If i use nice/renice to increase the priority of any process, will that process get a higher priority when it comes to I/O? If there are two processes waiting on I/O requests, will the process that has a higher priority have its I/O request processes first?
This came up in a discussion at work, and though I don't think that it will matter, would like to get some concrete answers.
No. Nice affects the order that processes on the run queue are selected by an idle cpu. There is a different priority value that sorta impacts I/O in that if two processes are sleeping waiting for the same disk block to arrive, the higher priority process will be awakened first. This depends on what the process was doing. Example: the lowest (worst) priority process finally gets a cpu and executes a mkdir() system call. Until the directory is created, this process will have a very high kernel priority (high enough to be unkillable). But its scheduling priority remains rather low. But two processes waiting for different disk blocks cannot influence which block arrives first. In fact, the disk driver re-orders disk operations to minimize head movement. As operations are queued to a disk drive, most modern disk drives do the same thing.
Note that sometimes numericly lower priority numbers are better than higher priority numbers. It is safer to talk about better and worse priority.
Thanks for the reply Perderabo. And I meant higher priority as better priority (and a lower nice value).