Date: Sat, 04 Oct 2003 17:54:45 -0400 From: Richard Coleman <richardcoleman@mindspring.com> To: Kris Kennaway <kris@obsecurity.org> Cc: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz> Subject: Re: Hyperthreading slowdown Message-ID: <3F7F41A5.7020202@mindspring.com> 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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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. > > Kris Sigh. No one is claiming HTT is magic performance beans. The 50% slowdown I'm talking about is between -j1 and -j2 BOTH ARE WHICH ARE USING HTT. It's just an interesting observation. That's all. Richard Coleman
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