The Dark Knight: Intel's Core i7
by Anand Lal Shimpi & Gary Key on November 3, 2008 12:00 AM EST- Posted in
- CPUs
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.
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Clauzii - Thursday, November 6, 2008 - link
I still use PS/2. None of the USB keyboards I've borrowed or tried out would work in 'boot'. Also I think a PS/2 keyboard/mouse don't lag so much, maybe because it has it's own non-shared interrupt line.But I can see a problem with PS/2 in the future, with keyboards like the Art Lebedev ones. When that technology gets more pocket friendly I'd gladly like to see upgraded but still dedicated keyboard/mouse connectors.
The0ne - Monday, November 3, 2008 - link
Yes. I have the PS2 keyboard on-hand in case my USB keyboard can't get in :)Strid - Monday, November 3, 2008 - link
Ahh, makes sense. Thanks for clarifying!Genx87 - Monday, November 3, 2008 - link
After living through the hell that were ATI drivers back in 2003-2004 on a 9600 Pro AIW. I didnt learn and I plopped money down on a 4850 and have had terrible driver quality since. More BSOD from the ati driver than I have had in windows in the past 5 years combined from anything. Back to Nvidia for me when I get a chance.That said this review is pretty much what I expected after reading the preview article in August. They are really trying to recapture market in the 4 socket space. A place where AMD has been able to do well. This chip is designed for server work. Ill pick one up after my E8400 runs out of steam.
Griswold - Tuesday, November 4, 2008 - link
You're just not clever enough to setup your system properly. I have two indentical systems sitting here side by side with the only difference being the video card (HD3870 in one and a 8800GT in the other) and the box with the nvidia cards gives me order of magnitude more headaches due to crashing driver. While that also happens on the 3870 machine now and then, its nowehere nearly as often. But the best part: none of the produces a BSOD. That is why I know you're most likely the culprit (the alternative is faulty hardware or a pathetic overclock).Lord 666 - Monday, November 3, 2008 - link
The stock speed of a Q9550 is 2.83ghz, not 2.66qhz.Why the handicap?
Anand Lal Shimpi - Monday, November 3, 2008 - link
My mistake, it was a Q9450 that was used. The Q9550 label was from an earlier version of the spreadsheet that got canned due to time constraints. I wanted a clock-for-clock comparison with the i7-920 which runs at 2.66GHz.Take care,
Anand
faxon - Monday, November 3, 2008 - link
toms hardware published an article detailing that there would be a cap on how high you are allowed to clock your part before it would downclock it back to stock. since this is an integrated par of the core, you can only turn it off/up/down if they unlock it. the limit was supposedly a 130watt thermal dissipation mark. what effect did this have in your tests on overclocking the 920?Gary Key - Monday, November 3, 2008 - link
We have not had any problems clocking our 920 to the 3.6GHz~3.8GHz level with proper cooling. The 920, 940, and 965 will all clock down as core temps increase above the 80C level. We noticed half step decreases above 80C or so and watched our core multipliers throttle down to as low as 5.5 when core temps exceeded 90C and then increase back to normal as temperatures were lowered.This occurred with stock voltages or with the VCore set to 1.5V, it was dependent on thermals, not voltages or clock speeds in our tests. That said, I am still running a battery of tests on the 920 right now, but I have not seen an artificial cap yet. That does not mean it might not exist, just that we have not triggered it yet.
I will try the 920 on the Intel board that Toms used this morning to see if it operates any differently than the ASUS and MSI boards.
Th3Eagle - Monday, November 3, 2008 - link
I wonder how close you came to those temperatures while overclocking these processors.The 920 to 3.6/3.8 is a nice overclock but I wonder what you mean by proper cooling and how close you came to crossing the 80C "boundary"?