Advanced Operating Systems: Lecture 19 - Mr. Farhan Zaidi

Advanced Operating Systems - Lecture 19: Characteristics of real-time operating systems. This lecture will cover the following: degradation of performance may have catastrophic consequences; features of real-time operating systems; real-time scheduling; static priority-driven preemptive;...

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) 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  byte alignment for GNU malloc (libc malloc) on Linux machines .. .Lecture? ?No.  19 Characteristics of Real­Time  Operating? ?Systems  Reliability   Degradation of performance may have catastrophic consequences Fail-soft operation   Ability... 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... 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