[CentOS] HT (Hyper Threading) on CentOS 5.5

Fri Mar 9 10:05:08 UTC 2012
Peter Kjellström <cap at nsc.liu.se>

On Friday 09 March 2012 08.44.53 Sorin Srbu wrote:
> What are some of the cases it would be practical/best to have it off?
> I know there might be some negative performance issues with certain
> applications, but would you notice it in day-to-day use?

Negative performance due to HT comes in several flavours. Some compute 
intensive loads simply perform a bit better with HT off (usually measureable 
but not really noticeable, 1-10% is expected here). Another way HT can cause 
you performance degradation is due to incorrect scheduling and/or pinning. 
What happens then is that two threads are (incorrectly) put on the same core 
instead of using two cores (impact: ~half performance).

On the positive side, if you have many (preferably independent) threads you'll 
often get both better throughput and better system response/latency (more 
"processors" available for the kernel to schedule on).

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