Latencies in Linux and FreeBSD kernels with different schedulers – O(1), CFS, 4BSD, ULE

This paper is a study of scheduler latencies in different versions of Linux 2.6 kernel with emphasis set on its usage in real-time systems. It tries to find the optimal kernel configuration leading to minimal latencies using some soft real-time tuning options. Benchmark tests under heavy load show differences between kernels and also between different scheduling policies. We compare Linux kernel 2.6.22 with a long-acting O(1) scheduler with the to date latest Linux kernel 2.6.28 that already uses a completely new CFS scheduler. Not only a scheduler, but also other kernel options can lead to latency reducing. We compare a kernel compiled for server employment and then tuned to act as a soft real time operating system. For better comparison we perform selected benchmarks also on FreeBSD 7.1 kernel compiled with the older 4BSD scheduler and with the newer ULE scheduler. ULE scheduler was improved in the version 7 of FreeBSD, so we compare it also with ULE in FreeBSD 6.3. The emphasis of this paper is set on finding a scheduler with minimal latencies on tested hardware.