How Anomalous Is Beladv's Anomaly?

Abstract

In a virtual memory system using demand paging, the page fault rate of a process varies with the number of memory frames allocated to the process. When an increase in the number of frames allocated leads to an increase in the number of page faults, Belady's anomaly is said to occur. In this study we used computer simulation to examine four conditions that affect the incidence of Belady's anomaly: (1) page replacement algorithm (FIFO vs. Random Page), (2) process size, (3) reference string length, and (4) memory frames allocated to the process. We found that over a wide range of process sizes and reference string lengths, Belady's anomaly occurred for up to 58.6% of the (random) reference strings under FIFO, and up to 100% of the reference strings for Random Page. Under conditions where anomalies occur most often, the average frame allocation level was around 75% of the process size for FIFO, but just over 50% of the process size for Random Page. Throughout the study, Belady's anomaly occurred so frequently that it no longer seems anomalous. This is especially true for the Random Page algorithm.