk I/O, ie if you're not actually doing real I/O, the I/O scheduler isn't involved at all. Only the process scheduler. And the process scheduler has no idea that you're "simulating" a hard disk - it has no information about what the processes are doing, just information about whether or not they're in need of CPU resources. (Again this is a simplified view of how things work).

So the process scheduler will not help you in re-ordering or coalescing your simulation of read requests. You need to implement that logic in your code. (Reading about I/O schedulers is a great idea.)

If you do submit real I/O, then doing the re-ordering yourself could improve performance in some situations, and indeed the I/O scheduler's algorithms for optimizing throughput or latency will affect the way your threads are scheduled (for blocking I/O anyway - asynchronous I/O makes it a bit more complicated still).