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.

The Chips Understanding Nehalem's Memory Architecture
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  • anand4happy - Sunday, February 08, 2009 - link

    saw many thing but this is the thing something dfferent

    sd4us.blogspot.com/2009/01/intel-viivintel-975x-express-955x.html
    Reply
  • nidhoggr - Monday, November 10, 2008 - link

    I cant find that information on the test setup page. Reply
  • nidhoggr - Monday, November 10, 2008 - link

    test not text :) Reply
  • puffpio - Wednesday, November 05, 2008 - link

    would you guys consider rebenchmarking?
    from the x264 changelog since the nehalem specific optimizations:
    "Overall speed improvement with Nehalem vs Penryn at the same clock speed is around 40%."
    Reply
  • anartik - Wednesday, November 05, 2008 - link

    Good review and better than Tom's overall. However Tom's stumbled on something that changed my mind about gaming with Nehalem. While Anand's testing shows minimal performance gains (and came to the not good for games conclusion) Tom's approached it with 1-4 GPU's SLI or Crossfire. All I can say is the performance gains with Nvidia cards in SLI was stunning. Maybe the platform favors SLI or Nvidia had a driver advantage in licensing SLI to Intel. Either way Nehalem and SLI smoked ATI and the current 3.2 extreme quad across the board. Reply
  • dani31 - Wednesday, November 05, 2008 - link

    I know it would't change any conclusion, but since we discuss bleeding edge Intel hardware it would have been nice to see the same in the AMD testbed.

    Using a SB600 mobo (instead of the acclaimed SB750) and an old set of drivers makes it look like the AMD numbers were simply pasted from an old article.
    Reply
  • Casper42 - Tuesday, November 04, 2008 - link

    Something I think you guys missed in your article/conslusion is the fact that we're now able to pair a great CPU with a pretty damn good North/South Bridge AND SLI.

    I found that the 680/780/790 featureset is plainly lacking and that the Intel ICH9R/10R seems to always perform better and has more features. If any doubt, look at Matrix RAID vs nVidia's RAID. Night and day difference, especially with RAID5.

    The problem with the X38/X48 was you got a great board but were effectively locked into ATI for high end Gaming.

    Now we have the best of both worlds. You get ICH10R, a very well performing CPU (even the 920 beats most of the Intel Quad Core lineup) AND you can run 1/2/3 nVidia GPUs on the machine. In my opinion, this is a winning combination.


    The only downside I see is board designs seem to suck more and more.

    With socket 1366 being so massive and 6 DIMM slots on the Enthusiast/Gamer boards, we're seeing not only 6 expansion slots (down from the standard of 7) but in most boards I have seen pics of, the top slot is an x1 so they can wedge it next to the x58 IOH which means your left with only 5 slots for other cards. Using 3 dual slot cards is out of the question without a massive 10 slot case (of which there are only like 3-5 on the market) and even if you can wedge 2 or 3 dual slot cards into the machine, you have almost zero expansion card slots should you ever need them.

    Then we get to all the cooling crap surrounding the CPU. ALL these designs rely on a top down traditional cooler and if you decide to use a highly effective tower cooling solution, all the little heatsink fins on the Northbridge and pwer regulators around the CPU get very little or no airflow. Now your in there adding puny little 40/60mm fans that produce more noise than airflow, not to mention that the DIMMs are hardly ever cooled in today's board designs.
    Call me a cooling purist if you will, but I much prefer traditional front to back airflow and all this side intake top exhaust stuff just makes me cringe. I personally run a Tyan Thunder K8WE with 2 Hyper6+ coolers and the procs and RAM are all cooled front to back. Intake and exhaust are 120mm and I have a bit of an air channel in which that airflow never goes near the expansion card slots below, which by the way have a 92mm fan up front pushing air in across the drives and another 92mm fan clipped onto the expansion slots in the back pulling it back out.

    I dont know how to resolve these issues, but I think someone surely needs to because IMHO its getting out of control.
    Reply
  • lemonadesoda - Tuesday, November 04, 2008 - link

    "Looking at POV-Ray we see a 30% increase in performance for a 12% increase in total system power consumption, that more than exceeds Intel's 2:1 rule for performance improvement vs. increase in power consumption."

    You cant use "total system power", but must make the best estimate of CPU power draw. Why? Because imagine if you had a system with 6 sticks of RAM, 4 HDDs, etc. you would have ever increasing power figures that would make the ratio of increased power consumption (a/b) smaller and smaller!

    If you take your figures and subtract (a guestimate of) 100W for non CPU power draw, then you DONT get the Intel 2:1 ratio at all!

    The figures need revisiting.
    Reply
  • AnnonymousCoward - Thursday, November 06, 2008 - link

    Performance vs power appears to linearly increase with HT. Using the 100W figure for non-CPU draw means a 25% power increase, which is close to the 30% performance.

    Unless we're talking about servers, I think looking at power draw per application is silly. Just do idle power, load power, and maybe some kind of flops/watt benchmark just for fun.
    Reply
  • silversound - Tuesday, November 04, 2008 - link

    great article, tomsharware reviews always pro intel and nvidia, not sure if they got pay $ to suppot them. anandtech is always neutral, thx! Reply

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