01 01 2022 E TS

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Advanced Operating Systems - Part A, January 1st 2022 Exercises and solutions Exam finalization Question 1.1 This is part A of the AOS’s exam and today you are finalising your entire grade. You must express now how you have finalised part B. The answer is mandatory for the entire exam. Exercise 2 — Open and closed questions (Points 15) Question 2.1 (Points 5) Describe the total store ordering (TSO) memory model and how it affects concurrent programming within the Linux kernel. Solution • In a multiprocessor, a memory model describes how the order of the operations on shared data done by one cpu is seen by another CPU. • In a TSO processor – The apparent ordering is impacted by the use CPU-local write queue to hide memory latency. – Stores are not seen immediately but, once seen, all cpus do agree on the (total) order in which they reached the shared memory • The effect is that loads might be reordered vs stores, e.g.: 1 store 1, X 2 load r1, Y 3 store 1, Y 4 load r2, X might behave like the following sequence 1 load r1, Y 2 load r2, X 3 store 1, X 4 store 1, Y • To force the code execution back into a sequentially consistent behavior, write memory fences (barrier) instructions must be used. 1 store 1, X 2 wmb 3 load r1, Y 4 store 1, Y 5 wmb 6 load r2, X Question 2.2 (Points 5) What is the purpose of the slab allocator in Linux? What is is overall structure? Solution • The slab allocator’s purpose is to efficiently allocate/deallocate kernel structures In fact, you can’t use a page for each structure because it would bring to considerable internal fragmentation. • An object cache is a set of slabs used to store objects of a specific type t. • The slab is an intermediary structure which consists of one or more contiguous page frames that contain both allocated and free…