Hyperthreading degrades performance?

Andre Oppermann andre at freebsd.org
Thu Aug 25 18:12:12 GMT 2005

Robert Watson wrote:
> On Thu, 25 Aug 2005, Justin R. Smith wrote:
>> This is in reply to the people who said this because of 
>> cache-contention. Has anyone benchmarked this?
>> There's an article
>> http://www.2cpu.com/articles/41_1.html
>> that benchmarks hyperthreading in Linux and shows a modest (~29%)
>> improvement in performance --- depending on applications (with java
>> showing degradation of performance). Perhaps the linux sheduler does
>> things differently...
> In in the last couple of years, I've seen some changes in how we 
> interact with HTT.  2-3 years ago, when benchmarking MySQL with and 
> without HTT, I saw a 30%+ drop-off when HTT was enabled.  Now, they come 
> out about the same.  I previously also saw no improvement with 
> buildkernel, but recently I've seen credible reports of build 
> improvements when running with HTT. So I think that things have changed 
> a bit as a result of significant scheduler improvements in the last few 
> years, as well as reduced lock contention.  A continued slight decrease 
> in performance for some benchmarks wouldn't surprise me, but seeing more 
> in the way of "break even" or even "improvement" strikes me as likely.  
> A thorough revisiting of the issue would be quite useful :-).

Don't forget better PIV revisions with larger instruction decoder caches,
better cache prefetching and branch prediction.  I doubt much of the
improvement is due to our SMP changes.  A real test to find out whether
it's our work or Intels would be to benchmark an old (pre Nacona) PIV
running 5.3R and 7.0-current vs. a new one doing the same.


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