Announcement
 Class this Wednesday (11/4) will be canceled due to
travel
ECE 455/555 Embedded System Design
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Recap from last two classes
 Hardware support
 CMOS, Clock gating, Supply shutdown, Dynamic voltage
scaling
 Dynamic power management
 Adjusts power mode to adapt to workload variations
 Baseline: greedy policy
• Break-even time TBE
 Energy saving
 Predictive techniques
 Power manager
 Advanced Configuration and Power Interface (ACPI)
 Holistic approach
ECE 455/555 Embedded System Design
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ECE 455/555
Embedded System Design
Operating Systems - I
Wei Gao
Fall 2015
Basic Functions
 OS controls resources:
 who gets the CPU;
 when I/O takes place;
 how much memory is allocated.
 Application programs run on top of OS services
 Challenge: manage multiple, concurrent tasks.
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Example: Sensor Motes
 Sensing
 Sampling (multiple) sensors in different rates
 Communication
 Send data
 Receive data
 Computation
 Data processing and aggregation
 Routing
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Life without Processes
Code turns into a mess:
 Spaghetti code
 do all tasks in a single piece of
code (with a loop)
 no separation of concern
 Control structure gets very
complex
 Hard to guarantee different
timing requirements
 Cannot handle varying execution
time or rate
 Cannot handle dynamic task
arrivals
time
A
B
C
A
C
ECE 455/555 Embedded System Design
A_code();
…
B_code();
…
if (C) C_code();
…
A_code();
…
switch (x) {
case C: C();
case D: D();
...
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Co-Routines Methodology
 Idea to realize controlled concurrency
 Separate tasks into different code segments (co-routines)
 Co-routines voluntarily give up control to other co-routines.
 Pattern of control transfers and context-switch are embedded in the
code.
Co-routine 1
ADR r14,co2a
co1a …
ADR r13,co1b
MOV r15,r14
co1b …
ADR r13,co1c
MOV r15,r14
co1c ...
Co-routine 2
co2a …
ADR r14,co2b
MOV r15,r13
co2b …
ADR r14,co2c
MOV r15,r13
co2c …
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Process
 A process is a unique execution of a program.
 Several copies of a program may run simultaneously or at
different times.
 A process has its own context:
 Data in registers, PC, status, memory.
 Stored in activation record
 OS manages processes.
 TinyOS: a task is similar to a process
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Processes and CPUs
 Activation record
 process context.
 Context switch:
 current CPU context goes
out
 new CPU context goes in
process 1
context
process 2
context
...
PC
registers
CPU
memory
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Cooperative Multitasking
 Improvement to co-routines:
 hides context switching mechanism;
 still relies on processes to voluntarily give up CPU.
 Each process allows a context switch at cswitch() call.
 Separate scheduler chooses which process runs next.
if (x > 2)
sub1(y);
else
sub2(y, 2);
cswitch();
proca(a, b, c);
Process 1
Student A
save_state(current);
p = choose_process();
load_and_go(p);
Scheduler
TA
ECE 455/555 Embedded System Design
proc_data(r, s, t);
cswitch();
If (val1 == 3)
abc(val2);
rst(val3);
Process 2
Student B
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Problems with Cooperative Multitasking
 Programming errors can keep other processes out:
 process never gives up CPU;
 process waits too long to switch, missing input.
Process2() {
x = global1; /* global1 is an input to the process */
while (x < 500)
x = aproc(global2); /* subroutine does its work */
cswitch();
}
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Preemptive Multitasking
 No more voluntary release of CPU
 Operating System (OS) is now in charge
Timer
 Most powerful form of multitasking:
interrupt
 OS controls when context switches;
 OS determines what process runs next.
 Use periodic timer interrupts to call OS to
switch contexts
interrupt
P1
OS
CPU
interrupt
P1
OS
P2
Flow of control with preemption
ECE 455/555 Embedded System Design
time
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Process Scheduling
 Who gets CPU?
