Nehalem.

Nuh - hay - lem

At least that's how Intel PR pronounces it.

I've been racking my brain for the past month on how best to review this thing, what angle to take, it's tough. You see, with Conroe the approach was simple: the Pentium 4 was terrible, AMD proudly wore its crown and Intel came in and turned everyone's world upside down. With Nehalem, the world is fine, it doesn't need fixing. AMD's pricing is quite competitive, Intel's performance is solid, power consumption isn't getting out of control...things are nice.

But we've got that pesky tick-tock cadence and things have to change for the sake of change (or more accurately, technological advancement, I swear I'm not getting cynical in my old age):



Could Nehalem ever be good enough? It's the first tock after Conroe, that's like going on stage after the late Richard Pryor, it's not an enviable position to be in. Inevitably Nehalem won't have the same impact that Conroe did, but what could Intel possibly bring to the table that it hasn't already?

Let's go ahead and get started, this is going to be interesting...
Nehalem's Architecture - A Recap

I spent 15 pages and thousands of words explaining Intel's Nehalem architecture in detail already, but what I'm going to try and do now is summarize that in a page. If you want greater detail please consult the original article, but here are the cliff's notes.


Nehalem

Nehalem, as I've mentioned countless times before, is a "tock" processor in Intel's tick-tock cadence. That means it's a new microarchitecture but based on an existing manufacturing process, in this case 45nm.

A quad-core Nehalem is made up of 731M transistors, down from 820M in Yorkfield, the current quad-core Core 2s based on the Penryn microarchitecture. The die size has gone up however, from 214 mm^2 to 263 mm^2. That's fewer transistors but less densely packed ones, part of this is due to a reduction in cache size and part of it is due to a fundamental rearchitecting of the microprocessor.

Nehalem is Intel's first "native" quad-core design, meaning that all four cores are a part of one large, monolithic die. Each core has its own L1 and L2 caches, and all four sit behind a large 8MB L3 cache. The L1 cache remains unchanged from Penryn (the current 45nm Core 2 architecture), although it is slower at 4 cycles vs. 3. The L2 cache gets a little faster but also gets a lot smaller at 256KB per core, whereas the lowest end Penryns split 3MB of L2 among two cores. The L3 cache is a new addition and serves as a common pool that all four cores can access, which will really help in cache intensive multithreaded applications (such as those you'd encounter in a server). Nehalem also gets a three-channel, on-die DDR3 memory controller, if you haven't heard by now.

At the core level, everything gets deeper in Nehalem. The CPU is just as wide as before and the pipeline stages haven't changed, but the reservation station, load and store buffers and OoO scheduling window all got bigger. Peak execution power hasn't gone up, but Nehalem should be much more efficient at using its resources than any Core microarchitecture before it.

Once again to address the server space Nehalem increases the size of its TLBs and adds a new 2nd level unified TLB. Branch prediction is also improved, but primarily for database applications.

Hyper Threading is back in its typical 2-way fashion, so a single quad-core Nehalem can work on 8 threads at once. Here we have yet another example of Nehalem making more efficient use of the execution resources rather than simply throwing more transistors at the problem. With Penryn Intel hit nearly 1 billion transistors for a desktop quad-core chip, clearly Nehalem was an attempt to both address the server market and make more efficient use of those transistors before the next big jump and crossing the billion transistor mark.