使用基本指令區塊字典減少指令匯流排耗能

Abstract

近年來，對於減少處理器耗能的需求越來越被重視。研究發現，系統在程式執行中所耗能量，有50% ~ 80% 是消耗在處理器與外部記憶體的傳輸上，即匯流排。故想要設計出更省電的系統，必須先從匯流排著手。匯流排上耗能是根據在其上傳送的資料位元變化所改變的。因此，指令匯流排的耗能，便是取決於處理器所對記憶體要求傳送的指令所造成的位元變化。 在這篇論文，我們提出了使用基本區塊 (basic-block) 指令字典的方法去減少指令匯流排的耗能。基本區塊是程式在執行時的特性之一，一旦處理器開始執行基本區塊內的指令，除非遇到中斷，否則區塊內所有指令都會被執行。我們的方法把常用的基本區塊放入指令字典中，這個字典有著比較小的耗能而且更接近處理器。而在原始程式中被替換的基本區塊則會被字典索引編碼取代。在執行到這些被放置到字典內的基本區塊時，只有字典索引編碼會在指令匯流排上面傳遞。藉著原本多道指令在指令匯流排上傳送變成只有一個編碼被傳送，我們可以減少許多的耗能。當遇到需要重複傳送相同的編碼的時候，我們也藉著應用編碼暫存機制去減少更多的耗能 實驗結果顯示，跟原始的系統來比較，我們可以達到平均91.2%的耗能減少。而由於我們的字典是存放在更靠近處理器，因此程式執行的時間跟原本的系統相比，也會減少大約83.6%的執行時間。

The requirement in reducing the power of a processor has grown dramatically over the past few years. Research show that 50%~80% power are dissipated on the transmission between CPU and Memory. Power consumed on buses are decided by numbers of bit toggles on the buses, and instructions transmitted on buses determine the numbers of bit toggles at instruction buses. In this thesis, we propose a basic-block instruction dictionary to reduce power consumption on instruction bus. Basic-block is one of the characteristics of program execution behavior that once a processor step into a basic-block, the whole block is executed except interrupt occurs. Our method tries to place the most frequently used basic-blocks into the instruction dictionary, which has smaller power consumption and is closer to CPU. The basic-blocks at original program are replaced by codewords indexing to the basic-blocks in the dictionary. We can reduce the power consumption when repeatedly executing on these basic-blocks because only the codewords are transferred in the instruction bus. A codeword lookaside buffer is also applied to provide more power reduction when accessing codeword repeatedly. Experimental results show that on average 91.2% power reduction can be achieved with comparing to the base system. By accessing the instruction at internal dictionary, we can also improve the execution time by 16.4%.