Computer Architecture by Hennessy John L. Patterson David A. & David A. Patterson

Author:Hennessy, John L., Patterson, David A. & David A. Patterson
Language: eng
Format: epub
ISBN: 9780123838735
Publisher: Elsevier Science
Published: 2013-01-11T05:00:00+00:00

a. [15] <5.3> Following the convention of Figure 5.11, let us divide the execution time into instruction execution, cache access, memory access, and other stalls. How would you expect each of these components to differ between system A and system B?

b. [10] <5.3> Based on the discussion of the behavior of the On-Line Transaction Processing (OLTP) workload in Section 5.3, what is the important difference between the OLTP workload and other benchmarks that limits benefit from a more aggressive processor design?

5.25 [15] <5.3> How would you change the code of an application to avoid false sharing? What might be done by a compiler and what might require programmer directives?

5.26 [15] <5.4> Assume a directory-based cache coherence protocol. The directory currently has information that indicates that processor P1 has the data in “exclusive” mode. If the directory now gets a request for the same cache block from processor P1, what could this mean? What should the directory controller do? (Such cases are called race conditions and are the reason why coherence protocols are so difficult to design and verify.)

5.27 [20] <5.4> A directory controller can send invalidates for lines that have been replaced by the local cache controller. To avoid such messages and to keep the directory consistent, replacement hints are used. Such messages tell the controller that a block has been replaced. Modify the directory coherence protocol of Section 5.4 to use such replacement hints.

5.28 [20/30] <5.4> One downside of a straightforward implementation of directories using fully populated bit vectors is that the total size of the directory information scales as the product (i.e., processor count × memory blocks). If memory is grown linearly with processor count, the total size of the directory grows quadratically in the processor count. In practice, because the directory needs only 1 bit per memory block (which is typically 32 to 128 bytes), this problem is not serious for small to moderate processor counts. For example, assuming a 128-byte block, the amount of directory storage compared to main memory is the processor count/1024, or about 10% additional storage with 100 processors. This problem can be avoided by observing that we only need to keep an amount of information that is proportional to the cache size of each processor. We explore some solutions in these exercises.

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