Hyperthreading slowdown

[Richard Coleman]

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.

Richard Coleman