CSR-PDOS: Out-of-Context Prefetching for L2 Caches

在多任务系统上，CPU每隔几毫秒循环一次多个进程。有些进程是内存密集型进程，但大多数是非密集型支持进程和其他用户程序，如文本编辑器、电子邮件客户端和即时消息传递程序。一个典型的系统通过几个内存密集型进程进行循环，每个进程替换来自其他进程的二级缓存中的几个缓存块。这导致了对L2的大量需求未命中，并阻碍了内存子系统的有效预取。本项目通过修改操作系统以在上下文之外，即当CPU正在执行非内存密集型进程时预取到L2，来研究对此问题的可能解决方案。(理想情况下，就在内存密集型进程被引入执行之前)。因此，当该进程恢复执行时，它将具有低L2高速缓存未命中，这反过来将保持存储器总线空闲，以便预取更多的块。上下文外预取的目标是从以按需获取为主的范例转变为以预取为主的范例，结合准确的预取预测器，这项工作有可能在系统范围内极大地缓解性能CPU-内存差距问题。该项目的预期结果是在全系统模拟器上实现脱离上下文的预取的Linux实现，以及一个网站，该网站记录了所有假设、源代码和超出相应研究文献发表的广泛项目结果。所有研究人员、操作系统设计师和处理器制造商都可以访问网站和研究成果，自由地将它们纳入他们的意愿。

On a multi-tasked system, the CPU cycles through a number of processes every few milliseconds. Some of the processes are memory-intensive, but most are non-intensive support processes and other user programs such as text-editors, email clients, and instant messengers. A typical system cycles through a few memory-intensive processes, each of which replaces several cache blocks from the level-2 cache from other processes. This results in a large number of demand misses for the L2, and ties up the memory subsystem preventing effective prefetching.This project investigates a possible solution to this problem by modifying the OS to prefetch to the L2 out of context, i.e. when the CPU is executing a non memory-intensive process. (ideally, just before the memory-intensive process will be brought in for execution). Thus, when this process resumes execution, it will have low L2 cache misses which in turn will keep the memory-bus free in order to prefetch even more blocks. Out-of-context prefetching aims to shift from a predominantly demand-fetched paradigm to a predominantly prefetched paradigm.In conjunction with accurate predictors for prefetching, this work has the potential to greatly mitigate the performance CPU-memory gap problem on a system-wide level. The expected outcome of the project is a Linux implementation of out-of-context prefetching on a full-system simulator, and a website that documents all the assumptions, the source code, and extensive project results beyond those published in the corresponding research literature. All researchers, operating system designers, and processor manufacturers will have access to the website and the research findings to freely incorporate them as they wish.