Java 虛擬機器的省電設計

Abstract

記憶體系統一直是嵌入式行動裝置中，悠關系統效能、能耗及系統成本的重要單位。隨著記憶體裝置技術的發展，兼具非揮發性、資料保存性高、低能耗優點的 Flash Memory 已經成為嵌入式行動裝置記憶體系統中不可或缺的重要元件。 NAND Flash 具有高密度、低成本的特性，隨著新技術的發展，將逐漸取代 NOR Flash 在嵌入式記憶體系統中所扮演 Code Storage 的角色。新的 NAND + RAM 架構下，系統程式及資料將儲存在 NAND Flash 中，而 RAM 則為工作時期資料暫存區。

Java 具有可攜性高的優點, 在嵌入式小型系統實作 Java 平台的需求是日趨重要的課題。Java 虛擬機器的效能，一直為使用者所垢病，更何況是資源有限之嵌入式系統，如何提升 Java 在嵌入式系統的效能一直是個重要的課題。本論文即基於 NAND + RAM 的嵌入式系統記憶體架構下，針對 NAND Flash 分頁循序讀取速度較慢的特性，提出我們 KVM 直譯器的三種改善法則。目標在於降低由 RAM 到 NAND 的分頁失誤率，使得 J2ME 程式在運作中，由 NAND Flash 所帶來的時間延遲及系統能耗降低。

Memory system is a main concern to embedded system engineers since it dominates the cost, power and performance of mobile embedded systems. In particular, emerging memory technology, the flash memory is becoming an indispensable component in mobile embedded systems due to its versatile features: non-volatility, solid-state reliability, low power consumption. NAND flash has the nature of high density and low cost. With the shadowing technique support, NAND flash will replace NOR flash for the embedded memory system code storage in nearly future. The more advanced solution is NAND+RAM. Neither NOR flash memory nor mask ROM is used in this architecture; it uses NAND flash memory for code storage. Copying the whole software executable binary code into RAM offers the best performance possible at run time. With the advantage of portability, it is more important to implement Java system for embedded devices. The performance is a major drawback of Java system, especially for resource-limited embedded devices. Therefore, if becomes an interesting topic to improve the Java performance for embedded devices. In our thesis, we present three KVM interpreter optimization schemes based on the NAND+RAM memory architecture and the drawback of NAND page sequential access latency. By reducing of page fault ratio from NAND to RAM, the J2ME execution performance and the NAND flash energy consumption will both be decreased.