Inside Intel's Haswell: What do 1.4 BEELLION transistors get you?
The brains and the brawn of the next Windows 8 slabs
Back in 2011, Intel pledged Haswell would cut the power consumed by a laptop when its lid was closed to five per cent, a promise it reiterated at IDF 2012 last September. Haswell employs a number of tricks in its bid to make good on the company’s promise and deliver what it claims is “Intel’s biggest increase in battery life generation on generation”. That means, it says, doubling or even tripling the battery life achievable with an Ivy Bridge chip, and a 20x stand-by power reduction when compared to a Sandy Bridge machine.
The shift to a smaller production process, from Sandy Bridge’s 32nm to 22nm, introduced last year with the Sandy Bridge re-spin, Ivy Bridge, helps a lot here. Intel has refined its 22nm process further, cutting the current leaking from its tri-gate transistors to between a half and a third of what it was in Ivy Bridge, and dropping the voltage required to drive each transistor. Bringing more IO control onto the processor die - all the digital display interfacing is now handled by the CPU - helps cut the system’s overall power draw a bit, but Haswell goes further.
First, it has an on-board voltage regulator, which Intel calls its “Fully Integrated Voltage Regulator (FIVR)”. This allows Haswell’s key sections - the CPU cores, the GPU, the IO and memory controllers, and the units that manage them all - to be fed from a single voltage input. Previously there were five separate inputs all fed from voltage regulators on the motherboard. Beyond reducing motherboard component counts and thus cost and power draw, the advantages of an integrated voltage regulator are faster power switching because there’s less oscillation around the target voltage when the switch takes place.
Separate voltage planes mean that deeper sleep states can be reached too, by completely shutting down parts of the core that aren’t required. “Everything that is not needed is turned off!” boast the engineers. The chip transitioned in and out of these states more quickly: Haswell is 25 per cent faster than Ivy Bridge in this regard, Intel has said in the past.
Placed on the chip, the FIVR can be closely tied into the processor’s power management system, which in Haswell uses the chip’s understanding of what it’s being asked to do in order to exploit more power-reduction opportunities. It can see, says Intel, that for a certain period of time parts of the chip - and even some system components - can be put into low-power states or turned off altogether to save energy. After the calculated duration, Haswell’s power manager - Intel calls it a Power Optimizer - wakes them up again.
Smarter power management isn’t merely central to making Haswell-based systems’ batteries last longer, but it also makes them more useful and responsive the way an ARM-powered mobile devices can be: constantly picking up messages, even when sleeping, and ready for use in a near instant. That’s a crucial facility laptops must gain if they aren’t to be overshadowed by tablets any more than they already are.
Haswell delivers three of power states that form a sub-set of the standard S0, ‘awake’ state that has been part of the ACPI (Advanced Configuration and Power Interface) specification for years. Instead of remaining fully awake in the S0 state, Haswell drops to the equivalent of existing S1 to S3 sleep mode power consumption levels but keeps an eye open, as it were, so it’s ready to handle user interaction. Think of it as a quick nap while the user pauses to read something on screen, or a deeper doze when the machine’s not being used. In each case the Power Optimizer can quickly bring everything back so the user doesn’t notice it hasn’t been entirely paying attention.
So when laptop’s lid is closed, the system will drop to a power consumption level existing machines reach only when hibernating - the S3 state - but the system is nonetheless sufficiently awake to be ready to use by the time the user has lifted the lid, and to periodically poll for messages and other net-delivered data. Haswell delivers “idle power approaching [that of] tablet CPUs”, claims Intel.
Latency data from compatible system components allows the Haswell Power Optimizer to know how long it takes each of them to wake up, ensuring the whole system - or at least those parts of it required to give the user immediate operation - are all ready at the same time. To gain the full benefit, then, notebook makers will have to equip their kit with components that can give Haswell that latency information, so expect Intel to exert even more control than it already does, by certifying which components give full Haswell Power Optimizer compatibility.
Latency informations also helps Power Optimizer schedule tasks to align them, resulting in burst of activity bookended by periods of longer power-saving inactivity. Windows 8 already supports this kind of operation on order to keep its new UI live tiles updated, a technique Microsoft calls Connected Standby, but Haswell extends it to Windows desktop apps too and integrates it into its more modern sleep state set-up.
Unlike Ivy Bridge, Haswell maintains separate clock signals for the cores, for the GPU, and for the L3 cache and the ring bus that links it to those other two modules. At the cost of cache performance, this means you can keep the cores slow when only the GPU needs to exchange data with the cache, saving power. Slowing down the core, because its frequency isn’t pegged to the GPU, means, conversely, there’s power left over to raise the GPU’s frequency, if it’s required.