Date: Mon, 13 Mar 2006 13:45:40 -0500 From: Bill Moran <wmoran@collaborativefusion.com> To: John Baldwin <jhb@freebsd.org> Cc: bseklecki@collaborativefusion.com, freebsd-amd64@freebsd.org Subject: Re: How is hyperthreading handled on amd64? Message-ID: <20060313134540.506b6b46.wmoran@collaborativefusion.com> In-Reply-To: <200603131052.29655.jhb@freebsd.org> References: <20060313085431.0eb059d9.wmoran@collaborativefusion.com> <200603131052.29655.jhb@freebsd.org>
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On Mon, 13 Mar 2006 10:52:28 -0500 John Baldwin <jhb@freebsd.org> wrote: > On Monday 13 March 2006 08:54, Bill Moran wrote: > > > > We've got some dell servers (Poweredge 2850 is specifically what > > the following tests were run on). Some of the behaviour we're > > seeing was unexpected. In particular, I'm rather fuzzy as to > > how an amd64 kernel handles hyperthreading. (Personally, I'm also > > a little fuzzy on whether this really is hyperthreading under amd64 > > at all) > > > > After building a kernel with SMP, I do see 2 logical processors, > > and top(1) shows both of them doing their thing. However, > > machdep.hypterthreading_allowed is set to 0. Is the amd64 kernel > > ignoring this value, or is top(1) reporting incorrectly? > > > > Additionally, I'm not seeing the performance I'm accustomed to on > > hypterthreaded machines. Usually, throughput scales when hyperthreading > > is enabled and more than one process are fighting for CPU. In my > > tests, it seems as if there is no scaling whatsoever. i.e.: On > > a standard hyperthreaded machine, running 1 ubench might yield a > > CPU value of 100,000. Running two ubench simultaneously will usually > > result in each of them returning ~90,000 or so. With the amd64, I > > end up with each one around 50,000 - as if there were no second > > logical CPU (despite the fact that top(1) shows both in use). > > > > dmesg attached. I can provide more information on request. > > Your dmesg is from a kernel that doesn't include 'options SMP' and is > thus only using 1 CPU. Really? I've triple-checked at this point and: 1) The kernel config file has "options SMP" 2) The running kernel was made from #1 3) top(1) shows cpus 0 and 1 I can't argue that the boot messages don't seem to show anything to indicate that a second logical CPU was found. -- Bill Moran Collaborative Fusion Inc.home | help
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