Traditional OS Processes and Scheduling
Lecture 15hb
Introduction to Embedded Systems
Summary of Previous Lecture
• Virtual memory
– Virtual addresses
– Page tables
– Page faults; thrashing
Introduction to Embedded Systems
Administrivia
• Quiz #3 in the second half of today’s lecture
– Will be open book/open notes
• Mid-Term Exam Will be open book/open notes
– Mid-term grades will be based on Quizzes #1-3 and Labs 1-2
Introduction to Embedded Systems
Outline of This Lecture
• Why also understand traditional OS processes and
scheduling?
• Unix processes
• Traditional Unix Scheduling
Introduction to Embedded Systems
Why Understand Traditional OS Notions?
• To understand what is possible,
• To understand what is done on traditional OSs,
• Most importantly, to understand how and why embedded
(real-time) operating systems are common in some aspects
and different in other aspects
Introduction to Embedded Systems
Dispatching
• The dispatcher (shortterm scheduler) is the entity
responsible for adding a process/thread to the run queue.
• It is invoked by the scheduler upon an event
– Examples include:
• End of a quantum
• End of a period
• End of a process
• End of a thread
• This thread of execution is explicitly invoked when
something happens
• The thread is, however, loaded up in memory.
– Where?
– What is the address space of this thread?
Introduction to Embedded Systems
Address Spaces
• Each heavyweight process has its own address space
• Each thread shares its address space and global data
structures with other threads in the same process
• There are provisions to share memory among heavyweight
processes: shared memory
– In Unix, you have to explicitly declare the shmem, get an id for the
area, attach yourself to it.
• Another way of sharing some info is between parent and
child processes
– They share the list of open files and the code of the process
(executable image)
Introduction to Embedded Systems
Forking and Processes
• When a Unix process calls a fork() at some point in the
code, a new (child) process becomes ready at the same point
of the parent process
• How does one define who is the parent (child) process?
• PID = fork() returns a process identifier (PID)
– In the child process, the value of the PID is zero
– In the parent process, PID contains the child's PID
• This PID is useful for the parent process to keep track of all
its children
– e.g., who is alive, who terminated
Introduction to Embedded Systems
What happens on a fork() call?
if (PID == 0) {
/* this is the child process;
* add your code here for a small fee
*/
}
else {
children[current_child] = PID;
current_child++;
/* this is the rest of the parent
* process; continue your code here
* E.g.: wait for your child process to
* come back.
*/
}
Introduction to Embedded Systems
UNIX Processes
• The bootstrap program will start the init process
• init is the parent of all processes
• It will fork off terminal processes (tty) by reading the
/etc/ttys file
• Each terminal has a process associated with it
– e.g., it is this process that prints the “prompt”, does validation of
userid/password and then waits for input
• Many of the user (shell) commands will fork off a new
process to work on the user commands.
Introduction to Embedded Systems
Unix
• On the other hand, some processes are initialized by the
kernel itself (after all the main functions were put in place
correctly).
• In Unix, processes that run in the background, such as
printing or the ftp manager, are called daemons
– These daemons complement the kernel functionality
• Everything in Unix is a process and the kernel is stiff (single
locus of execution) but efficient
Introduction to Embedded Systems
Unix Scheduling
• The problem with multiple queues is starvation
– This can be solved by aging, which means that the longer a process
executes, the higher its priority
– Unix scheduling is round-robin with multilevel feedback
– It computes the priority of the processes according to the user
directives, resource usage and aging
• User directives: nice command (positive integer)
• Resource usage is computed by the kernel
– Aging is computed by increasing the priority of the process every
quantum regardless of anything else
Introduction to Embedded Systems
Unix Scheduling (cont)
• The algorithm for setting the priority, at each quantum:
– CPU_usage = CPU_usage/2
– priority = CPU_usage/2 + base_priority +
nice_level
– base_priority is typically set to 70
– nice_level is typically 0, unless nice command is used
– CPU_usage starts out as 0, clearly
– Initial priority starts out the same as base_priority
• Does this algorithm actually give higher priority to aging
processes?
• Does it take into consideration resource usage?
Introduction to Embedded Systems
Example of UNIX scheduler
• Quantum 1sec, base priority 60, nice 0, init priority 0
• Clock interrupts the system 60 times/sec (or 60 times/quantum)
– At each interrupt, CPU usage is incremented
– Thus, in one quantum, CPU usage increases by 60
• At every quantum, kernel performs calculations
– CPU = decay(CPU) = CPU/2
– Process priority = (CPU/2) + 60
time
procA
pr CPU
procB
pr CPU
procC
pr
CPU
0
1
2
3
4
5
60
75
67
63
76
68
60
60
75
67
63
76
60
60
60
75
67
63
0
60,30
15
7
67,33
16
0
0
60,30
15
7
67,33
0
0
0
60,30
15
7
Introduction to Embedded Systems
More on Unix Scheduling
• Priorities in Unix are divided into Kernel and User
• Some processes executing with Kernel priority are nonpreemptable
– e.g., diskrelated system calls
• Processes executing with User priority are preemptable
– user processes, or system functions executing for the user
• Unix grants higher priorities to processes waiting for lowerlevel algorithms (e.g., disk read/write).
– why?
• UNIX disregards the job characteristics
– e.g., IObound or CPUbound processes when scheduling
Introduction to Embedded Systems
The “top” Utility on Unix (/usr/bin/top)
Introduction to Embedded Systems
Summary of Lecture
• Dispatching
• Unix as an example of a traditional general-purpose OS
– Unix processes and threads
• Unix Scheduling
• Looking at “top”
• GOOD LUCK FOR QUIZ #3!!
Introduction to Embedded Systems