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November 24, 2008

最近乱翻一些论文,发现龙芯还是在走RISC的乱序执行的路。做CPU体系结构的人都知道,乱序执行是能耗大户,而且对于芯片设计的复杂度也是很大的挑战。龙芯现在走的乱序执行的路,似乎和龙芯的商业目标相左。

现在龙芯在商业化上面的目标,是要成为低价笔记本/终端的芯片。这样的定位,其实对于性能的要求,远没有对于能耗的要求高。也就是说,可以牺牲部分性能,来降低能耗。

2002年的时候,我做过一个项目,是用模拟器来计算RISC芯片的能耗,并对体系结构的改变进行能耗上面的评估。当时的结论是,对于典型的应用,如果能够把纯乱序执行变为有限制的乱序执行,那么可以达到30%左右的能耗优化,而性能的影响小于10%。

Intel新的ATOM架构里面,采用了类似的思想,就是一般的指令序列是顺序执行;对于最常见的容易产生流水线空泡的指令序列,即长周期指令(浮点指令)后面紧跟一个短周期指令(整数指令)的序列,才进行乱序优化。

优化的方法是:先判断后面的整数指令是否对前面的浮点指令有数据依赖,没有的话才进行乱序执行。这是典型的80-20分析方法,将有限的能耗预算,用于优化最常见的影响性能的指令序列。

2002年做研究的时候用的基准应用程序,有很多科学计算的东西,龙芯的基准程序集合,因为是针对日常应用,应该有更多的整数指令序列,对乱序执行的要求应该会更少。换句话说,龙芯的乱序执行部件,应该有很多时候都是闲置在那里,空耗能量的。如果改成有限的乱序执行,应该能节省很多能耗。

相应的为了补偿减弱/取消乱序执行带来的性能减弱,可以考虑增加cache的容量。Cache的设计相对芯片逻辑更简单一些。

不知道新的龙芯是否能借鉴这种设计思想。

3 Comments »

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