lec15_16_sync - Resources and Resource Access Control (only...

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1 Resources and Resource Access Control (only on Fixed-Priority System) - Chapter 8 - Overview • Resource access (Mutual Exclusive) – Priority inversion – Unbounded priority inversion • Resource access control protocol – Priority inheritance protocol – Priority ceiling protocol
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2 Mutually exclusive resource sharing Task1 (High) Task2 (Low) CPU Rd Wr Rd Add Task1 Task2 release release A A=0 Add Wr A=1 A=1 Rd Wr Rd Add Task1 Task2 release release try lock BLOCK lock unlock Add A=0 A=1 Wr A=2 Mutually exclusive resource sharing (Another example) Task1 (High) Task2 (Low) CPU COM Plant C R C R Task1 Task2 R2 R1 Plant C R C R Task1 Task2 R2 R1 Plant release release release release try lock BLOCK lock unlock
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3 Unbounded Priority Inversion When a high priority task is delayed by lower priority tasks, it is said that priority inversion has occurred and the high priority task is blocked by the lower priority task. Priority inversion occurs during synchronization. When tasks synchronize, we expect delays due to the use of mutual exclusion. And we expect that the delay due to mutual exclusion is a function of the duration of the critical sections. When the duration of priority inversion is not bounded by a function of the duration of critical sections, unbounded priority inversion is said to occur. Unbounded Priority Inversion τ 1:{. ..P(S). ..V(S). ..} τ 3:{. ..P(S). ..V(S). ..} τ1 (h) τ2( m ) τ3 (l) Legend S Locked Executing Blocked Legend S Locked Executing Blocked time S Locked S Unlocked B B S Locked S Unlocked Attempt to Lock S
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Mars Pathfinder (Rover) What really happened on Mars? The Mars Pathfinder mission was widely proclaimed as "flawless" in the early days after its July 4 th , 1997 landing on the Martian surface . Successes included its unconventional "landing" -- bouncing onto the Martian surface surrounded by airbags, deploying the Sojourner rover, and gathering and transmitting voluminous data back to Earth, including the panoramic pictures that were such a hit on the Web. But a few days into the mission, not long after Pathfinder started gathering meteorological data, the spacecraft began experiencing total system resets, each resulting in losses of data. The press reported these failures in terms such as "software glitches" and "the computer was trying to do too many things at once". VxWorks provides preemptive priority scheduling of threads . Tasks on the Pathfinder spacecraft were executed as threads with priorities that were assigned in the usual manner reflecting the relative urgency of these tasks. Pathfinder contained an "information bus", which you can think of as a shared memory area used for passing information between different components of the spacecraft. A bus management task ran frequently with high priority to move certain kinds of data in and out of the information bus . Access to the bus was synchronized with mutual exclusion locks (mutexes). You can read more at
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This note was uploaded on 07/02/2008 for the course COMPUTER S 664 taught by Professor Leechangkyu during the Spring '08 term at Korea University.

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lec15_16_sync - Resources and Resource Access Control (only...

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