linux-kernel - Re: How to detect multi-core and/or HT-enabled CPUs in 2.4.x and 2.6.x kernels

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Date:	Wed, 27 Dec 2006 16:13:00 +0100
From:	Arjan van de Ven <arjan@...radead.org>
To:	knobi@...bisoft.de
Cc:	linux-kernel@...r.kernel.org
Subject: Re: How to detect multi-core and/or HT-enabled CPUs in 2.4.x and
	2.6.x kernels

> 
>  one piece of information that Ganglia collects for a node is the
> "number of CPUs", originally meaning "physical CPUs".

Ok I was afraid of that.

>  With the
> introduction of HT and multi-core things are a bit more complex now. We
> have decided that HT sibblings do not qualify as "real" CPUs, while
> multi-cores do.

I think that decision is a mistake, and is probably based on experiences
with the first generation of HT capable Pentium 4 processors.

The original p4 HT to a large degree suffered from a too small cache
that now was shared. SMT in general isn't per se all that different in
performance than dual core, at least not on a fundamental level, it's
all a matter of how many resources each thread has on average. With dual
core sharing the cache for example, that already is part HT. Putting the
"boundary" at HT-but-not-dual-core is going to be highly artificial and
while it may work for the current hardware, in general it's not a good
way of separating things (just look at the PowerPC processors, those are
highly SMT as well), and I suspect that your distinction is just going
to break all the time over the next 10 years ;) Or even today on the
current "large cache" P4 processors with HT it already breaks. (just
those tend to be the expensive models so more rare)

I would strongly urge you to reconsider this decision; if you want to
show "sockets" that sounds reasonable, or even if you want to do it on
the "bus sharing" level (FSB/HT), but HT.. just sounds wrong.

-- 
if you want to mail me at work (you don't), use arjan (at) linux.intel.com
Test the interaction between Linux and your BIOS via http://www.linuxfirmwarekit.org

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