Fork/Wait and Multicore Frequency Scaling: a Generational Clash

The complexity of computer architectures has risen since the early years of the Linux kernel: Simultaneous Multi-Threading (SMT), multicore processing, and frequency scaling with complex algorithms such as Intel® Turbo Boost have all become omnipresent. In order to keep up with hardware innovations, the Linux scheduler has been rewritten several times, and many hardware-related heuristics have been added. Despite this, we show in this paper that a fundamental problem was never identified: the POSIX process creation model, i.e., fork/wait, can behave inefficiently on current multicore architectures due to frequency scaling. We investigate this issue through a simple case study: the compilation of the Linux kernel source tree. To do this, we develop SchedLog, a low-overhead scheduler tracing tool, and SchedDisplay, a scriptable tool to graphically analyze SchedLog's traces efficiently. We implement two solutions to the problem at the scheduler level which improve the speed of compiling part of the Linux kernel by up to 26%, and the whole kernel by up to 10%.