Research and Advances

A comparison of next-fit, first-fit, and best-fit

“Next-fit” allocation differs from first-fit in that a first-fit allocator commences its search for free space at a fixed end of memory, whereas a next-fit allocator commences its search wherever it previously stopped searching. This strategy is called “modified first-fit” by Shore [2] and is significantly faster than the first-fit allocator. To evaluate the relative efficiency of next-fit (as well as to confirm Shore's results) a simulation was written in Basic Plus on the PDP-11, using doubly linked lists to emulate the memory structure of the simulated computer. The simulation was designed to perform essentially in the manner described in [2]. The results of the simulation of the three methods show that the efficiency of next-fit is decidedly inferior to first-fit and best-fit when the mean size of the block requested is less than about 1/16 the total memory available. Beyond this point all three allocation schemes have similar efficiencies.

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Research and Advances

The reallocation of hash-coded tables

When the space allocation for a hash-coded table is altered, the table entries must be rescattered over the new space. A technique for accomplishing this rescattering is presented. The technique is independent of both the length of the table and the hashing function used, and can be utilized in conjunction with a linear reallocation of the table being rescattered. Moreover, it can be used to eliminate previously flagged deletions from any hash-coded table, or to change from one hashing method to another. The efficiency of the technique is discussed and theoretical statistics are given.

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