Optimizing the Idle Task and Other MMU Tricks

In highly cached and pipelined machines, operating system performance, and aggregate user/system performance, is enormously sensitive to small changes in cache and TLB hit rates. We have implemented a variety of changes in the memory management of a native port of the Linux operating system to the PowerPC architecture in an effort to improve performance. Our results show that careful design to minimize the OS caching footprint, to shorten critical code paths in page fault handling, and to otherwise take full advantage of the memory management hardware can have dramatic effects on performance. Our results also show that the operating system can intelligently manage MMU resources as well or better than hardware can and suggest that complex hardware MMU assistance may not be the most appropriate use of scarce chip area. Comparative benchmarks show that our optimizations result in kernel performance that is significantly better than other monolithic kernels for the same architecture and highlight the distance that micro-kernel designs will have to travel to approach the performance of a reasonably efficient monolithic kernel.