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Operating systems

The service nodes (i.e. login and I/O) run SUSE Linux. The compute nodes run a lightweight Linux kernel called the Cray Linux Environment, or CLE. CLE was formally known as Compute Node Linux (CNL) and some documentation may still refer to this. CLE is essentially a stripped down version of Linux (c.f. Blue Gene's compute node kernel, CNK). It is designed to be extremely lightweight so as to limit the number of interruptions from the operating system. The rationale is to keep the compute nodes as uninterrupted by the operating system as possible by outsourcing the usual operating system tasks to dedicated additional hardware. The benefits of this are potentially excellent scalability up to large task counts which should results in minimal variation of application run time. The downside of this reduced kernel is that some services which a small number of applications expect or rely on, e.g. access to the X11 libraries, are not available on the compute nodes.