How 2 switch kernel CPU mode between Threaded / UnThreaded

CaptainBlood · Advocate Joined: 24 Jan 2010 Posts: 3999

Hi,

Should I achieve this with 2 differents .config?
Or should kernel command line suffices?
I expect "UnThreaded" to be time saving while emerging single threaded ebuilds.

Thks 4 ur attention

Ant P. · Watchman Joined: 18 Apr 2009 Posts: 6920

Your expectations are broken, unless you plan to run that emerge as the init process on a machine with no consoles or background processes at all.

CaptainBlood · Advocate Joined: 24 Jan 2010 Posts: 3999

Thks for advice... :oops:

Anyhow the question remains:
Is it possible to configure kernel (via .config or command line) so that the 2 cores are taken into account, and that each core isn't splitted into 2 logical ones?

Thks 4 ur attention, interest & support

NeddySeagoon · Posted: Sun Feb 15, 2015 9:57 pm Post subject:

CaptainBlood,

Turn off Hyperthreading support in the BIOS and in the kernel.

I don't see a command line option in /usr/src/linux/Documentation/kernel-parameters.txt
Maybe there is something in the kernel help?
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NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

CaptainBlood · Advocate Joined: 24 Jan 2010 Posts: 3999

Yamakuzure · Posted: Wed Feb 18, 2015 10:32 am Post subject:

BIOS: Here you can (normally) set whether HyperThreading is supported or not. To use it, it must be enabled.
Kernel: Either build with the SMT Scheduler (CONFIG_SCHED_SMT) or not.

Disabling the SMT-Scheduler disables HyperThreading, no matter what you have set in your BIOS.
Enabling the SMT-Scheduler without enabling support in the BIOS only adds overhead.

On my laptop (i7, 4 cores) the overall performance is worse without HyperThreading. Nowadays it is a too rare occasion that a single process hogs only one logical CPU for disabling HyperThreading to give me any benefit.
This might, of course, be completely different on your setup.
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Edited 220,176 times by Yamakuzure

CaptainBlood · Advocate Joined: 24 Jan 2010 Posts: 3999

Hi,
No such an option in laptop BIOS.

So seems like there are little chances to have dual core cpu involved without each being splited into 2 logical CPUs.

While doing

s4e8 · Guru Joined: 29 Jul 2006 Posts: 311

just add kernel param maxcpus=<half_of_#cpu>, it only work on single cpu package or some 2way server.

CaptainBlood · Advocate Joined: 24 Jan 2010 Posts: 3999

Yes that's an issue, only 1 CPU until next reboot.
Not an option, sorry. :wink:

CaptainBlood · Advocate Joined: 24 Jan 2010 Posts: 3999

s4e8 · Guru Joined: 29 Jul 2006 Posts: 311

On single CPU system, logical cpus always sorted by coreid threadid, ie: all physical cores at first, threaded-cpus next.
On two CPU system, some m/b sorted by coreid socketid threadid and others by coreid threadid socketid. First method upper half cpus is threaded, but second method, upper half cpus is second socket's.