uC/OS-II之動態記憶體管理

Abstract

在嵌入式與即時系統中，爲了執行日漸複雜的應用程式，對於更多記憶體的需求越趨增加，而且通常藉由分頁或者虛擬記憶體來實現。然而，在這樣的系統中，分頁錯誤大幅地影響了記憶體存取的效能，而且造成了不可預期的系統反應時間，故降低分頁錯誤率對改進系統效能相當關鍵。在本論文中，我於 uC/OS-II 中提出了一套基於分頁機制建構的動態記憶體管理方案。根據程序的優先權來分配分頁給程序，使得高優先權程序的分頁更能被保留於記憶體中，而低優先權程序的分頁則容易被置換掉。如此，避免了分頁時常地重新分配，降低了分頁錯誤率。在實驗中，分頁錯誤率可以降低到 66%。此外，我亦測試了在不同的情況之下，效能如何受到影響。結果顯示出此優先權的考量改善了整體分頁錯誤率的效能，特別是在大記憶體存取範圍、具備許多程序，以及低老化機制門檻值的環境中。相信加入優先權考量的動態記憶體管理能有效地增加相當多嵌入式與即時系統的整體效能。

There is an increasing demand for more memory to satisfy complex execution of applications even in embedded or real-time systems. The implementation of more memory is usually done by using paging or virtual memory. However, page faults in such systems impact the performance of memory access significantly and result in unpredictable response time. So, reducing page fault rate is critical to improve the system performance. In this thesis, a dynamic memory management scheme based on paging on μC/OS-II is proposed. Memory pages are allocated to tasks relying on task priorities, so pages of high-priority tasks are more likely to be kept in memory, while those of low-priority tasks may be easier replaced. By this way, pages are prevented from being re-allocated frequently, resulting in a lower page fault rate. In my experiment, the page fault rate could reduce 66\%. More experiments under different scenarios are also designed to test how the performance would be influenced. The results show that such prioritized scheme improves the overall performance of page fault rate, especially with wide memory access range, more number of tasks, and low aging strategy. I believe dynamic memory management with task priority can effectively improve the overall performance of many embedded and real-time systems.