The Battle of the Schedulers: FreeBSD ULE vs. Linux CFS


Justinien Bouron, Sebastien Chevalley, Baptiste Lepers, and Willy Zwaenepoel, EPFL; Redha Gouicem, Julia Lawall, Gilles Muller, and Julien Sopena, Sorbonne University/Inria/LIP6


This paper analyzes the impact on application performance of the design and implementation choices made in two widely used open-source schedulers: ULE, the default FreeBSD scheduler, and CFS, the default Linux scheduler.

We compare ULE and CFS in otherwise identical circumstances. We have ported ULE to Linux, and use it to schedule all threads that are normally scheduled by CFS. We compare the performance of a large suite of applications on the modified kernel running ULE and on the standard Linux kernel running CFS. The observed performance differences are solely the result of scheduling decisions, and do not reflect differences in other subsystems between FreeBSD and Linux.

There is no overall winner. On many workloads the two schedulers perform similarly, but for some workloads there are significant and even surprising differences. ULE may cause starvation, even when executing a single application with identical threads, but this starvation may actually lead to better application performance for some workloads. The more complex load balancing mechanism of CFS reacts more quickly to workload changes, but ULE achieves a better load balance in the long run.

Open Access Media

USENIX is committed to Open Access to the research presented at our events. Papers and proceedings are freely available to everyone once the event begins. Any video, audio, and/or slides that are posted after the event are also free and open to everyone. Support USENIX and our commitment to Open Access.

@inproceedings {215951,
author = {Justinien Bouron and Sebastien Chevalley and Baptiste Lepers and Willy Zwaenepoel and Redha Gouicem and Julia Lawall and Gilles Muller and Julien Sopena},
title = {The Battle of the Schedulers: {FreeBSD} {ULE} vs. Linux {CFS}},
booktitle = {2018 USENIX Annual Technical Conference (USENIX ATC 18)},
year = {2018},
isbn = {978-1-931971-44-7},
address = {Boston, MA},
pages = {85--96},
url = {},
publisher = {USENIX Association},
month = jul

Presentation Audio