The Dark Knight: Intel's Core i7by Anand Lal Shimpi & Gary Key on November 3, 2008 12:00 AM EST
- Posted in
Nehalem's Weakness: Cache
Intel opted for a very Opteron-like cache hierarchy with Nehalem, each core gets a small L2 cache and they all sit behind one large, shared L3 cache. This sort of a setup benefits large codebase applications that are also well threaded, for example the type of things you'd encounter in a database server. The problem is that the CPU launching today, the Core i7, is designed to be used in a desktop.
Let's look at a quick comparison between Nehalem and Penryn's cache setups:
|Intel Nehalem||Intel Penryn|
|L1 Size / L1 Latency||64KB / 4 cycles||64KB / 3 cycles|
|L2 Size / L2 Latency||256KB / 11 cycles||6MB* / 15 cycles|
|L3 Size / L3 Latency||8MB / 39 cycles||N/A|
|Main Memory Latency (DDR3-1600 CAS7)||107 cycles (33.4 ns)||160 cycles (50.3 ns)|
*Note 6MB per 2 cores
Nehalem's L2 cache does get a bit faster, but the speed doesn't make up for the lack of size. I suspect that Intel will address the L2 size issue with the 32nm shrink, but until then most applications will have to deal with a significantly reduced L2 cache size per core. The performance impact is mitigated by two things: 1) the fast L3 cache, and 2) the very fast on die memory controller. Fortunately for Nehalem, most applications can't fit entirely within cache and thus even the large 6MB and 12MB L2 caches of its predecessors can't completely contain everything, thus giving Nehalem's L3 cache and memory controller time to level the playing field.
The end result, as you'll soon see, is that in some cases Nehalem's architecture manages to take two steps forward, and two steps back, resulting a zero net improvement over Penryn. The perfect example is 3D gaming as you can see below:
|Intel Nehalem (3.2GHz)||Intel Penryn (3.2GHz)|
|Age of Conan||123 fps||107.9 fps|
|Race Driver GRID||102.9 fps||103 fps|
|Crysis||40.5 fps||41.7 fps|
|Farcry 2||115.1 fps||102.6 fps|
|Fallout 3||83.2 fps||77.2 fps|
Age of Conan and Fallout 3 show significant improvements in performance when not GPU bound, while Crysis and Race Driver GRID offer absolutely no benefit to Nehalem. It's almost Prescott-like in that Intel put in a lot of architectural innovation into a design that can, at times, offer no performance improvement over its predecessor. Where Nehalem fails to be like Prescott is in that it can offer tremendous performance increases and it's on the very opposite end of the power efficiency spectrum, but we'll get to that in a moment.