Confused about HyperThreading and Performance

[Terry Lambert]

Daniel Ellard wrote:
> Can someone point me at some non-marketing documentation about
> hyperthreading on the latest Intel chips?  I'm seeing some strange
> performance measurements and I would like to figure out what they
> mean.

Go out to Intel's web site's "developer" section, and look for
"SMT".  There is a lot of literature.

The reason you are seeing a performance drop is contention for
shared resources that the scheduler doesn't know are shared.

SPECmark and similar benchmarks tend to get worse numbers on
every OS when SMT is enabled, due to contention.

The only model which will work is a hierarchical affinity and
negaffinity model, and I am not away of an OS that is not also
treating the hardware as NUMA which works this way (and none of
those run on Intel chips; mostly, they run on real NUMA hardware).

-- Terry