Re: [SLUG] Hyperthreading

[Peter Chubb <peterc@xxxxxxxxxxxxxxxxxx>]

>>>>> "Simon" == Simon Rumble <Rev> writes:

Simon> I have an Intel Xeon 3 gig CPU and have hyperthreading turned
Simon> on in the BIOS.  I've been trying to work out what the
Simon> advantages and disadvantages of this are.

Simon> The CPU appears as two CPUs to the machine, which means that
Simon> non-threaded apps don't appear to use the whole CPU.  Is this a
Simon> correct assumption?  For example, using Devede to convert
Simon> video, the transcode process only uses 50% of CPU in top.  If I
Simon> run another CPU-intensive process, the CPU usage in top goes
Simon> close to 100%.

This is an incorrect assumption.  Linux treats each thread for the purposes of
calculating CPU usage as a separate core. So even if one (hyper)thread
is using 100% of the processor, it'll show up as 50% usage.

If you have more than one running thread/process, hyperthreading
typically gives between 1 and 30% increase in throughput (compared
with the same processor with SMT turned off).  If you have
a single thread, then there should be no performance penalty (except
that you can probably gain performance by configuring ou kernel a a
uniprocessor kernel, and avoiding all the locking overhead).

--
Dr Peter Chubb  http://www.gelato.unsw.edu.au  peterc AT gelato.unsw.edu.au
http://www.ertos.nicta.com.au           ERTOS within National ICT Australia