RFC: Simplfying hyperthreading distinctions

[Slawa Olhovchenkov]

On Fri, Mar 06, 2015 at 03:44:06PM -0500, John Baldwin wrote:

> Currently we go out of our way a bit to distinguish Pentium4-era 
> hyperthreading from more recent ("modern") hyperthreading.  I suspect that 
> this distinction probably results in confusion more than anything else.  
> Intel's documentation does not make near as broad a distinction as far as I 
> can tell.  Both types of SMT are called hyperthreading in the SDM for example.  
> However, we have the astonishing behavior that 
> 'machdep.hyperthreading_allowed' only affects "old" hyperthreads, but not 
> "new" ones.  We also try to be overly cute in our dmesg output by using HTT 
> for "old" hyperthreading, and SMT for "new" hyperthreading.  I propose the 
> following changes to simplify things a bit:
> 
>   1) Call both "old" and "new" hyperthreading HTT in dmesg.
> 
>   2) Change machdep.hyperthreading_allowed to apply to both new and old HTT.
>      However, doing this means a POLA violation in that we would now disable
>      modern HTT by default.  Balanced against re-enabling "old" HTT by default
>      on an increasingly-shrinking pool of old hardware, I think the better
>      approach here would be to also change the default to allow HTT.

>   3) Possibly add a different knob (or change the behavior of
>      machdep.hyperthreading_allowed) to still bring up hyperthreads, but leave
>      them out of the default cpuset (set 1).  This would allow those threads
>      to be re-enabled dynamically at runtime by adjusting the mask on set 1.
>      The original htt settings back when 'hyperthreading_allowed' was
>      introduced actually permitted this via by adjusting 'machdep.hlt_cpus' at
>      runtime.
> 
> What do people think?

Do you have expiriment with 3)? And compare with HTT/SMT disabled in
BIOS?
My expirense (for may workload) with SMT is very bad -- unperdicable
performance in pair threads don't allow to build high (and prdicable) performance
system.