 Usually priority-based
• Priority can be based on deadline, requested time, or…
 When context switch occurs?
 Low priority tasks are running while high priority tasks are
waiting
 We have two classes to introduce different
scheduling algorithms
 Key issue of real-time operating systems
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TinyOS: Tasks vs. Interrupts
 Tasks do intensive computations
 Unpreemptable FIFO scheduling
 Bounded number of pending tasks
 Events handle interrupts
 Interrupts trigger lowest level events
 Events can signal events, call commands, or post tasks
 Two priorities
 Interrupt
POST
events
 Tasks
Tasks
Preempt
FIFO
commands
commands
Interrupts
Hardware
ECE 455/555 Embedded System Design
Time
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POSIX
 IEEE standards designed to provide application
portability between Unix variants.
 IEEE 1003.1 defines a Unix-like OS interface.
 IEEE 1003.2 defines the shell and utilities
 IEEE 1003.4 defines real-time extensions.
 Supported by many operating systems
 Variants of UNIX: AIX, HP-UX, Solaris, Linux
 Many commercial RTOS, e.g., VxWorks
 Windows provides similar services
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Processes in POSIX
Create a process with fork:
 parent process keeps executing the old program;
 child process executes a new program.
Process A
Process A
Process B
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Create a New Process using fork()
 A process can create a child process by making a
copy of itself
 Parent process is returned with the child process ID
 Child process gets a return value of 0
child_id = fork();
if (child_id == 0) {
/* child operations */
} else {
/* parent operations */
}
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Overlay a Process using execv()
 Child process usually runs different code
 Use execv() to overlay the code of a process
 Parent uses wait() to wait for child process to finish and then
release its memory
childid = fork();
if (childid == 0) {
execv(“mychild”,childargs);
exit(0);
} else {
parent_stuff();
wait(&csstatus);
exit(0);
}
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Process States
 A process can be in one of three states:
 executing on the CPU;
 ready to run;
 waiting for data.
executing
gets
CPU
Scheduler
preempted
needs
data
gets data
and CPU
gets data
ready
waiting
needs data
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Process Scheduling: Embedded vs.
General-Purpose
 Workstations try to avoid starving processes of CPU
access.
 Fairness = access to CPU.
 Embedded systems must meet deadlines.
 Low-priority processes may not run for a long time.
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Priority-Driven Scheduling
 Every process has a priority.
 CPU goes to the highest-priority ready process
 Variants
 Fixed vs. dynamic priority
 Preemptive vs. non-preemptive
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Scheduling Example
 Each process has a fixed priority (1 highest);
 P1: priority 1; P2: priority 2; P3: priority 3.
P3 released
P1 released
P2 released
P2
0
P1
10
20
P2
30
P3
40
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time
60
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The Scheduling Problem
 Can we meet all deadlines?
 Must be able to meet deadlines in all cases.
 How much CPU horsepower do we need to meet our
deadlines?
 Timing violations: What happens if a process doesn’t finish by
its deadline?
 Hard deadline: system fails if missed.
 Soft deadline: user may notice, but system doesn’t necessarily fail.
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Example: Space Shuttle Software
Error
 Space Shuttle’s first launch was delayed by a
software timing error:
 Primary control system PASS and backup system BFS.
 BFS failed to synchronize with PASS.
 A change to one routine added unnoticed delay that threw
off start time calculation.
 1 in 67 chance of timing problem.
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Summary
 OS: manage multiple, concurrent tasks
 Process
 Co-routines methodology
 Multitasking with context switch
 Processes in TinyOS and POSIX
 fork(), execv()
 Process states and management
 Process scheduling
 Priority-driven scheduling
 Space shuttle software error
 Inter-process communication
 Shared memory
 Message passing
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Reading
 Textbook: 6.1-6.3
 Recommended reading:
Operating System Concepts, A. Silberschatz, B. Galvin
and G. Gagne, 8th edition, 2008.
(the famous “dinosaur” book)
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