Managing PCI-express slots in Mac Pros

Mac Pro computers come with three unused PCI-express expansion slots for adding extra graphics cards, RAID controllers, multimedia, and I/O controllers, among other options, to expand your Mac's capabilities. In most Mac Pros the PCI-express slots are set up such that the first two are capable of x16 speeds, and the last two are only capable of x8 speeds at a maximum. Despite these capabilities, on early systems all slots cannot simultaneously run at their maximum speeds.

PCI-express works by dedicating a set number of "lanes" in each controller to the various devices, running the lanes simultaneously in parallel. In addition, each lane is a serial interface, allowing for bidirectional data transfer. These features allows for greater throughput and customization than previous PCI technologies, but does come with the drawback that some cards may not get the number of slots they demand.

For instance, if you have two graphics cards in the first and second slots, you can either run both slots at x8 speeds or run one at x1 speeds and one at x16 speeds. This means that if you have two x16 cards in your system, you will need to compromise the performance of one card in order to run the other at its optimal speed (for systems before the 2009 models). It also means that you may need to reorganize your cards to get an optimal number of lanes assigned to the card.

In early Mac Pros, the system will automatically evenly distribute the number of available lanes to expansion cards. In the case of the first two slots (capable of x16 speeds), if you install two graphics cards then both will run at x8 speeds; however, you can use Apple's Expansion Slot Utility to assign the number of lanes used by each PCI-express slot. You can therefore force the system to dedicate more lanes to a specific card. This utility will not work in later Mac Pro models, but should be useful for those who adopted early versions of the computer.

The utility will present a number of available PCI-express configurations that you can choose from, which will be changed upon restarting the computer. If your installed expansion cards are requesting more PCI-express lanes than are made available to them, the utility will open and request you set a more compatible configuration.

In this utility, you can set the option to never show the utility again, but if you need to change your PCI-express configuration you can launch the Utility anytime from the /System/Library/CoreServices/ folder.

For more information on this utility, read this Apple knowledgebase article: http://support.apple.com/kb/HT2838

For more information on PCI and PCI-express in Apple products, including power constraints and speed capabilities, see this developer document: PCI Product-Specific Details

UPDATE: Corrected some ambiguity about which models can use this utility

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