Disabling HyperThreading?

Hubert Tournier hubert at frbsd.org
Wed May 5 23:26:28 UTC 2010


Jack L.-3 wrote:
> sysctl machdep.hyperthreading_allowed=0 will disable it.

Thank you for this lead.

It gives the following result on my system:
sysctl: unknown oid 'machdep.hyperthreading_allowed'

Searching a little bit, i found 2 references to this OID in the kernel
source code:

Maybe it's missing because (as i read in these files):
SCHED_ULE doesn't allow enabling/disabling HT cores at run-time.

I'm not sure i could use the OID in /boot/loader.conf however?

I also see in the source code that the default is to enable HT cores:
static int      hyperthreading_allowed = 1;

So i will test a custom kernel with this disabled.

For reference, here is the CPU i use:
CPU: Intel(R) Xeon(R) CPU           X5650  @ 2.67GHz (2666.65-MHz K8-class
  Origin = "GenuineIntel"  Id = 0x206c2  Stepping = 2
  AMD Features=0x2c100800<SYSCALL,NX,Page1GB,RDTSCP,LM>
  AMD Features2=0x1<LAHF>
  TSC: P-state invariant
FreeBSD/SMP: Multiprocessor System Detected: 24 CPUs
FreeBSD/SMP: 2 package(s) x 6 core(s) x 2 SMT threads

Best regards,

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