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In [[computer science]], '''coalescing''' is a part of [[memory management]] in which two adjacent free blocks of [[computer memory]] are merged.
In [[computer science]], '''coalescing''' is the act of merging two adjacent free blocks of memory. When an application frees memory, gaps can fall in the [[memory segment]] that the application uses. Among other techniques, coalescing is used to reduce [[Fragmentation (computer)|external fragmentation]], but is not totally effective. Coalescing can be done as soon as blocks are freed, or it can be deferred until some time later (known as deferred coalescing), or it might not be done at all.▼
When a [[Computer program|program]] no longer requires certain blocks of memory, these blocks of memory can be [[Free a memory ___location|freed]]. Without coalescing, these blocks of memory stay separate from each other in their original requested size, even if they are next to each other. If a subsequent request for memory specifies a size of memory that cannot be met with an integer number of these (potentially unequally-sized) freed blocks, these neighboring blocks of freed memory cannot be allocated for this request. Coalescing alleviates this issue by setting the neighboring blocks of freed memory to be contiguous without boundaries, such that part or all of it can be allocated for the request.
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Coalescence and related techniques like heap compaction, can be used in [[Garbage collection (computer science)|garbage collection]].
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