[Info-vax] Disable hyper-threading from the DCL prompt

[Michael Moroney]

Stephen Hoffman <seaohveh at hoffmanlabs.invalid> writes:

>Beyond that htread.exe tool, the SET CPU /NOCOTHREAD command is also 
>available per 
><https://view.officeapps.live.com/op/view.aspx?src=http%3A%2F%2Fde.openvms.org%2FTUD2008%2FHyperthread_Support_in_OpenVMS_V8.ppt>, 
>though it does not appear that this DCL command was documented in the 
>help libraries — more fodder for VSI to ponder.

Interesting.  I did not know about that.

>The command is honored on an OpenVMS V8.4 test server, without errors.  
>Whether it works as intended, I did not check — the test server does 
>not support hyperthreads.

I just tried it, it appears to be equivalent to a $ STOP/CPU on the
cothread CPU.  Whether there's a difference, I don't know, but SHOW
CPU shows the cothread as stopped.

Selective STOP/CPU (or SET CPU /NOCOTHREAD) commands on processors 
assigned to Fastpath for a device may help, since a performance
hit can happen if one thread owns a spinlock and its cothread is spinning
waiting for that spinlock.  That's why the bigger hammer of turning off
(all) threads often helps.