Turn off hyperthreading on dual core Atom?

Brett Glass brett at lariat.net
Mon Aug 29 17:24:15 UTC 2011

I'm building a few systems using dual core Atom processors, and 
have noted that when the system boots up it says it has four CPUs: 
2 actual cores and 2 virtual ones. But performance is a bit 
unsteady, and I'm wondering if it's going to be better to turn 
hyperthreading off.

With hyperthreading, the FreeBSD scheduler simply acts as if there 
are 4 CPUs. Each "CPU" gets clock interrupts (which add overhead), 
and the scheduler is naive about the fact that two of the "CPUs" 
are not separate chips and could be held up if its mate has a heavy 
load. I do not know if the supposed higher utilization of the 
resources on each chip (including executing one thread while the 
CPU waits for data for another) is worth it. What has your experience been?

--Brett Glass

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