What is the difference between an on-line and off-line algorithm?

Question

These terms were used in my data structures textbook, but the explanation was very terse and unclear. I think it has something to do with how much knowledge the algorithm has a

Answer 1

Cache Miss problem: In a computer system, cache is a memory unit used to avoid the speed mismatch between the faster processor and the slowest primary memory. The objective of using cache is to minimize the average access time by keeping some frequently accessed pages in the cache. The assumption is that these pages may be requested by the processor in near future. Generally, when a page request is made by the processor then the page is fetched from the primary or secondary memory and a copy of the page is stored in the cache memory. Suppose, the cache is full, then the algorithm implemented in the cache has to take immediate decision of replacing a cache block without knowledge of future page requests. The question arises: which cache block has to be replaced? (In worst case, it may happen that you replace a cache block and very next moment, the processor request for the replaced cache block). So, the algorithm must be designed in such a way that it take immediate decision upon the arrival of an incoming request with out advance knowledge of entire request sequence. This type of algorithms are known as ONLINE ALGORITHM