Scaling Linux to New Heights: the SGI Altix 3000 System
Other Linux developers often ask, “What kind of changes did you have to make to get Linux to run on that size system?” or “Isn't Linux CPU scaling limited to eight or so processors?” Answering these questions involves examining further what SGI is using as its software base, the excellent changes made by the community and the other HPC-related enhancements and tools provided by SGI to help make Linux scale far beyond the perceived limit of eight processors.
On the SGI Altix 3000 system, the system software consists of a standard Linux distribution for Itanium processors and SGI ProPack, an overlay product that provides additional features for Linux. SGI ProPack includes a newer 2.4-based Linux kernel, HPC libraries highly tuned to exploit SGI's hardware, NUMA tools and drivers.
The 2.4-based Linux kernel used on the SGI Altix 3000 system consists of the standard 2.4.19 kernel for Itanium processors (kernel.org), plus other improvements. These improvements fall into one of three categories: general bug fixes and platform support, improvements from other work occurring within the Linux community and SGI changes.
The first category of kernel changes is simply ongoing fixes to bugs found during testing and the continued improvements for the underlying platform and NUMA support. For these changes, SGI works with the kernel team's designated maintainer to get these changes incorporated back into the mainline kernel.
The second category of kernel improvements consists of the excellent work and performance patches developed by others within the community that have not been accepted officially yet or were deferred until the 2.5 development stream. These improvements can be found on the following VA Software SourceForge sites: “Linux on Large Systems Foundry” (large.foundries.sourceforge.net) and the “Linux Scalability Effort Project” (sourceforge.net/projects/lse). We used the following patches from these projects: CPU scheduler, Big Kernel Lock usage reduction improvements, dcache_lock-usage reduction improvements based on the Read-Copy-Update spinlock paradigm and xtime_lock (gettimeofday) usage reduction improvements based on the FRlock locking paradigm.
We also configured and used the Linux device filesystem (devfs, www.atnf.csiro.au/people/rgooch/linux/docs/devfs.html) on our systems to handle large numbers of disks and I/O busses. Devfs ensures that device path names persist across reboots after other disks or controllers are added or removed. The last thing a system administrator of a very large system wants is to have a controller go bad and have some 50 or more disks suddenly renumbered and renamed. We have found devfs to be reliable and stable in high-stress system environments with configurations consisting of up to 64 processors with dozens of fibre channel loops with hundreds of disks attached. Devfs is an optional part of the 2.4 Linux kernel, so a separate kernel patch was not needed.
The third category of kernel change consists of improvements by SGI that are still in the process of getting submitted into mainline Linux, were accepted after 2.4 or will probably remain separate due to the specialized use or nature of the patch. These open-source improvements can be found at the “Open Source at SGI” web site (oss.sgi.com). The improvements we made included: XFS filesystem software, Process AGGregates (PAGG), CpuMemSets (CMS), kernel debugger (kdb) and a Linux kernel crash dump (lkcd).
In addition, SGI included its SCSI subsystem and drivers ported from IRIX. Early tests of the Linux 2.4 SCSI I/O subsystem showed that our customers' demanding storage needs could not be met without a major overhaul in this area. While mainstream kernel developers are working on this for a future release, SGI needed an immediate fix for its 2.4-based kernel, so the SGI XSCSI infrastructure and drivers from IRIX were used as an interim solution.
Figures 7-9 illustrate some of the early performance improvements that were achieved with Linux on the SGI Altix 3000 system using the previously described changes. Figure 7 compares XFS to other Linux filesystems. (Note, for a more detailed study on Linux filesystem performance, see “Filesystem Performance and Scalability in Linux 2.4.17”, 2002 USENIX Annual Technical Conference, which is also available at oss.sgi.com). Figure 8 compares XSCSI to SCSI in Linux 2.4, and Figure 9 shows CPU scalability using AIM7.
While SGI is focused more toward high-performance and technical computing environments—where the majority of CPU cycles is typically spent in user-level code and applications instead of in the kernel—the AIM7 benchmark does show that Linux can still scale well with other types of workloads common in enterprise environments. For HPC application performance and scaling examples for Linux, see the Sidebar “Already Solving Real-World Problems”.
Figure 10 shows the scaling results achieved on an early SGI 64-processor prototype system with Itanium 2 processors running the STREAM Triad benchmark, which tests memory bandwidth. With this benchmark, SGI demonstrated near-linear scalability from two to 64 processors and achieved over 120GB per second. This result marks a significant milestone for the industry by setting a new world record among a microprocessor-based system, which was achieved running Linux within a single-system image! This impressive result also demonstrates that Linux can indeed scale well beyond the perceived limitation of eight processors. For more information on STREAM Triad, see www.cs.virginia.edu/stream.
When you look at the list of kernel additions included in SGI ProPack the list is actually surprisingly small, which speaks highly of Linux's robust original design. What is even more impressive is that many of these and other changes are already in the 2.5 development kernel. At this pace, Linux is quickly evolving as a serious HPC operating system.
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