[concurrency-interest] To Hyper-Thread or not

Nathan Reynolds nathan.reynolds at oracle.com
Fri Dec 23 15:40:48 EST 2011

Intel has said that older hyper-threaded cores sometimes showed a gain 
and sometimes showed a loss in throughput.  They also said that newer 
hyper-threaded cores show a 30-50% gain in throughput for the workloads 
they tested.  I am not sure where the older vs newer line is.  A Nehalem 
and Westmere CPU is definitely in the newer category.

I am not sure of the impact of hyperthreading on context switches.

I don't think JRockit or HotSpot do any thread scheduling or pinning.  I 
think the kernel scheduler determines where each thread runs.  JRockit 
Virtual Edition does thread scheduling since it is a thin "kernel" for 
running JRockit as a virtualization guest.

As for detecting shared data/contention, that was the point of 
@Contended email chain on this list a while back.  Intel and Oracle have 
worked on a solution for true/false sharing.  The current Intel CPUs 
don't quite expose enough information for a fully automated solution.

Nathan Reynolds 
<http://psr.us.oracle.com/wiki/index.php/User:Nathan_Reynolds> | 
Consulting Member of Technical Staff | 602.333.9091
Oracle PSR Engineering <http://psr.us.oracle.com/> | Server Technology

On 12/23/2011 12:57 PM, Rahul Saksena wrote:
> Assuming we have 8 physical cores (intel xeon SMP) and the java 
> process has more than 12 threads running  through out its life cycle 
> (no shared variables),
> Would you enable hyperthreading and have 16 logical cores?
> Does enabling hyperthreading result in far less number of context 
> switches in this case?
> Does the jvm recognize that these threads have no shared 
> data/contention and pin/bind it to different cores?
> Thanks,
> Rahul.
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