[Gllug] Best option for a lot of compute power

[adam at thebowery.co.uk]

On Mon, Jun 07, 2004 at 09:21:29AM +0100, Huw Lynes wrote:
> On Sun, 06 Jun 2004 17:27:12 +0100
> John Winters <john at sinodun.org.uk> wrote:
> 
> > The front runner at the moment is Tablix, which has good support for
> > running on a whole LAN-full of machines. 
> > 
> > I am therefore considering putting in a bid for a powerful multi-cpu
> > workstation.  The obvious choice seems to be between a couple of
> > hyperthreaded processors (giving almost 4 effective processors) 
> 
> hyper-threading has always sucked badly when we've bench-marked it. We turn it
> off by default on all our Xeon boxes. If tablix has been compiled with icc you
> might see a different story though. 

I presume many of your benchmarks have been when running single threaded
processes which each use *large* amounts of ram and all the available cpu and 
which have a rather large licensing cost per CPU (and therefore they see a
single cpu in a machine with hyperthreading enabled as 2 cpus so double the
license cost) which is a really bad scenario for using hyperthreading. (It is
the equivalent of swapping a farm of 100 servers with lots of Ram for a farm of
200 servers which are half the speed of the originals and half the Ram each when
the software you are running needs as much cpu and ram as it can possibly get)

The first thing I did in a similar situation was to turn off the hyperthreading 
capabilities. I think that hyperthreading on Xeons is supposed to work much 
faster when you have several multithreaded apps which don't need lots of cpu 
all the time or all of the available ram and then you should see the mythical
performance boost. 

Judging by what John has said about the characteristics of the software I think
that hyperthreading would probably make the software run slower for similar
reasons, but this may not be the case when dealing with out software or systems.
(but I would still have bought opterons over Xeons anyhow as they are insanely 
fast and good bang for buck)

Adam
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