QPI, Turbo Mode

Intel QuickPath Interconnect

Rather than relying on the FSB for yet another processor, Intel has developed their QuickPath Interconnect (QPI) to link the CPU to the outside world.

QPI is a high-speed, point-to-point interconnect that provides connections between the CPU to memory, CPU-to-CPU, and CPUs to the I/O hub. The QuickPath interconnect boasts up to 6.4 Gigatransfers/second links (one link is the equivalent of 12.8GB/sec of bandwidth), since it’s bi-directional, QPI effectively delivers 25.6GB of total bandwidth. In comparison, Conroe’s 1066MHz FSB topped out at 8.5GB/sec of peak bandwidth.

Integrated memory controller

Nehalem sports an integrated triple-channel memory controller that supports DDR3 memory exclusively. Memory clocks are limited to just two speeds: 800MHz DDR3 and 1066MHz DDR3. Nehalem can run with faster DDR3-1333 and DDR3-1600 memory, but in this case the modules would be underclocked to run at 1066MHz (unless of course you decide to OC).

By integrating the memory controller on the processor die, memory latency is dramatically reduced.

Obviously with a triple-channel memory controller you’ll have to install memory modules in groups of three rather than in pairs. Like Phenom, Nehalem’s memory controller supports NUMA (non-uniform memory architecture). In a multi-socket system each CPU will have its own local memory so you’ll need six modules for say a 2P server to deliver optimal performance.

Turbo Mode

One lesson Intel’s learned over the years is just how slow the software industry is to adapt to the multi-core CPU world we live in today. Games for instance are just now being written with dual-core in mind, there are only a handful of titles that truly take advantage of quad-core. As Intel goes from two, to four, and eventually eight processing cores in the future, there’s potential that many of these additional cores will sit idling completely untapped by the software. With this in mind Intel has developed a new power control unit (PCU) right onto the CPU die. The PCU is solely responsible for power management, actively monitoring the cores for aspects such as utilization and temperature. The PCU can then completely shut off cores that aren’t being used, helping to reduce overall CPU power consumption. This brings us to Turbo Mode.

In cases where cores aren’t being used, the PCU can shut down those cores and selectively overclock the core(s) that are being taxed. Say for instance you’re running a single-threaded game. In this case the PCU shuts off three processing cores and overclocks the one core you’re using. All this is completely invisible to the OS and the end user, providing a performance boost without any user intervention on your part.

The PCU will OC the active core by up to two clock speed bumps (+266MHz) max, so your 2.66GHz CPU becomes a 2.93GHz processor. If the PCU detects that your power usage, current, or temps are too high at that level, it will automatically drop you down to just one speed bump (+133MHz), knocking you down to 2.80GHz.

If you can keep your power usage and core temps down, the PCU will potentially run all four cores at 266MHz over your CPU’s base clock frequency. Keep in mind that this also applies when OC’ing, so if you’ve overclocked your CPU 533MHz over stock in BIOS, Turbo Mode will OC the processor core(s) another 133MHz or 266MHz above that, netting a clock speed boost of up to 800MHz.

Turbo Mode is a feature that can be completely adjusted in BIOS, so if you’re reluctant to OC your processor you can turn it off, or if you’re an enthusiast who loves to OC, you can tweak Turbo Mode settings in your motherboard’s BIOS to get just the right clock speed. We’ll be discussing Turbo in more depth further in this article.