Agreed. 10% was a round number, it is really only 7% to 8%.
With normal production loads I rarely see more than 2 processes actually 
executing. Since the NetBSD scheduler does not distinguish between logical 
and physical processors it can, and often does, schedule two threads on the 
logical processors in one package and leave the other processor idle.
In general processes appear to remain on the BSP and the other logical 
processor in the same package. This causes interrupts serviced by the BSP 
to suffer the performance degradation from having a thread executing on the 
other logical processor.

Looking at the per logical processor performance hit of about 45% with HT I 
wonder what the motivation for HT was in the first place.  I have certainly 
decided not to use HT on my production systems.

To make HT useful the scheduler would have to have knowledge of the 
physical reality behind the logical processors and maybe carefully place 
related threads on the correct processors. At the very least the scheduler 
would have to distribute processes across physical processors before 
starting to use the "secondary" processor in each package.

I am cc'ing this to tech-smp since that's a more appropriate place than 
current-users for this thread.


--On 20 January 2005 22:48 +0100 Manuel Bouyer <bouyer@antioche.eu.org> 
wrote:

> On Wed, Jan 19, 2005 at 05:15:34PM +0000, Peter O'Kane wrote:
>> Good point. Further results here running make -j8 to build a 2.0 kernel:
>>
>> Real	User	System	Configuration
>> 288.12	255.15	35.97		Single processor no HT
>> 251.72	440.61	49.55		Single processor with HT
>> 165.58	260.00	40.74		Twin physical processors no HT
>> 151.54	435.04	69.11		Twin physical processors with HT
>>
>> again running make -j16
>>
>> 160.9	263.44	44.24		Twin physical processors no HT
>> 146.46	452.66	78.69		Twin physical processors with HT
>>
>> and finally make -j32
>>
>> 158.82	260.33	42.63		Twin physical processors no HT
>> 146.23	465.37	77.8		Twin physical processors with HT
>>
>> Note that the user/system time required to do the job remains fairly
>> constant at about 250-260/30-45 seconds with HT off. With HT enabled the
>> user/system time required rises to about 440-460/50-70 seconds.
>> It appears that HT gives about 10% overall performance boot for many
>> tasks  but the effective processor speed seen by a single thread is
>> 55%-60% of the  speed of the processor in non HT mode.
>
> This matches what I've noticed when I tested a single-CPU P4 system
> (although I though the gain was more like 6-7% than 10%).
>
> --
> Manuel Bouyer <bouyer@antioche.eu.org>
>      NetBSD: 26 ans d'experience feront toujours la difference
> --
>
> !DSPAM:41f02aef14861293653217!
>
>



Peter O'Kane                            E-mail:peter.okane@it.nuigalway.ie
Information Technology Department,      Voice: +353 91 492527
National University of Ireland, Galway. Fax: +353 91 494501