Intel Core i7 870 & Core i5 750 Review - The Science Bit Review

As mentioned previously, these two new CPU ranges are based on the same Nehalem microarchitecture as Core i7 900 (which in turn was very similar to that of the Core microarchitecture) so the actual parts that do the calculations are identical and the fundamentals of the rest of the CPU are the same.

Essentially, through improvements in micro-op handling, the addition of SSE4.2 instructions, and an increase in the number of execution units, Nehalem offered a significant increase in performance at the micro-op level over the Core microarchitecture that came before it. In other words, whether you’re dealing with single or multi-threaded applications, Nehalem can run them more efficiently (i.e. faster) than Core.

Other changes included a move to a three level cache system, improved power management, and the introduction of Turbo Mode and all these features have been carried over from Bloomfield to Lynnfield. Simultaneous Multithreading (two threads can run on each core), QPI, and an integrated memory controller (IMC) were all also introduced with Nehalem but it’s these features that have either had the chop or been modified in the move from Bloomfield to Lynnfield.

Starting with the IMC, Lynnfield CPUs will still have one, and it still interfaces with DDR3 memory, but it has been reduced from triple channel to dual channel. Assuming you’re using 1,333MHz memory this means a reduction in memory bandwidth from 31.2GB/s to 20.8GB/s, which is quite a drastic reduction that brings it below the theoretical level of AMD’s latest systems. Nevertheless, it is still markedly more than could be easily achieved with the fastest Penryn systems, which still compete with AMDs current fastest systems in terms of overall performance, so clearly memory bandwidth isn’t the be all and end all.

The next big change requires us to look at the whole system architecture to explain. With Bloomfield, as well as bringing the memory controller onboard, Intel introduced a new interconnect between the CPU and other parts of the system. Called QuickPath Interconnect (QPI), it talks to the X58 IOH ”northbridge” which in turn interfaces with PCI-Express graphics and the rest of the IO subsystem through the ICH ”southbridge” (hard drives, USB, audio, etc). This link is very fast, providing up to 25.6GB/s of bandwidth, which means it can support, amongst other things, 36 lanes of PCI-Express attached to the X58 IOH. However, this amount of bandwidth is seldom going to be used by most systems so for Lynnfield Intel decided to take a different approach.

Instead of QPI, Lynnfield integrates 16 lanes of PCI-Express graphics communication onto the CPU itself and all other communication is done via a DMI link, which can deliver up to 2GB/s. This may sound like a drastic drop but everything running through the DMI was running through the DMI on Bloomfield so there should be no performance drop here. The only really concern is graphics performance.

While 16 lanes is perfectly adequate for any single graphics card (even the dual-GPU monsters like ATI’s HD 4890 X2 and nVidia’s GTX 295) it is going to limit the potential for running multiple cards in CrossFire or SLI. While these modes will still be supported by means of a 2×8 configuration, there may be situations where multiple high-end graphics cards are bandwidth limited. This is a question we shall return to shortly in another article.

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