Subj : Re: Nice value and thread priority
To   : comp.programming.threads
From : Loic Domaigne
Date : Sun Jul 24 2005 10:13 pm

Hello Vinay,

> I am trying to understand the scheduling in Linux.

[snip]

> Can anybody please explain me the relationship between the nice value
> and the priority needed for thread scheduling in real time.

As of Linux v2.6, the kernel uses internally 140 priorities for the 
scheduling (referred in the sequel as scheduling priority), ranging from 
1 to 140. The kernel selects the KSE with the lowest priority to run 
first. Note that these priorities are only known by the kernel. The 
relationship with priority / nice value as defined by POSIX is given below.

Real-time KSE have a scheduling priority ranging from 1 to 99. The 
relationship with POSIX priority is 100-p, where p is the priority as 
defined by POSIX. In other words, a KSE with the lowest POSIX priority 
(1 on linux) has a scheduling priority of 99 and the KSE with the 
highest POSIX priority (99 on Linux) has a scheduling priority of 1.

Non real-time KSE maps linearly from 100 to 140 depending on the nice 
value following the formula 120+n, where n is the nice value (the nice 
value is comprised between -20 and 19 on Linux). In other words, a 
traditional KSE with a default nice value of 0 has a scheduling priority 
of 120.

Note that the priority of a non-real time KSE is dynamically adjusted 
using a heuristic that includes, among other, the nice value and the 
current KSE activity. On the other hand, the priority of a real-time KSE 
is never adjusted dynamically by the kernel. This assures that a 
real-time KSE always preempts a non-real time one.

More information about scheduling for Linux kernel v2.6 might be found 
in the excellent book from Robert Love "Linux Kernel Development", §3 
pp31-5, ISBN 0-672-32512-8.


HTH,
Loic.

.