Hyperthreading slowdown

[Kris Kennaway]

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
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