Linux memory management on larger machines

A large amount of work has gone into the memory management subsystem during the 2.5 series of Linux kernels, and it is more stable under a wide variety of workloads than the 2.4 VM (virtual memory subsystem). Many scalability problems have been solved, making memory management perform much better on larger machines (meaning either with more than 1GB of RAM, or more than one processor, or both). Some of these changes also benefit smaller machines.

During the 2.4 series of kernels, the main Linux distributions diverged massively from the mainline kernel, particularly in the area of VM. This causes ongoing maintainance problems, and wasted duplicated effort in problem solving and feature implementation. Many of the enhancements made by the distributions have been brought back into the mainline kernel during the 2.5 series, under the leadership of Andrew Morton, providing a solid base for future development, and a greater potential for co-operative work.

This paper discusses the changes made to the Linux VM system during 2.5 that will significantly impact larger machines. It also covers changes that are proposed for the future, most of which are currently available as separate patches. Larger machines also have to cope with a larger number of simultaneous tasks - I have focused on up to 5000.

For the sake of simplicity, clarity, and brevity, we assume an IA32 machine with PAE mode (3 level pagetables) and normal memory layout settings throughout the paper. Unless otherwise specified, measurements were taken on a 16-CPU NUMA-Q system (PIII/700MHz/2MB L2 cache) with 16GB of RAM.

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