fa09_cs433_hw5_sol - CS433: Computer Systems Organization...

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CS433: Computer Systems Organization Fall 2009 Homework 5 Assigned: Nov/8 Due in class Dec/1 Total points: 32 for undergraduate students, 40 for graduate students. Instructions: Please write your name, NetID and an alias on your homework submissions for posting grades (If you don’t want your grades posted, then don’t write an alias). We will use this alias throughout the semester. Homeworks are due in class on the date posted.
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1. [8 points] Snooping Cache Coherence Protocols I We are adding a Owned state to the basic MSI snooping cache protocol (Figure 4.7). More specifically: • One node at most may have a block of data in the owned state; if another node has the data as well, it should be in the shared state. • When a read miss occurs and another processor has the data in the Owned or Modified, that processor provides the data and the block, in that processor, transitions to (or stays in) Owned. • When a write miss occurs and another processor has the data in the Owned or Modified state, that processor provides the data and the block, in that processor, transitions to Invalid. • Main memory is updated when a block in the modified/dirty state or owned state is replaced. • A processor write to Owned state will transition the state to Modified. (a) [5 points] Draw the state transition diagram. Solution: This is the MOSI Protocol. Grading: -1 for each mistake. Diagram may be more complicated if book diagram was used as the reference.In this case, just ignore the extra information.
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(b) [3 points] How does adding this new state improve performance over the basic MSI snoopin protocol? Solution: Reduces the number of reads to memory. The node that has the data in state Owned will provide the data if someone reads it, not memory. Recall the data is not modified. Memory will only provide the data if it is not in state Owned or Modified in any cache. Grading: 1 point for correct answer, 2 point for explaination.
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2. [10 points] Snooping Cache Coherence Protocols II (a) [2 points] How does the Valid Exclusive state in the MESI (Illinois) Protocol improve performance over the basic MSI scheme? Solution: It reduces the amount of bus traffic. When a block is in the exclusive state, the node with the block does not need to send out any information on a write since the node knows it has the only copy. Grading: 1 point for correct answer, 1 point for explaination. (b) [2 points] In the MESI (Illinois) Protocol, why can’t a node transition from the shared state to the excusive state if all other nodes have had a particular block evicted from the cache, leaving that node with the only copy in memory? Solution: When a block is in the shared state, it has no information about how many other nodes might possibly have the same block. This information is not broadcasted. Grading:
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This note was uploaded on 04/18/2010 for the course CS 433 taught by Professor Harrison during the Fall '08 term at University of Illinois, Urbana Champaign.

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fa09_cs433_hw5_sol - CS433: Computer Systems Organization...

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