cvs commit: src/sys/amd64/amd64 mp_machdep.c src/sys/i386/i386 mp_machdep.c

Julian Elischer julian at
Sat Nov 10 07:21:15 PST 2007

Maxim Sobolev wrote:
> Bruce Evans wrote:
>> Off is a good default since hyperthreading seems to be a pessimization
>> in most casts.
> Well, actually it all depends on workload and scheduling. I believe ULE 
> is expected to improve things quite a bit in scheduling department. 
> However, even with ol'good SCHED_BSD we measured noticeable performance 
> increase (15-25%) in CPU intensive real-world tasks  with HTT enabled on 
> 2-way SMP Xeon system. It was in 2004, both CPUs and schedules should 
> have been improved since that time.
> -Maxim

Vicor (where I used to work) noticed recently that newer systems were 
getting much less work done thann their older bretheren.
On investigation they found that it was because the newer
systems had HT turned off..  Their workload is a mix of FP and Int crunching
in image processing and just happens to be the near perfect HT workload.
With 2 x HT processors (4 logical processors), they could get about 3.7
times the throughput of a single processor in tests I did in 04 or so.

However for most other loads HT turned  out to be at best a wash and at worst 
a slight loss, so having it off will help the average user I think.

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