Evaluation of design alternatives for a directory-based cache coherence protocol in shared-memory multiprocessors

University dissertation from Lund : Lund University

Abstract: In shared-memory multiprocessors, caches are attached to the processors in order to reduce the memory access latency. To keep the memory consistent, a cache coherence protocol is needed. A well known approach is to record which caches have copies of a memory block in a directory and only notify the caches having a copy when a processor modifies the block. Such a protocol is called a directory-based cache coherence protocol. This thesis, which is a summary of seven papers, identifies three problems in a directory-based protocol, and evaluates implementation and performance aspects of some design alternatives. The evaluation methodology is based on program-driven simulation. The write-invalidate policy, which is used in the baseline protocol, forces all other copies of a block to be invalidated when a processor modifies the block. This leads to a cache miss each time a processor accesses an invalidated block. To reduce the number of cache misses, a competitive-update policy is proposed in this thesis. The competitive-update policy is shown to reduce both the read stall and execution times as compared to write-invalidate under a relaxed memory consistency model. However, update-based policies need more buffering and hardware support in the caches. In the baseline protocol, the implementation cost of the directory is proportional to the number of caches. To reduce this cost, an alternative directory organization is proposed which distributes the directory information among the caches sharing the same memory block. To achieve a low write latency, the caches sharing a block are organized in a tree. The caches are linked into the tree in parallel with application execution to achieve a low read latency. The hardware-implemented directory controller in the baseline protocol may lead to high design complexity and implementation cost. This thesis evaluates a design alternative where the controller is implemented using software handlers executed on the compute processor. By using efficient strategies and proper architectural support, this design alternative is shown to be competitive with the baseline protocol. However, the performance of this alternative is more sensitive to other design choices, e.g., block size and latency tolerating techniques, than the baseline protocol.

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