Operating System

Because the Xeon Phi has full-fledged cores, not just highly optimized special-purpose computing units, it can run its own operating system. Intel leverages this ability to manage the board's resources and simplify software development.

When the host computer boots, the Xeon Phi first appears as a normal PCI device; the processor on the board is inactive. To activate the card, the host system's system management controller helps load an initrd image with a built-in BusyBox into the Xeon Phi's memory.

The Linux kernel used on the Xeon Phi differs only slightly from an ordinary x86 kernel; the necessary adjustments are comparable to those for an ARM image. After the image is transferred, the processor is started, and Linux is booted on the card for the first time. The coprocessor either uses initrd directly as the root filesystem, or it loads a filesystem from the host computer to the memory card, or it uses NFS to retrieve a filesystem.

Data Exchange

The PCIe bus allows the host system to write data to a memory expansion card. Conversely, expansion cards can also write to the memory of the host computer. However, writing directly to the memory of the host system is extremely awkward for an application programmer because this kind of low-level data transfer usually only takes place at the driver level. Intel therefore provides the Symmetric Communications Interface (SCIF), a library that includes an easy-to-use interface for low-level data transfer at the memory level. SCIF is the most efficient way of exchanging data between the host computer and the Xeon Phi card, and it also provides a means for transferring the root file system to the memory of the card.

Networking via Virtio

Intel has implemented additional data exchange options. The most important of these options integrates the card into a network. Intel uses the Virtio framework [5], among other things, for network access. Virtio provides virtual Ethernet interfaces on both the host system and the card's operating system, with the data traveling across the PCIe bus. The Ethernet interfaces operates in typical Linux style. In other words, the virtual Ethernet interface on the host operating system can connect to a physical port on the host computer and the Linux running on the card can join the local network via the virtual network interface (Figure 2).

Figure 2: Virtual Ethernet interfaces mean that the Xeon Phi can integrate into the local network.

Following the same principle, Intel has also implemented a virtual serial port and a virtual block device. The virtual serial port is designed to transfer the boot log, debug messages, and other status information to the host computer. The block device is actually intended to provide the Linux swap space on the card, but if you modify the init scripts supplied by Intel appropriately, it also provides a root filesystem and thus basically a fourth option for booting the card.