Intel Core i7 Mobile CPU (Clarksfield) Review

Intel Core i7 Mobile CPU (Clarksfield) ReviewIntroduction and Specifications

For quite some time now, Intel has been the undisputed leader when it comes to laptop CPU performance. This advantage comes from the fact that Intel has successfully scaled its desktop processor technology for use in the mobile platform. That trend continues with the latest mobile processor platform being unleashed today from Intel: the "Clarksfield"-based Core i7 Mobile processor family and the new PM55 Express Chipset.

This marks the first time that the "Nehalem" Core i7 microarchitecture has been ported over to the mobile side. The fact that the mobile version of Nehalem makes its debut only two weeks following the launch of Intel's Core i5 ("Lynnfield") mainstream desktop CPU and P55 Express chipset is no coincidence. As it turns out, the Core i7 Mobile processor die is identical to the desktop version, but uses mobile packaging--as far as the microarchitecture is concerned, Clarksfield and Lynnfield are essentially the same thing. The lower power demands of Lynnfield (as opposed to the original Nehalem architecture) are in large part what enable it to also be used in notebooks.

What this means is that Core i7 Mobile-based notebooks will see a number of the same benefits that Core i7 desktops already have, such as integrating the memory controller into the processor die, using a three-level cache hierarchy, utilizing Hyper-Threading technology, and taking advantage of Intel Turbo Boost Technology. What Lynnfield/Clarksfield adds to the equation is on-die PCI Express connectivity, getting rid of the Northbridge chip, and improving the Intel Turbo Boost Technology (from the original Nehalem architecture). Additionally, whereas only the previous Intel Core 2 mobile ("Penryn") microarchitecture had primarily dual-core offerings, all of the Core i7 Mobile solutions are quad cores. The three Penryn-based quad-core mobile processors (the 2.53GHz Core 2 Extreme QX9300, 2.26GHz Core 2 Quad Mobile Q9100, and 2.0GHz Core 2 Quad Mobile Q9000) are made from two dual-core chips merged together in a single CPU package, while Clarksfield uses a single-chip (monolithic) design.

Core i7 Mobile processors are available in three versions. The flagship processor is the Core i7-920XM, which has a base speed of 2.0GHz and goes up to 3.2GHz using Turbo Boost. The middle CPU is the Core i7-820QM, which has a base speed of 1.73GHz and can go up to 3.06GHz with Turbo Boost. Last, but not least, is the Core i7-820QM, with a base speed of 1.6GHz and maximum Turbo Boost speed of 2.8GHz. The Core i7-920XM and Core i7-820QM both have 8MB of shared L3 Smart Cache, while the Core i7-720QM has 6MB of L3 Smart Cache. The other notable difference between these three processors is that the Core i7-920XM has a maximum TDP of 55W, while the Core i7-820QM and Core 720QM both have a maximum 45W TDP. (For comparison, the three quad-core Penryn processors all have a maximum TDP of 45W).

...................................................................................................

Intel Core i7 Mobile CPU (Clarksfield) Review

Intel Core i7 Mobile CPU (Clarksfield) Review
Core i7 Mobile Processor Details

The Core i7 has a die size of approximately 296mm2 and contains roughly 774-million transistors. The die includes four physical execution cores, three levels of cache, an integrated memory controller, and PCI Express interconnects.

Lynnfield/Clarksfield Die Map

Since the Core i7 Mobile processor integrates the memory controller and PCI Express interconnects onto the processor, there is no need for a traditional Northbridge chip. This means that the PM55 chipset acts primarily as a Southbridge chip, handling most of the device I/O. With no Northbridge chip to communicate with, the Core i7 Mobile processor doesn't need the Intel QuickPath Interconnect (QPI) that the Core i7-900 series ("Bloomfield") processors use to communicate with the chipset. Instead, the Core i7 Mobile uses the slower Direct Media Interface (DMI) interconnect to communicate with the PM55 chipset. Another major difference between the Core i7 Mobile and the Core i7 900-series, is that unlike the Core i7 900-series support for triple-channel DDR3 memory, the Core i7 Mobile instead supports just dual-channel DDR3. The Core i7 Mobile processor supports configurations of 16 PCI Express 2.0 lanes per GPU or two sets of 8 PCI Express 2.0 lanes.

With the memory controller and PCI Express interconnects on the processor die,
the Core i7 Mobile doesn't need a Northbridge chip.

The Core i7-920XM and Core i7-820QM processors have the same three-level cache architecture as all other existing Core i7 and Core i5 processors. The cache configuration is made up of a 4 x 32K instruction L1 cache, 4 x 32K data L1 cache, 4 x 256K L2 cache, and an 8MB L3 cache that is shared between all four cores. The Core i7-720QM has the same L1 and L2 cache amounts, but instead has 6MB L3 shared cache.

All Core i7 Mobile CPUs have three levels of cache; the
Core i7-920XM and Core i7-820QM have 8MB of shared
L3 cache (the Core i7-720QM has 6B of shared L3 cache).

Also, as with all other Core i7 processors, the Core i7 Mobile includes Hyper-Threading support. Hyper-Threading allows each core to process two simultaneous threads. As a result, the Core i7 Mobile can run up to eight simultaneous threads.

With Hyper-Threading, the Core i7-920MX has four physical execution cores and
four virtual cores, for a total of eight cores available for multi-threaded workloads.

As you can see in the Windows 7 Resource Monitor screenshot above, the system sees a total of eight cores: four physical execution cores plus four virtual cores via Hyper-Threading.

..............................................................................................................

Intel Core i7 Mobile CPU (Clarksfield) Review

Intel Core i7 Mobile CPU (Clarksfield) Review
Intel Turbo Boost Technology

Intel Turbo Boost Technology has been part of the Core i7 (Nehalem) architecture from day one. However, Lynnfield and Clarksfield bring a significant update to the Turbo Boost technology, making it an even more flexible and powerful performance booster.

A graphical example of how Core i7 Mobile Turbo Boost works.

The Core i7 Mobile processor is actually capable of three different speeds above its base speed, depending on the nature of the active single or multi-threaded workloads (and as long as the processor is operating within specified thermal and power limits). When a single-threaded workload is detected, the Core i7-920XM's clock speed on just a single execution core increases up to 3.2GHz, while the remaining three cores remain inactive and operate and close to zero power. When a lightly-threaded workload is detected, only two cores are active and they operate at up to 3.06GHz. With a high-threaded workload, all four cores are active and operate at up to 2.26GHz.

A single-threaded workload (left) vs. a heavily-threaded workload (right).

The image on the left was recorded while Cinebench R10 was running its single-threaded workload. As you can see in Windows 7's Resource Monitor, CPU2 is getting pegged heavily. There is also some activity on CPU1, CPU5, and CPU7, which can be attributed to Windows 7 background tasks and other apps running in the background. The Intel Turbo Boost Technology Monitor gadget shows the processor running at 3.2GHz. While this screenshot shows that the Core i7-920XM is capable of scaling up to 3.2GHz, the chip actually ran at 3.07GHz during the vast majority of the test run.

The screenshot on the right was captured while Cinebench R10 was running its multi-threaded workload. Resource Monitor shows all four physical execution cores and all four Hyper-Threading virtual cores completely saturated, with the Intel Turbo Boost Technology Monitor showing the processor running at 2.0GHz. Curiously, the processor did not run at the 2.26GHz speed that Turbo Boost supports for highly-threaded workloads, and instead ran at the processor's 2.0GHz base speed.

....................................................................................................