Memory reservation and shared page management for real-time systems

Abstract

Memory reservations are used to provide real-time tasks with guaranteed memory access to a specified amount of physical memory. However, previous work on memory reservation primarily focused on private pages, and did not pay attention to shared pages, which are widely used in current operating systems. With previous schemes, a real-time task may experience unexpected timing delays from other tasks through shared pages that are shared by another process, even though the task has enough free pages in its own reservation. In this paper, we first describe the problems that arise when real-time tasks share pages. We then propose a shared-page management framework which enhances the temporal isolation provided by memory reservations in resource kernels that use the resource reservation approach. Our proposed solution consists of two schemes, Shared-Page Conservation (SPC) and Shared-Page Eviction Lock (SPEL), each of which prevents timing penalties caused by the seemingly arbitrary eviction of shared pages. The framework can manage shared data for inter-process communication and shared libraries, as well as pages shared by the kernel's copy-on-write technique and file caches. We have implemented and evaluated our schemes on the Linux/RK platform, but it can also be applied to other operating systems with paged virtual memory.