所謂Belady現象是指：採用FIFO算法時，如果對—個進程未分配它所要求的全部頁面，有時就會出現分配的頁面數增多但缺頁率反而提高的異常現象。
基本介紹
- 中文名：貝萊迪現象
- 外文名：belady
- 採用：FIFO算法
- 異常現象：分配頁面數增多，缺頁率反而提高
- FIFO：先進先出頁面置換算法
belady,Belady現象,
belady
In computer storage, Bélády's anomaly proves that it is possible to have more page faults when increasing the number of page frames while using FIFO page replacement algorithm. László Bélády demonstrated this in 1969.
In common computer memory management, information is loaded in specific sized chunks. Each chunk is referred to as a page. The central processor can only load a limited number of pages at a time. It requires a frame for each page it can load. A page fault occurs when a page is not found, and might need to be loaded from disk into memory.
When a page fault occurs and all frames are in use, one must be cleared to make room for the new page. A simple algorithm is FIFO. Whichever page has been in the frames the longest is the one that is cleared. Until Bélády's anomaly was demonstrated, it was believed that an increase in the number of page frames would always provide the same number or fewer page faults.
Belady現象
所謂Belady現象是指：採用FIFO算法時，如果對一個進程未分配它所要求的全部頁面，有時就會出現分配的頁面數增多但缺頁率反而提高的異常現象。 Belady現象可形式化地描述為：一個進程戶要訪問M個頁，OS分配N個記憶體頁面給進程P；對一個訪問序列S，發生缺頁次數為PE(S，N)。當N增大 時，PE(S，N)時而增大時而減小。Belady現象的原因是FIFO算法的置換特徵與進程訪問記憶體的動態特徵是矛盾的，即被置換的頁面並不是進程不會訪問的。 先進先出算法(FIFO)。選擇裝入最早的頁面置換。可以通過鍊表來表示各頁的裝入時間先後。FIFO的性能較差，因為較早調入的頁往往是經常被訪問的頁，這些頁在FIFO算法下被反覆調入和調出，並且有Belady現象.
