1. CS703 - Advanced Operating Systems
By Mr. Farhan Zaidi

2. Lecture No. 19

3. Characteristics of Real-Time 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

4. Features of Real-Time 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

5. Features of Real-Time 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

6. Real-Time Scheduling Static table-driven
Determines at load or even compile time when a task begins execution
Static priority-driven preemptive
Traditional priority-driven scheduler is used
Dynamic planning-based
Feasibility determined at run time
Dynamic best effort
No feasibility analysis is performed

7. Deadline Scheduling Information used Ready time Starting deadline
Completion deadline
Processing time
Resource requirements
Priority
Subtask scheduler

8. Rate Monotonic Scheduling
Assigns priorities to tasks on the basis of their periods
Highest-priority task is the one with the shortest period

9. 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

10. Process Memory Image Allocators request additional heap memory
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)

11. Assumptions Memory is word addressed (each word can hold a pointer)
Free word
Allocated block
(4 words)
Free block
(3 words)
Allocated word

12. 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