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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