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Subject: Re: methods of hyperthreading on or off
From: John Nielsen <lists@jnielsen.net>
In-Reply-To: <aAYtYBDlikQ8BvPW@int21h>
Date: Mon, 21 Apr 2025 12:56:35 -0600
Cc: freebsd-hackers@freebsd.org
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To: void <void@f-m.fm>
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On Apr 21, 2025, at 5:34=E2=80=AFAM, void <void@f-m.fm> wrote:
>=20
>=20
> I'd like to turn off hyperthreading on some (most) freebsd machines,
> the rationale being greater network throughput.
>=20
> HT can be turned off in the bios and/or via a read-only sysctl
> in /boot/loader.conf (machdep.hyperthreading_allowed=3D"0")
>=20
> Which method is superior, and why?

=E2=80=9CSuperior=E2=80=9D is entirely subjective in this case. The =
outcome will the largely the same with either method; your machine will =
not use hyperthreads. If you disable it in the BIOS then the OS will =
never see them. If you disable it via the tunable then the OS will never =
use them. The one benefit I can think of is that the tunable is easier =
to change remotely since you typically need console access to manage =
BIOS settings.

As to whether this will help whatever network throughput issue you=E2=80=99=
re seeing (or if there could be a better way to approach that) I have no =
idea.

JN