Date: Sun, 5 Oct 2003 14:31:49 +0200 From: Wilko Bulte <wkb@freebie.xs4all.nl> To: Kris Kennaway <kris@obsecurity.org> Cc: freebsd-hackers@freebsd.org Subject: Re: Hyperthreading slowdown Message-ID: <20031005123149.GB11276@freebie.xs4all.nl> In-Reply-To: <20031004200435.GA60432@rot13.obsecurity.org> References: <Pine.LNX.4.58.0310041623250.6065@artax.karlin.mff.cuni.cz> <20031004190251.GA60026@rot13.obsecurity.org> <3F7F1D63.2010703@mindspring.com> <20031004200435.GA60432@rot13.obsecurity.org>
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On Sat, Oct 04, 2003 at 01:04:35PM -0700, Kris Kennaway wrote: > On Sat, Oct 04, 2003 at 03:20:03PM -0400, Richard Coleman wrote: > > Kris Kennaway wrote: > > >On Sat, Oct 04, 2003 at 04:39:03PM +0200, Mikulas Patocka wrote: > > >>I installed FreeBSD 4.9RC1 on P4 3GHz with hyperthreading and I see > > >>drastic slowdown when kernel with hyperthreading is booted. For example > > >>program compilation took this time: > > >> > > >>hyperthreading kernel, make -j 1 --- 1:09 > > >>hyperthreading kernel, make -j 2 --- 0:42 > > >>singlethreading kernel, make -j 1 --- 0:45 > > >>singlethreading kernel, make -j 2 --- 0:41 > > >> > > >>Compilation does very few system calls so when I compile with only one > > >>process (-j 1), it should be as fast as with singlethreading kernel. Do > > >>you have any idea why is it so slow? > > > > > >Do you realise that hyperthreading != a secret extra CPU in your system? > > > > > >Kris > > > > I didn't see anywhere in the message where he implied that. To me, the > > interesting thing is that there is such a larger difference between the > > compile time for -j1 and -j2 when using hyperthreading as compared to > > the difference between -j1 and -j2 for a single threaded kernel. It's > > over a 50% slowdown. > > Yes, that's because (as discussed in the archives) the kernel treats > it like an extra, completely decoupled physical CPU and schedules > processes on it without further consideration. This is presumably the > cause of the slowdown, because it's only efficient to use the virtual > CPU under certain workload patterns. HTT is not magic performance > beans. Right. And in addition it makes the system considerably more power hungry.. I measured both with and without SMP on my P4/2.4G HTT CPU. -- | / o / /_ _ wilko@FreeBSD.org |/|/ / / /( (_) Bulte
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