Hyperthreading degrades performance?

Robert Watson rwatson at FreeBSD.org
Thu Aug 25 16:37:13 GMT 2005

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 :-).

Robert N M Watson

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