Disable hyperthreading at startup.

fredolic · n00b Joined: 11 Jul 2005 Posts: 18

Hi,

I can't seem to remember how to pass options to /sys on startup. I have checked my notes, and tried to search online for such a ridicule question, but I am stuck.

So here it goes: I want to have hyperthreading enabled in the kernel, but I want to disable it at startup. Hence, I need to pass "echo off > /sys/devices/system/cpu/smt/control". I must probably deal with some file in "/etc/conf.d", but I can't seem to remember. How can I do that?

I am using openrc.

Thank you.

fedeliallalinea · Posted: Wed Apr 15, 2020 7:51 am Post subject:

You can made it adding nosmt to GRUB_CMDLINE_LINUX variable in /etc/default/grub
_________________
Questions are guaranteed in life; Answers aren't.

Ionen · Developer Joined: 06 Dec 2018 Posts: 2861

If "still" want to use sysfs, personally I use /etc/local.d for these with OpenRC. See /etc/local.d/README (the "local" service is already enabled by default I believe and merely does nothing if no scripts found)

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

fredolic · n00b Joined: 11 Jul 2005 Posts: 18

Dear all,

Thank you very much for all the replies.

I was looking for something like /etc/local.d. I think this is the most flexible approach.

I didn't knew that disabling it in the BIOS also had an effect at the hardware level (if you can say it this way). I will explore it.

But my idea is to have something flexible, that I can move to hyperthreading or not, based on the work that I am done. I sometimes have to run scientific software (MCMC simulations that need maximum clockspeed, and not to move a lot between processors, and not to share resources at the core level), and hyperthreading does not seem to be a good idea. So disabling it at the BIOS or at GRUB is less convenient for me.

I will try to make local.d/local.start work.

Thank you, again, everyone.

Naib · Posted: Sat Apr 18, 2020 12:06 am Post subject:

*IF* you are after guaranteed resource have you considered Cgroups?
you can lock processes to specific cores, dedicate RAM, network etc...

This is the approach I do to manage resources on a simulation box I manage.
_________________

fredolic · n00b Joined: 11 Jul 2005 Posts: 18

No, I haven't, @Naib.

I have never thougth of cgroups as working in a personal computer. A first quick overview seems not evident to configure. Do you think it it worth trying? Would it be possible to say like "use these 3 CPUs for this R/JAGS/Stan process and this amount of memory" relatively easy or is it more complicated than that? Would it make a noticeable difference in computation time?

Thank you very much for the suggestion.

xdarma · Posted: Sat Apr 18, 2020 6:23 am Post subject:

Perhaps "taskset" can be useful. It can bind a process to a list of cpus.

If you want to avoid multithreading you can define only odd or even cpus.
For example: taskset -c 1,3,5,7 /usr/bin/myfavouritgame

It's part of sys-apps/util-linux package.

szatox · Advocate Joined: 27 Aug 2013 Posts: 3442

Why not just disable HT in bios? I mean, what is that thing you're doing that actually benefits from HT? To me it looks a bit like a corner case / marketing BS.
But... You can disable CPU cores via /sys. With exception of CPU0, since the boot cpu can't be disabled.

Naib · Posted: Sat Apr 18, 2020 10:11 am Post subject:

szatox · Advocate Joined: 27 Aug 2013 Posts: 3442

Yes, I know it's easy to lock a particular process to a CPU.
Is it also possible to lock every other process out of that CPU? Like in "every" other process, not "every process I found and manually banned from this core"

Naib · Posted: Sat Apr 18, 2020 11:48 am Post subject: