[clug] Dual core or dual processor?

[Ian McCulloch]

On Mon, 27 Nov 2006, Hugh Fisher wrote:

[...]

> My question is, will a dual processor system perform
> significantly better than one dual core CPU on this
> kind of workload? My first guess would be yes, but I
> really have no idea how a 'hyperthreaded' system
> actually schedules threads and/or manages cache and
> memory.

Dual core != hyperthreaded.  Hyperthreading is a single CPU, which has two 
(or more) instruction streams sharing the same functional units (ie. the 
things that actually do the computation).  If you have a suitably mixed 
workload, this enables, for example, one thread to do some integer 
operations while another thread is using the floating-point unit, all on 
the same CPU.  For typical number crunching workloads, hyperthreading only 
slows things down because all threads want to access the floating point 
unit at the same time, while competing for the memory caches.

A dual core machine is the same as a dual processor system, the only 
difference being that the two processors are on the same physical piece of 
silicon.  Hence the latencies and bandwidth for communication between the 
two processors are (or should be) much improved.

I guess it ultimately depends on the design of the interconnect, but in 
principle a dual core system should perform better, or at least as good, 
in every way, compared with a dual processor system.

Cheers,
Ian