A potential bug reported against the Completely Fair Scheduler suggested that it was causing a network slowdown, measured with the 'Iperf' bandwidth performance benchmarking tool. The performance hit was quickly tracked to the previously discussed changes in how CFS handles sched_yield(). When it was suggested that this was a bug in the new process scheduler, Ingo explained:

"I had a quick look at the source code, and the reason for that weird yield usage was that there's a locking bug in iperf's 'Reporter thread' abstraction and apparently instead of fixing the bug it was worked around via a horrible yield() based user-space lock."

He then submit a small patch to fix the bug and remove the call to sched_yield() resulting in, "iperf uses _much_ less CPU time. On my Core2Duo test system, before the patch it used up 100% CPU time to saturate 1 gigabit of network traffic to another box. With the patch applied it now uses 9% of CPU time." He added playfully, "sched_yield() is almost always the symptom of broken locking or other bug. In that sense CFS does the right thing by exposing such bugs =B-)" Stephen Hemminger pointed out that a similar patch had been submitted to the Iperf project last month as it caused an identical problem with FreeBSD's scheduler.