Core i5 750 - Core i7 860 and 870 processor review - QPI and Multi Channel memory

Quickpath interconnect (QPI)

The Core i5 and i7 processors sport both the memory controller and system I/O integrated onto the CPU die and therefore eliminates the Intel Front Side Bus (FSB) altogether. In place of the FSB, one or more high speed, point-to-point buses called Quick Path Interconnect (QPI) are used, formerly known as Common Serial Interconnect Bus or CSI. QPI features higher bandwidth than the traditional FSB and is better suited to system scaling. We quickly close one eye and hint you towards AMD. See, AMD's HyperTransport links are somewhat similar to what Intel is doing today. High speed point-to-point inter-component or processor connectivity/communications. Intel was tied to the frontside bus that started to interfere with performance. The QPI architecture will allow Intel to connect dual, tri- or even quad-channel memory directly to the the processor's integrated memory controller.

Intel also added PCI Express links directly into the CPU die. This will for sure deliver much more bandwidth for high performance graphics cards and remove any bottleneck issues with other system components.

So you are asking how will that work in relation to say overclocking. Well, in fact it's still there, yet functioning in a different way. Imagine a single pumped FSB (now called base clock) at 133 and apply a flexible multiplier to it, say with a range of 12 to 25. Depending on the workload the processor it will dynamically alter that multiplier. So in an idle state you'd see an effective 12x133 Mhz = 1596 MHz processor clock frequency. This is how Core i5 and i7 achieves its clock.

On full load a Core i7 processor would jump to 133MHz x 25mp = 3.3 GHz. QPI is one of the new extendable building blocks of the Nehalem CPU architecture.

One small change though. On the X58 motherboards QPI is also linked to the X58 chipset. With Lynnfield processors and P55 a DMI link from the processor to the P55 chipset is made. Other than that, it's all the same.

Back to a Dual channel memory controller

With the arrival of Core i7 870, 860 and i5 750 Intel made the decision to go back to a dual-channel memory interface. Don't get too worried here, as the point-to-point controller is so much faster than the old limited FSB based controllers.

Today's launched processors come with DDR3 1333 support and much like the previous Nehalem i7 processors are 64-bit memory wide controllers which are connected directly with the processor's silicon. As a result this design brings a bandwidth utilization of as much as 90%, a nice jump from the FSB based 50-60% utilization for sure.

The new controller of course supports both registered (server market) and unregistered (consumer) memory DIMMs. The controller is fast... very fast, and supports DDR3-800, DDR3-1066, DDR3-1333 JEDEC standards, yet has room for future scalability, in fact you will have no problems running your memory at 1600 or even 1866 MHz. In fact we have already seen an ASUS board that could do 2500+ MHz (!) on the memory, overclocked of course.

In this review we'll be using an 8GB 1600 MHz Corsair Dominator memory kit. Corsair provided this kit specifically for this review. It is horribly sweet memory as it's arranged as 2x4 = 8 GB of memory.

Similar to the Core i7 900 series memory controllers you'll be limited to 1.65 DIMM/QPI voltage for it to run properly.

For this test we'll be using a Corsair Dominator DDR3 kit. Lynnfield optimized, low 1.65 Volts 1600 MHz at CAS 8.