Dual Caches for Dual-core Chips

DominoTree writes "The dual-core chips that AMD and Intel plan to bring to market next year won't be sharing their memories. A version of Opteron coming in 2005 and Montecito, a future member of Intel's Itanium family also slated for next year, will both have two processor cores, the actual unit inside a processor that performs the calculations, and each core will have separate caches."

Comments

Note: Here, Single is Better

[Anonymous Coward]

In case it's not obvious to those who didn't read the article all the way through, it's a better thing when the memory is shared (single cache) rather than separate (dual cache). But that is harder to design, so for these first-generation dual-core chips from Intel and AMD, they are using separate caches for each core. (IBM's dual core Power4 processor has a unified cache.) At some point down the road, they will likely unify them to increase performance.

Re:Note: Here, Single is Better

[spuzzzzzzz]

Are there situations where two caches might be better? For example, a multi-threaded application with two memory-intensive threads, each locked down onto a specific CPU?

Not really. The problem with 2 caches is duplication. It is quite probable that both cores will want to work on the same thing, in which case cache space will be wasted. It also creates timing complications when one core wants to write to its cache because the other core will have to be told to invalidate its relevant cache entry. On the other hand, you could create a single cache with double the size. This would make sharing memory between CPUs simpler and it wouldn't significantly increase access times (so the situation you mentioned wouldn't be affected). The argument for double caches is about cost, scalability and design simplicity, not performance.

Different core models

[SIGALRM]

The dual-core chips that Advanced Micro Devices and Intel plan to bring to market next year won't be sharing their memories

As I understand it, the rationale behind Opteron's "Direct Connect" dual-core architecture is to make it easier to place two processor cores on the same silicon die. It's also a power-consupmtion issue, as the two processors can run at lower clock speeds. However, unlike Intel's design, Direct Connect features an integrated memory controller and hypertransport interconnects that connect the processor to the I/o port or directly to another processor.

Re:Licensing Issues?

[Ianoo]

When hyperthreading was released, the industry had to cope with similar issues. Those of us using operating systems with artificial limits imposed on the number of possible processors used in a system had to wait for software updates to fix detection. I'm sure that the same thing will happen again, undoutedly there will be some flag in a register somewhere that identifies whether a processor is part of a dual-core chip or just a single CPU on its own. The OS or software can just read this in and work out whether there is sufficient licensing to use them.

Re:Confused

[ERJ]

Kinda. I could see a couple advantages though:

1) Fast interconnect between chips. Instead of having to transfer data over the bus, if the CPU needed info from the other CPU it could transfer over a high speed connection without having to involve other parts of the machine (bus). AMD already has a sort of high speed interconnect to their multi-cpu motherboards instead of splitting like intel does but I would imagine that this would still be faster.

2) Less motherboard room needed. You don't need dual cooling fans, dual power / interface lines and have more room overall on the motherboard.

Re:How is this different from a two processor syst

[hawkbug]

It's not much different - that's the point. 2 processors in a single socket, saves a lot of money production wise, and that should pass onto the consumer. AMD has said their's is backward comaptible, and that's huge. You already got a single cpu opteron workstation? Well now you can have a dual cpu one for the price of a single cpu upgrade. That kicks ass.

Re:Itanium? (somewhat off-topic)

[Anonymous Coward]

Despite what Sun has to say on the matter, Itanium system and processor sales have been increasing steadily since 2H,2000prior to that, there was a big lull in demand because few wanted to buy underperforming Itanium 1 machines when the Itanium 2 was expected rather soon (and announced relatively early).

Today, in contrast, there _doesn't_ appear to a lull in demand for Itanium 2 machines, even though Montecito (Itanium 3) has been announced in a fair bit of detail. That's because for some applications (in HPC, high-end database work, certain EDA/CAD/CAE work, and ultra-high-reliability computing) Itanium 2 systems are basically unbeatable. They also run some OSes which are very important to some organizations, such as HP-UX and OpenVMS.

Long story short, the Itanium 1 was something of a flop, the Itanium 2 is really pretty decent, and everyone is expecting the Itanium 3 to offer pretty decent _price/performance_, in addition to best-bar-none performance when it is released next year.

Re:mmmm cores

[iNiTiUM]

Sure you can
Oh you want one for the AMD64?
How about these?