Sunday, March 15, 2009

Intel Core i7, Part 2 : QPI

Northbridge, Southbridge, Socket, FSB and ... QPI?
In the early 90's, AMD was making top notch CPU and, in some cases, they were up to two times faster than the Pentium 2 and Pentium 3 processors. While years went up, AMD went down when intel announced the Pentium 4 with breakthrough technologies like hyper-threading and SSE2 support. We are now in year 2003. Intel business is going very well and they are dominating the market. AMD was falling down so they needed a solution or they where to die. That's when they introduced two new technologies to the consumers, 64 bits architectures and HyperTransport. The first one should have been enough to put AMD back into the course but, even today, only 1/8 of our computers use a 64bit operating system so that wasn't the solution. HyperTransport, on the other side, did a very good job at reviving them. This technology aimed at replacing the FSB that was in our computers since the Intel 8088, released in 1979. The motive is simple : "A 25 years old technology does not have place in a 2003 computer."

HyperTransport is a ultra-fast link between the CPU and the computer's memory. AMD's solution is to move the memory controller form the northbridge to the processor. While they didn't completely removed the FSB, this initiative was frightening 6 years ago. When the Athlon 64 3200+ got into the market, they managed to produce a CPU that was on par with the Intel 3.2gHz Pentium 4 but was half the price and used a 2gHz clock instead of a 3.2gHz one. Those little details made computers more power efficient and reduced the amount of emitted heat.

A urge breakthrough indeed and AMD went even further in 2005 when they released the first dual core desktop CPU on the market. When Intel users needed to pay for a dual socket machine which cost a lot, AMD users could get better performances for half and even quarter of the price.

Now Intel was in trouble. They were in the same position that AMD was two years before. They rushed to provide 64 bit architecture, dual-core and even quad-core processor but something... yes... something was missing. Even if Intel quad-core processors were better than any AMD's in 2006, they wanted to show their superiority. They got trough every technology wall by creating 45 nm processors but still... something was missing.

In the last 4 months, Intel started to produce the Core i7. The fastest processors on the planet and what make it so fast is the QPI; Intel 5 years late response to AMD's HyperTransport. Instead of simply taking the memory controller into the CPU, Intel completely removed the now 30 years old FSB and replaced it by a brand new technology : the Quick Path Interconnect.

Not just a FSB replacement
What make the QPI so spacial is that's it's not just a very effective FSB replacement. It is a very fast (Quick) way (Path) to connect internal computer component (Interconnect). Who talked about CPU, nothbridge or memory here? It can connect everything that need a lot of bandwidth. For now, the base line Core i7 only have one QPI that is used to make a direct link between the CPU and the X58 chipset but hi-end Xeon processors have two of them.

Those processors are on dual socket motherboard and the northbridge always was a massive bottleneck on this kind of architecture. They needed snoop-filter caches and a very complicated routing system to ensure that multi-threaded application worked correctly. Worse, those problem even were in quad core systems since they were using the same routing as dual socket system.

With the QPI, there is no need to go though the FSB to the chipset and back on the FSB to the second processor just to know if he is working on the data the first one needed. They now have a direct inter-processor link to do those operations minimizing cost by simplified routing and making dual socket system as fast as multi core system.

Specs time!
AMD's current implementation of HyperTransport use revision 3.0, up to 2.6gHz, 16bit link in every processors thus providing 20.8 GB/s of bandwidth. Intel, on the other side is using it's first revision of the QPI which is 16bit links at 3.2gHz. This little difference is enough to make a big jump in performances and provide 25.6 GB/s (23% more bandwidth). They basically double the bandwidth of a traditional 1600mHz FSB like the one used in the previous Intel x48 chipset.

Part 2, Conclusion
With the arrival of this new point-to-point link technology, Intel is pushing our computers to extreme performances for a very good price and enable a new era of HPC, server, workstation and desktop class computers. In the end, by using the new QuickPath technology, we are not only using faster bus but also using more of them and using them more wisely. It definitely mark the end of a 30 years old era.

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