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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 777
746
CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
is easy to make a new task structure for each cloned thread and have it point either
to the old thread’s scheduling, memory, and other data structures or to copies of
them. The fact that such fine-grained sharing is possible does not mean that it is
useful, however, especially since traditional UNIX versions do not offer this func-
tionality.
A Linux program that takes advantage of it is then no longer portable to
UNIX.
The Linux thread model raises another difficulty. UNIX systems associate a
single PID with a process, independent of whether it is single- or multithreaded. In
order to be compatible with other UNIX systems, Linux distinguishes between a
process identifier (PID) and a task identifier (TID). Both fields are stored in the
task structure. When
clone
is used to create a new process that shares nothing with
its creator, PID is set to a new value; otherwise, the task receives a new TID, but
inherits the PID. In this manner all threads in a process will receive the same PID
as the first thread in the process.
10.3.4 Scheduling in Linux
We will now look at the Linux scheduling algorithm.
To start with, Linux
threads are kernel threads, so scheduling is based on threads, not processes.
Linux distinguishes three classes of threads for scheduling purposes:
1. Real-time FIFO.
2. Real-time round robin.
3. Timesharing.
Real-time FIFO threads are the highest priority and are not preemptable except by
a newly readied real-time FIFO thread with even higher priority. Real-time round-
robin threads are the same as real-time FIFO threads except that they have time
quanta associated with them, and are preemptable by the clock.
If multiple real-
time round-robin threads are ready, each one is run for its quantum, after which it
goes to the end of the list of real-time round-robin threads. Neither of these classes
is actually real time in any sense. Deadlines cannot be specified and guarantees are
not given. These classes are simply higher priority than threads in the standard
timesharing class. The reason Linux calls them real time is that Linux is confor-
mant to the P1003.4 standard (‘‘real-time’’ extensions to UNIX) which uses those
names. The real-time threads are internally represented with priority levels from 0
to 99, 0 being the highest and 99 the lowest real-time priority level.
The conventional, non-real-time threads form a separate class and are sched-
uled by a separate algorithm so they do not compete with the real-time threads. In-
ternally, these threads are associated with priority levels from 100 to 139, that is,
Linux internally distinguishes among 140 priority levels (for real-time and non-
real-time tasks). As for the real-time round-robin threads, Linux allocates CPU
time to the non-real-time tasks based on their requirements and their priority levels.
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