[Nottingham] Linux HT

[Jon Masters]

Hi,

Does anyone have a particularly possitive experience of the Linux
implementation of Hyperthreading in Intel's Xeon (Netburst) CPUs?

I am reviewing a box at the moment and am disappointed with performace of
two 2.4GHz Xeon CPUs when running under the supplied Linux configuration
(Redhat 8, 2.4.18-26.8.0smp). This could be due in part to the Linux
kernel configuration which Redhat supply and other related factors.

It would seem logical to schedule a single process per real CPU and
several threads of execution context per logical CPU though this is not
what is happening in practice and the cache contention must be nasty.

I have read Intel's architecture papers and understand roughly how the
implementation of logical architectural units is done though there is
always a chance someone here will be much more aquainted with the Linux
kernel side of the HT support. I can see why the Linux implementation
would have been done as it initially has been - and it is early days.

The idea behind Hyperthreading is to improve the performance of threaded
applications at a cost of about 10% increased die usage. The execution
context of the Intel Architecture is duplicated but not the supporting
caching and related mechanisms. This would be great in situations where
standard locality of code and data dictates that highly threaded
applications will benefit from increased throughput, but only for threads.

Someone tell me their experience please.

Jon.