A bit-parallel search algorithm for allocating free space

Abstract

File systems that allocate data contiguously often use bitmaps to represent and manage free space. Increases in the size of storage to be managed creates a need for efficient algorithms for searching these bitmaps. We present an algorithm that exploits bit-parallelism, examining all bits within a processor word at the same time, to improve search performance. Measurements of our implementation show that these techniques lead to a 14 times increase in the rate at which bitmap pages can be searched on a 64-bit processor. Trace-driven experiments indicate that overall allocation performance increases by a factor of 3 to 6 on a 32-bit processor. As processors mature, registers become wider and the degree of bit-level parallelism increases, which makes the performance improvements of our search algorithm more substantial.