Date: Fri, 6 Mar 2015 23:55:20 +0300 From: Slawa Olhovchenkov <slw@zxy.spb.ru> To: John Baldwin <jhb@freebsd.org> Cc: 'Andriy Gapon' <avg@freebsd.org>, freebsd-arch@freebsd.org Subject: Re: RFC: Simplfying hyperthreading distinctions Message-ID: <20150306205520.GA95179@zxy.spb.ru> In-Reply-To: <1640664.8z9mx3EOQs@ralph.baldwin.cx> References: <1640664.8z9mx3EOQs@ralph.baldwin.cx>
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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.
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