316 IBM eX5 Implementation Guide7.5 PCIe adapters and riser card optionsThis section describes considerations for determining how to use your PCIe slots, dependingon the types of PCIe riser cards that you have installed. The x3690 X5 is designed to functionin a wide variety of tasks, starting from a high-performance graphics workstation up to astorage or processing node in a high-performance cluster.7.5.1 Generation 2 and Generation 1 PCIe adaptersAll of the PCIe slots in the x3690 X5 are at the Generation 2 (Gen2) specification. Gen2 offersadditional error correction and addressing advancements, so that all of the slots of this serverexchange data twice as fast as servers with Gen 1 PCIe slots.Table 7-3 describes the theoretical limits of each of the common types of PCIe adapters.Remember that theoretical limits are based on the mathematics of the frequency and datawidth of the bits that are transmitted over the interface. Theoretical limits do not take intoaccount the communications needed to maintain the protocols required to interface betweenintelligent devices. Theoretical limits also do not take into account the inability to maintain asteady flow of data in full duplex.Table 7-3 Theoretical data transfer limits of Gen1 PCIe slot types versus Gen2 PCIe slot typesPCIe adapters connect to the processors via the I/O hub. The purpose of the I/O hub is tocombine the various data streams from each of the PCIe slots into a single aggregate link tothe processors by using a dedicated QPI link to each processor. The data transfer rate of theQPI link is negotiated between the processor and the I/O hub.Table 4-5 on page 130 shows the QPI link speeds based on the types of installed processors.The I/O hub supports the highest QPI link speed on the table, which is 6.4 gigatransfers persecond (GT/s). You can adjust the QPI link speed to conserve power by booting intoF1-Setup, then selecting System Settings Operating Modes, and setting QPI LinkFrequency to values other than the default Max Performance, as shown in Figure 7-14 onpage 317.PCIe slot type Generation 1 limit Generation 2 limitx1 500 MBps 1 GBpsx4 2 GBps 4 GBpsx8 4 GBs 8 GBpsx16 8 GBps 16 GBpsx32 16 GBps 32 GBps
