An Evaluation of the Linux Virtual Memory Manager to Determine Suitability for Runtime Variation of Memory
Muthukumaraswamy Sivakumar, Vijay
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Systems that support virtual memory virtualize the available physical memory such that the applications running on them operate under the assumption that these systems have a larger amount of memory available than is actually present. The memory managers of these systems manage the virtual and the physical address spaces and are responsible for converting the virtual addresses used by the applications to the physical addresses used by the hardware. The memory managers assume that the amount of physical memory is constant and does not change during their period of operation. Some operating scenarios however, such as the power conservation mechanisms and virtual machine monitors, require the ability to vary the physical memory available at runtime, thereby making invalid the assumptions made by these memory managers. In this work we evaluate the suitability of the Linux Memory Manager, which assumes that the available physical memory is constant, for the purposes of varying the memory at run time. We have implemented an infrastructure over the Linux 2.6.11 kernel that enables the user to vary the physical memory available to the system. The available physical memory is logically divided into banks and each bank can be turned on or off independent of the others, using the new system calls we have added to the kernel. Apart from adding support for the new system calls, other changes had to be made to the Linux memory manager to support the runtime variation of memory. To evaluate the suitability for varying memory we have performed experiments with varying memory sizes on both the modified and the unmodified kernels. We have observed that the design of the existing memory manager is not well suited to support the runtime variation of memory; we provide suggestions to make it better suited for such purposes. Even though applications running on systems that support virtual memory do not use the physical memory directly and are not aware of the physical addresses they use, the amount of physical memory available for use affects the performance of the applications. The results of our experiments have helped us study the influence the amount of physical memory available for use has on the performance of various types of applications. These results can be used in scenarios requiring the ability to vary the memory at runtime to do so with least degradation in the application performance.
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