Intel QPI & HyperTransport
QPI and HyperTransport are point-to-point connectors used to connect two devices. Many sites still refer to them as buses though due to the fact that they are a set of wires on the motherboard transferring data. Both systems are very fast but have different clock cycles.
QPI transmits data in flits of 80 bits, which takes two clock cycles. QPI runs off a multiplier of the base clock (133MHz for Intel), currently either x18 or x24, giving us speeds of 2.4 or 3.2GHz. All this boils down to theoretical maximum transfer rates of 19.2GB/s for the 2.4GHz version, and 25.6GB/s for the 3.2GHz.[1]
HyperTransport has separate data paths for input and output, enabling the processor to read and write at the same time. It also employs double data rate technology to squeeze two transfers per clock, and has a variable bit-width between two and 32 bits. Thus 3.2GHz double data rate by 32 bits gives us 3.2 x 2 x 32 bits per second in each direction, which when divided by 8 to convert into bytes, reaches the headline 51.2GB/s maximum figure, 25.6GB/s in each direction. [2]
Techradar.com advises against overclocking either system as a method for gaining additional speed. “QPI is currently faster, but HyperTransport is ultimately more flexible. Both are extremely capable, and can quite happily handle a single-processor desktop PC with room to spare.”[3]
QuickPath Interconnect is mainly designed to connect multiple processors to each other and to the input/output controller. HyperTransport does that but can also be used for add-on cards and as a data transfer mechanism in routers and switches. HyperTransport is also an open technology which may give it a significant advantage over QuickPath Interconnect which is an Intel technology.[4]
Determining which of these systems would be best is difficult as they are each others competition and have many similarities. QPI is faster, HyperTransport is a more flexible technology. Several of these articles are a few years old so they may not be offering the most up to date analysis either. So I think it would really come down to what you are trying to accomplish. From what I’ve read I’m going to choose HyperTransport as it seems to have more opportunities that would allow someone to have some additional options for data transfer besides just increasing speed.
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[1] http://www.techradar.com/us/news/computing-components/quickpath-interconnect-vs-hypertransport-909072
[2] http://www.techradar.com/us/news/computing-components/quickpath-interconnect-vs-hypertransport-909072
[3] http://www.techradar.com/us/news/computing-components/quickpath-interconnect-vs-hypertransport-909072
[4] http://www.windowsnetworking.com/articles-tutorials/common/QuickPath-Interconnect.html 