Meet the Core i5 and the Core i7-800 series
Much has been written already about Intel's naming schemes for Nehalem-derived products, so I won't rehash any of those arguments. What you need to know is that Lynnfield introduces a new LGA1156 socket type and is thus incompatible with older Core i7 motherboards. Lynnfield chips will initially slot into two product lines, the Core i5-700 series and the Core i7-800 series. Bloomfield CPUs will coexist as higher-end specimens and members of the Core i7-900 series. Intel has three initial models based on Lynnfield, although I'd expect more to come eventually.
|Core i5-750||4||4||2.66 GHz||8MB||2.13 GHz||95W||$199|
|Core i7-860||4||8||2.8 GHz||8MB||2.4 GHz||95W||$285|
|Core i7-870||4||8||2.93 GHz||8MB||2.4 GHz||95W||$555|
Note that the lone Core i5 processor supports only four threads. In other words, Hyper-Threading has been disabled in this chip to differentiate it from the more expensive models. Another way the Core i5 differs from it siblings is its 2.13GHz uncore speed. That clock is important because it contributes to another Core i5-750 limitation: the max memory speed, without overclocking the base system clock, is 1333MHz. With a 2.4GHz uncore, the Core i7-800-series chips can hit 1600MHz memory speeds without extra help. The uncore speed also determines the clock speed of the L3 cache, another little Lynnfield control knob that will impact performance, if not tremendously.
Still, at $199, the Core i5 is squarely in the spotlight as the product with the broadest potential audience of the bunch. The closest competition from AMD is the Phenom II X4 955, which is selling for $189 at Newegg right nowa big discount off list price and a signal that AMD has anticipated Lynnfield's debut in its pricing. Intel has less of an incentive to make outgoing Core 2 products attractive in the face of the Core i5; the Core 2 Quad Q9550 is selling for $219.99 at Newegg presently, and the firm says it has no plans to reduce Core 2 prices upon the Core i5's introduction.
At $249, the Phenom II X4 965 is probably the closest competitor to the Core i7-860 from AMD. Other CPUs in its price range include the Core 2 Quad Q9650 at $320 online and the Core i7-920 at $280.
One rung up, well, AMD has no real rival to the Core i7-870, although the i7-870 does match up pretty closely against the Core i7-950 at $570. Keep these match-ups in mind as we move into our test results.
Now, forget what you just read about clock speeds. I've been holding out on telling you about one of Lynnfield's most notable features because I wanted to get that product information into the mix first. Like all Nehalem-derived products, Lynnfield chips have an onboard microcontroller dedicated to power management. This controller governs dynamic clock speed scaling schemes like SpeedStep for power savings, and it can be programmed via firmware to implement different algorithms, to tune for higher performance or lower power consumption, and so on.
This microcontroller contributes to Nehalem-based chips' impressive low power draw at idle, but it also enables a nifty little feature called Turbo Boost, which may be familiar from the Core i7-900 series. Turbo Boost can opportunistically raise clock speeds beyond their usual peaks, momentarily and dynamically, using the same P-state mechanism as SpeedStep.
The idea here is to take advantage of any additional thermal headroom available when the processor is under loadeither partially or fully. Uniquely, Nehalem includes a switch that can shut off power to an idle core entirely, eliminating even the leakage power that core would otherwise consume. Shutting down a core in this way opens up additional thermal headroom, so the remaining, engaged cores can ramp up their clock speeds and boost performance. Even with all four cores active, a chip may have some additional thermal headroom, and Turbo Boost can take advantage.
Bloomfield chips have Turbo Boost, but it's a relatively conservative version. With one thread active, a Core i7-900-series chip can raise its clock speed by up to two "ticks" or increments of the 133MHz base clock. With two or more threads loading up cores, the chip can go up to 133MHz above stock. That nets you a little bit more performance, especially because a Core i7-975 Extreme rated at 3.33GHz will spend a lot of its time at 3.46GHz, but it's not exactly eye-popping.
With its Lynnfield products, Intel has become much more aggressive with Turbo Boost tuning. The table below outlines the clock speeds possible with Turbo Boost doing its thing in the various Lynnfield models.
|Peak Turbo Boost speed|
|4 active cores||3 active cores||2 active cores||1 active core|
|Core i5-750||2.66 GHz||2.8 GHz||2.8 GHz||3.2 GHz||3.2 GHz|
|Core i7-860||2.8 GHz||2.93 GHz||2.93 GHz||3.33 GHz||3.46 GHz|
|Core i7-870||2.93 GHz||3.2 GHz||3.2 GHz||3.46 GHz||3.6 GHz|
The two Core i7-800-series processors have the most aggressive Turbo Boost tuning. What you're looking at here could amount to a pretty substantial jump in performance for single- and dual-threaded applications, includingyepgames. The Core 2 Quad Q9650 tops out at 3GHz. Between the clock-for-clock performance gain and the jump to 3.46 or 3.6GHz, the Core i7-870 should be markedly faster with such applications.
Interestingly enough, Turbo Boost speeds are not guaranteed by Intel, will depend on the thermal properties of the individual chip in question, and as I understand it, are also dependent on having good CPU cooling. However, my experience with various Nehalem-derived processors suggests that the clock speed on the product label isn't the speed at which the CPU will typically run under load. You can probably expect something more, with one tick up as a functional minimum.
This development also means that CPU performance is no longer exactly deterministic, which creates some emotional issues for me as a lab testing guy. One understands that taking as much performance as the thermal headroom will allow is a sensible behavior, especially now that thermals are the primary CPU performance constraint. Still, one senses there is no going back, and things will only become more complicated from here.
Another outgrowth of the Lynnfield Turbo mechanism is that we cannot use higher-end chips to exactly replicate the performance of lower-end products. We found this out when we investigated simulating a Core i7-860 with a Core i7-870. Although we know what the correct Turbo Boost ratios are for the i7-860, the ratios on the i7-870 cannot be modified via the BIOS, either on our Gigabyte P55-UD6 testbed motherboard or on the Intel DP55KG. As a result, we unfortunately don't have performance results for the Core i7-860 in the following pages. We'll have to acquire a specimen of the actual product in order to test its performance.
A little special sauce for Windows 7
You may have noticed that Lynnfield and Windows 7 are hitting the market not far from one another, and PCs based on both should be common during the fall buying season. Intel says it has worked with Microsoft on several specific optimizations for Windows 7, the most intriguing of which is a feature called "SMT parking."
The basic notion behind SMT parking is that the Windows scheduler will attempt to schedule threads so that all physical cores are occupied before any core gets two threads scheduled on its two front-ends (or logical cores). Since Hyper-Threading involves some cache partitioning and other forms of resource sharing, this is a potentially important feature. We've seen scheduler quirks cause poor and oddly unpredictable performance on Core i7 processors in the past. Based on our limited experience testing with Windows 7 and a cadre of SMT-enabled processors for this review, our initial impressions of SMT parking are positive. We've seen performance results for executables that rely on the Windows scheduler for thread allocation that match the performance of executables with explicit, SMT-aware thread affinity built in. Our initial sense is that SMT parking blunts some potential disadvantages of Hyper-Threading, making it more of an unqualified win, even on the desktop.
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