E5-2699v4 - SMT (Simultaneous Multithreading) - Enabled or Disabled

localhost

So, on this new colo server when I started installing AlmxLinux, a small warning popped on the bottom of the installer screen:

Clicking on details takes me to:

I am curious whether its advisable to disable SMT (Simultaneous Multithreading)? Has anyone seen major performance degradation?

This is the first time I am seeing this warning, so bit unsure what way to go.

Thanks

nullnothere

There are mostly good enough mitigations at the kernel level to leave SMT enabled. There will be a performance drop if SMT is disabled (the actual drop will vary but it is fair to expect 15-40% drop depending on the work load characteristics).

If this is a "self" hosted machine that is going to be used by "trusted" processes, you should be safe to leave it enabled. Even most hosting providers leave it enabled (with appropriate kernel fixes of course) - and that kind of a hosted environment is where damage is most pronounced as there are potentially many untrustworthy processes running on the host node.

localhost

@nullnothere said:
There are mostly good enough mitigations at the kernel level to leave SMT enabled.

Is it possible to identify if the 5x kernel is having enough mitigations?

mfs

@localhost said: Is it possible to identify

https://github.com/speed47/spectre-meltdown-checker can give some hints about some of the multithreading CVEs popped up in the last years; interpret the results with a pinch of salt.

nullnothere

@localhost said:

@nullnothere said:
There are mostly good enough mitigations at the kernel level to leave SMT enabled.

Is it possible to identify if the 5x kernel is having enough mitigations?

In general, if it is a distro shipped/supplied kernel, mitigations are (by default) applied and enabled at boot. You have to explicitly disable them via boot time kernel arguments/parameters - so you should be safe.

Also, cat /proc/cpuinfo will tell you a little more about potential cpu bugs and mitigations that are in place.