以先進先出記憶體為基礎之一致性虛擬共享記憶體系統

Abstract

虛擬共享記憶體系統結合了共享記憶體與訊息傳遞兩種多處理機系統，以 提供共享記憶體程式模式和較佳的系統延展性。虛擬共享記憶體系統中主 要的問題為如何維持各節點上記憶區塊的整體一致性。在本論文中，於一 個以先進先出記憶體為連結所建構之虛擬共享記憶體系統下，提出一個運 用弱一致性語意的記憶體一致性方法。在弱一致性模式之下，某些維持記 憶體一致性的動作可以延遲到下一個同步點，而不影響程式執行的正確性 。如此的方法可降低記憶區塊在節點間被來回傳遞的機率。在維持記憶體 一致性的動作中，處理機有時處於等待回應的狀態。在這段等待期間，可 藉由使處理機預先執行其後的指令而提高系統的效能。根據解析模型的分 析，使用本論文所提之方法可大大地減少為維持記憶體一致性所引發的動 作，進而改善系統的整體效能。 Virtually shared memory (VSM) is a cost-effective approach to adapting the well-understood shared-memory programming model to message-passing multicomputers, attempting to provide the best of both worlds: ease of programming and better system scal- ability. The major problem with a VSM system is to maintain the coherence of distributed data entities. In this thesis, a memo- ry coherence scheme that exploits weak consistency semantics is proposed for a VSM system based on FIFO RAM links. Under weak consistency model, invalidation of memory blocks can be delayed until the next synchronization point. Such a scheme is called lazy invalidation, and is effective in reducing memory thrashing. In addition, the overall system performance can be further improved by overlapping a look-ahead computing and the waiting period required for a memory miss. According to the analytical modeling, the memory miss rate can be greatly reduced by using our scheme.