Re: HARSH article on TheRegister - Jobs lied about Intel speed, doesn't care about computers now ipods are so successfull

In article <BFFBBEC6.14F1E0%chrisridd@xxxxxxx>,
Chris Ridd <chrisridd@xxxxxxx> wrote:
>On 24/1/06 11:08, in article 43mg4kF1oj4sgU1@xxxxxxxxxxxxxx, "Ian McCall"
><ian@xxxxxxxxxx> wrote:
>
>> Certainly if I were an iMac G5 owner, I wouldn't be considering dumping
>> my machine for the Intel. But then, that doesn't mean the Intel is a
>> bad box, just that it's a good incremental upgrade with more headroom
>
>That's an incremental upgrade to one CPU; don't forget the new iMacs have 2
>CPUs.

They have 2 cores, not 2 CPUs. It very much depends on the application
whether you will get the scalability in performance. Multi-threaded
applications will generally do well. There will be scenarios where that
doesn't happen, because of contention between the cores for the shared
L2 cache or the shared access to main memory.

This is one place that Opteron wins over Xeon; Xeon machines tend to be
real SMP machines, where all CPUs share the access to the memory, and
can end up contending. Opteron machines are cc-NUMA (cache-coherent
non-uniform memory access) where each socket has local memory for which
it does not have to fight with the other processors. This is an
inherently more scalable design (which is why many of the largest single
machines in the world are cc-NUMA in design, such as the SGI Altix
architecture, which scales to hundreds of CPUs in a single machine).
The downside of course, is that if the OS or application gets it wrong,
and allocates memory that actually belongs to another CPU, the
performance suffers badly. I saw this recently on a dual-core opteron,
where the core enumeration by Linux was exactly wrong, and all memory
was being allocated on the worst possible node. The performance was
pretty poor. The problem was fixed in the Linux kernel pretty quickly.
The machine in question was a quad-CPU dual-core Opteron box (i.e. 8
cores altogether).

In July I get to play with a machine which will have 40 dual-core
Opteron CPUs in a single machine. Should be fun. :-)

As far as Macs are concerned, the cc-NUMA/SMP distinction doesn't really
matter at the moment, with only 2-4 processing cores, SMP works pretty
well anyway.

Tim

.

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