5) Quick fit
Yet another allocation algorithm is quick fit, which maintains separate lists for some of the more common sizes frequently requested. So, it maintains a separate linked list for this frequently request process size.
For example, it might have a table within entries, in which the first entry is a pointer to the head of a list of 4-MB holes, the second entry is a pointer to a list of 8-MB holes, the third entry a pointer to 12-MBholes, and so on. Holes of, say, 21-MB, could be put either on the 20-MB list or on a special list of odd-sized holes.
But maintain the separated linked list for frequently use process size is not a good approach because de-allocation might have a big problem. It is quick but not at all efficient.
So, among all first fit is the best one, and most widely used is first fit.
50 percent rule:
Depending on the total amount of memory storage and the average process size, external fragmentation may be a minor or a major problem. Statistical analysis of the first fit, for instance, reveals that, even with some optimization, the given N allocated blocks, another 0.5 N blocks will be lost to fragmentation. That is, one-third of memory may be unusable. This property is known as the 50-percent rule.