HPCwire: SGI Expanding the Reach of Linux

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October 06, 2006

SGI Expanding the Reach of Linux

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Steve Neuner, the director for Linux engineering at SGI, has been pushing Linux up the scalability ladder for the better part of the 21st century. In August of this year, SGI announced that they were able to run a single system image of the Linux OS over 1024 processors on an Itanium-based Altix 4700 supercomputer. How was this feat accomplished? This week at the Gelato Itanium Conference and Expo (ICE) in Singapore, Neuner presented a session that described the Linux kernel modification that helped to make this possible. HPCwire caught up with him before the conference to ask him about the Linux improvements and where the future of single system image scalability is headed.

HPCwire: Can you give us a brief time line of how Linux has scaled from 8 processors to 1024 processors over the last five years?

Neuner: In the summer of 2001, we built an early 32 processor prototype system in the lab. SGI used it extensively to begin identifying and fixing scaling issues. This development system was later increased to 64 processors, which became our initial configuration limit for a single system image of the Linux kernel when we launched SGI Altix in February of 2003. A year later, that limit was increased to 256 processors.

Later in February of 2005, we started shipping the 2.6 Linux kernel, which was a major step forward that enabled support for 512 processor systems. In August of this year, this limit was increased to our now current limit of 1024 processors.

HPCwire: Can you describe the types of changes that were made to the Linux 2.6 kernel to get a single image of the OS to run on a 1024-processor system?

Neuner: The changes usually fall into one of two categories. The first is getting the system to boot and recognize all the hardware. This typically involves increasing the size of data structures throughout the kernel that contain information related to the amount of nodes, processors, or memory on a NUMA system. SGI uses a hardware simulator to find and fix most of these problems before we have a system of that size in the lab. For example, when engineering received the first 1024 processor system for testing, it booted right up the very first time.

Once Linux can boot and run on a larger system, the next category of fixes is getting Linux to perform well. This work often involves running benchmark tests and various HPC applications, so hot-locks, cache lines, timing windows, and race conditions can be exposed and pin-pointed in order to improve Linux's efficiency on very large systems.

Surprisingly, most of the changes going from 512 processors to 1024 processors fell into the first category of enabling the kernel to recognize and boot on a 1024 processor system. It turned out that the performance scaling work done earlier with our 512p system paid off since issues were already found and fixed. So going from 512p to 1024p became more of a testing and validation exercise. As a result, we were able to officially support 1024 processors for our customers a year ahead of plan.

HPCwire: Can you talk about some of the other 2.6 Linux kernel enhancements that have been added for HPC functionality?

Neuner: As processor counts increase, so does memory. Significant improvements in 2.6 were made in memory handling and supporting larger memory sizes. Some examples in this area include support for over 10 TB of memory, improved node locality and NUMA awareness in various kernel memory allocations mechanisms, 4-level page table, page migration, out-of-memory error handling improvements, and fault containment of double-bit uncorrectable memory errors.

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