Advanced Operating Systems: Lecture 19 - Mr. Farhan Zaidi

CS703 ­ Advanced 
Operating Systems
By Mr. Farhan Zaidi


Lecture No. 
19


Characteristics of Real­Time 
Operating Systems


Reliability




Degradation of performance may have
catastrophic consequences

Fail-soft operation




Ability of a system to fail in such a way as to
preserve as much capability and data as possible
Stability

Features of Real­Time Operating 
Systems






Fast process or thread switch
Small size
Ability to respond to external interrupts
quickly
Multitasking with inter-process
communication tools such as semaphores,
signals, and events


Features of Real­Time Operating 
Systems








Use of special sequential files that can
accumulate data at a fast rate
Preemptive scheduling base on priority

Minimization of intervals during which
interrupts are disabled
Delay tasks for fixed amount of time
Special alarms and timeouts


Real­Time Scheduling


Static table-driven




Static priority-driven preemptive




Traditional priority-driven scheduler is used

Dynamic planning-based




Determines at load or even compile time when a
task begins execution

Feasibility determined at run time

Dynamic best effort


No feasibility analysis is performed


Deadline Scheduling


Information used








Ready time
Starting deadline
Completion deadline
Processing time
Resource requirements
Priority
Subtask scheduler


Rate Monotonic Scheduling




Assigns priorities to tasks on the basis of
their periods
Highest-priority task is the one with the
shortest period


Dynamic Memory Allocation
Application
Dynamic Memory Allocator
Heap Memory


Explicit vs. Implicit Memory Allocator






Explicit: application allocates and frees space
 E.g., malloc and free in C
Implicit: application allocates, but does not free space
 E.g. garbage collection in Java, ML or Lisp

Allocation




In both cases the memory allocator provides an abstraction of
memory as a set of blocks
Gives out free memory blocks to application


Process Memory Image
kernel virtual memory

memory invisible to
user code

stack

%esp

Memory mapped region for
shared libraries

Allocators request
additional heap memory
from the operating
system using the sbrk
function.

the “brk” ptr
run-time heap (via malloc)
uninitialized data (.bss)
initialized data (.data)

program text (.text)
0


Assumptions


Memory is word addressed (each word can hold a pointer)

Allocated block
(4 words)

Free block
(3 words)

Free word
Allocated word


Constraints


Applications:





Can issue arbitrary sequence of allocation and free requests
Free requests must correspond to an allocated block

Allocators







Can’t control number or size of allocated blocks
Must respond immediately to all allocation requests
 i.e., can’t reorder or buffer requests
Must allocate blocks from free memory
 i.e., can only place un-allocated blocks in free memory
Must align blocks so they satisfy all alignment requirements
 8 byte alignment for GNU malloc (libc malloc) on Linux
machines