Re: [9fans] hyperthreading

[William Josephson <jkw@eecs.harvard.edu>]

On Sun, Dec 28, 2003 at 05:12:32PM -0500, Joel Salomon wrote:
> ron minnich said:
> > hyperthreading: don't waste your time.
> >
> > hype-r-threading.
>
> I'm to assume then that hyperthreading is distinct from having multiple
> "cores" on a single die (what IBM is doing with some of their machines),
> or is this the "hype" you are referring to? Multiple CPUs on one silicon
> chip sounds cool to me.

Look up the literature on SMT (symetric multithreading).
In the case of Intel, the two virtual CPUs do not have
the full set of resources available to a single real CPU.
The result is a modest improvement in performance for a
few specific workloads and has little or negative
performance impact on many others (cf.  the P4 core
generally -- it is a good example of microbenchmarking
gone awry in my experience).  I have found that with some
Unix variants, typically those with underwhelming
schedulers for interactive workloads, it helps a bit with
keeping the shell and windowing system zippy under load.

 -WJ