Power management and, uh, forced induction
Like AMD's native quad-core Phenom, the Core i7 can raise and lower the clock speed of each of its processor cores independently and dynamically in response to demand. Unlike the Phenom, though, the Core i7 doesn't use separate power planes for the cores and the "uncore." Instead, Intel has put a switch between each core and the voltage regulator output, and power can be shut off to any individual core that goes into the deepest idle state, C6, transparently to software and to the other cores. Because power to the core is shut off, Intel claims even leakage power is eliminated, making that core's power consumption approximately zero. In the event that all four cores become idle, then the uncore can go into a C6 state, as well, in which most uncore logic is stopped and I/O drops into a low-power mode.
Controlling all of this wizardry in the Core i7 is a dedicated, on-chip microcontroller for power management. This microcontroller is programmable via firmware and can be made to use different algorithms to optimize for, say, the lowest possible power use or for low latencies when stepping up from low-power states. No doubt Intel will use this capability to tune products for diverse segments, giving mobile processors different behaviors than, say, high-performance desktop parts like the ones we're reviewing here.
One trick that this microcontroller enables is the oh-so creatively named "Turbo mode" built into the Core i7. This feature pushes the active cores beyond their baseline clock frequencies when the CPU isn't at full utilization. Turbo mode operates according to some simple rules. In the event that a single-threaded application is occupying one core while the rest are idle, Turbo mode will raise clock speeds by as much as two full "ticks" beyond the baseline. For instance, for our Core i7-965 Extreme processor, Turbo mode could raise the multiplier from 24 to 26, or the core clocks from 3.2 GHz to 3.46 GHz, since the base clock in Core i7 systems runs at 133 MHz. With two or more threads active, Turbo mode will only raise clock speeds by one tick. All of this happens automatically using the same basic P-state mechanism as SpeedStep.
The additional clock frequency headroom comes from the fact that a less-than-fully-occupied Core i7 may not run up against the limits imposed by its thermal design power, or TDPthe chip's specified power envelope. We've seen a processor running eight instances of Prime95 stay at "one tick up" for a sustained period of time with good cooling. Then again, Intel has set CPU core voltages individually at the factory for some time now, and it's quite possible that some chips may not be able to sustain Turbo acceleration within their specified power envelopes for any length of time. As I understand it, that may simply be the luck of the draw, with only the baseline clock speed guaranteed.
Interestingly enough, because the Core i7-965 Extreme Edition doesn't have a locked upper multiplier, the CPU can be overclocked by tweaking the Turbo mode settings in the BIOS. Intel's DX58S0 "Smackover" (uh huh) motherboard exposes control over the maximum clock multipliers for one, two, three, and four occupied cores, as well as the ability to adjust the TDP limit in watts and the current limit in amps. You'll probably want a good aftermarket cooler if you plan to play with these settings. If that's too fancy for your tastes, one may also choose to disable Turbo mode and overclock via the usual ways, as welleither by raising the multiplier on an Extreme Edition or by cranking up the base clock on any Core i7.
Pricing and availability
Although we are publishing our review of the Core i7 today, products won't be selling to consumers immediately. Instead, Intel has given us the nebulous target of "in November" for product availability. Beyond that, I have no more information than you about when to expect these things in stores. I can, however, give you pricing and model information. Like this:
|Model||Clock speed||North bridge speed||QPI speed||TDP||Price|
|Core i7-965 Extreme||3.2 GHz||3.2 GHz||6.4 GT/s||130 W||$999|
|Core i7-940||2.93 GHz||2.13 GHz||4.8 GT/s||130 W||$562|
|Core i7-920||2.66 GHz||2.13 GHz||4.8 GT/s||130 W||$284|
All three of the Core i7 processors coming this month are "Bloomfield" chips, so they all have quad cores, three memory channels, and 8 MB of L3 cache onboard. As you can see, though, they do differ in terms of the clock speed of the L3 cache and of what I've labeled the "North bridge speed." That's basically the clock speed of the "uncore," but things get a little hairy from there. The uncore includes several elements, including the QPI links, the L3 cache, and the memory controller. Each of these elements may run at different multipliers from the base clock. For the Core i7-965 Extreme, the relationship is straightforward: everything runs at 3.2 GHz, including the QPI link, hence its 6.4 GT/s data rate. In the 940 and 920, the cores run at one speed, the QPI link at another (2.4 GHz), and, as I understand it, the memory controller and L3 cache both run at 2.13 GHz.
One of the implications of the slower memory controller frequency in the Core i7-920 and -940 is that, at least on our Intel "Smackover" board, one cannot achieve DDR3 memory speeds beyond 1066 MHz without overclocking the base system clock, which presents a real risk of instability. The multipliers just aren't available in the BIOS to go beyond that. We'll have to see how that works out in practice with enthusiast motherboards from the big names that, uh, aren't Intel, but it appears DDR 1066 MHz may be a practical limit without overclocking for the 920 and 940, which is a shame.
Expect to see even more variety from Nehalem-derived processors in the future, because the architecture is designed to be modular. Intel may vary the core count, cache sizes, number of QPI links, the presence of Hyper-Threading, and the number of memory channels in future products. We also expect them to integrate a graphics core into some parts. Given what we've learned about uncore clocking flexibility, I'd expect some variance there, too. Intel may choose to, say, clock down various parts of the uncore, such as the L3 cache, in lower end or mobile products in order to save on power or to improve yields.
Unfortunately, more affordable variants of Nehalem may be a long time in coming. We know that mainstream desktop Nehalem derivatives are expected to have only two DDR3 memory channels and possible integrated graphics, but those products may not arrive until well into next year. Until then, the Core i7 may remain a rather pricey option, because even the 920 is wedded to motherboards based on the premium X58 chipset. You may, though, want to check out our review of two of the first X58 boards right here.
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