在硬式即時系統下基於工作量延緩之任務間動態電壓調整演算法

Abstract

這幾年來，個人數位助理與手機等手持裝置越來越普遍。因為這些裝置是由電池供電，所以能源節省是一個重要的問題。動態電壓調整策略根據處理器的工作量動態地調整處理器的頻率及電壓，是一種低耗能的設計技術。一個動態電壓調整演算法的好壞取決於如何準確地估計寬裕時間 (slack time)。本論文提出一個基於工作量延緩之任務間動態電壓調整演算法，稱為dwDVS。dwDVS有兩個特色，第一是為每一任務保留一個時間區間，即使是在最壞的情況下，每一個任務都能夠在此時間區間內執行完它的工作量。用此種方法，可以有效率地估算從優先權較低的任務中所能夠利用的寬裕時間；第二是延緩這些被保留的時間區間，使這些時間區間儘可能靠近其對應任務之截止期限。用此種方法，處理器的頻率即使是在沒有寬裕時間可利用的情況下也可以調降。模擬結果顯示，相較於Static [1]、laEDF [1] 及 DRA [2]，dwDVS節省了 40-70%、10-20% 及 3-10% 的能源消耗，且與最佳理論值Bound至多只有12%的差距。

Hand-held devices such as personal digital assistants (PDAs) and cellular phones are getting more and more popular in recent years. Energy consumption is a critical issue because these devices are battery powered. Dynamic voltage scaling (DVS) is a low-power design technique that adjusts the CPU frequency and voltage levels dynamically based on CPU workloads. The performance of a DVS algorithm largely depends on how to estimate slack time accurately. In this thesis, we propose a deferred-workload-based inter-task DVS algorithm (dwDVS), which has two features. The first is that we reserve a time interval for each task to execute and its workload can be completed in this time interval even in the worst-case condition, which means that the actual workload (execution time) of each task is equal to its worst-case execution time. In this way, we can estimate the slack time from lower priority tasks more aggressively. The second is that we defer these reserved time intervals, which means that a reserved time interval will be shifted to the deadline of its corresponding task as close as possible. In this way, the operating frequency can be reduced even without slack time. Simulation results show that the proposed dwDVS reduces the energy consumption by 40-70%, 10-20%, and 3-10% compared with the static voltage scaling (Static) [1], laEDF [1], and DRA [2] algorithms, respectively, and approaches theoretical low bound (Bound) by an margin of at most 12%.