preallocates

Preallocation is the practice of reserving resources in advance rather than allocating them on demand. The goal is to reduce latency, prevent fragmentation, and guarantee availability under peak load. Preallocation can apply to memory, storage, file handles, network resources, and other scarce assets.

In computing, preallocation often means reserving memory capacity before filling a data structure, such as allocating a minimum capacity for a dynamic array. This can prevent repeated reallocations as the structure grows and improve performance. In storage systems and databases, preallocation involves reserving disk space or data blocks ahead of time, using mechanisms such as file preallocation calls to reduce fragmentation and improve write throughput.

Benefits of preallocation include lower allocation overhead, better cache locality, and more predictable performance. Drawbacks include potential waste if reserved resources remain unused, increased peak memory or disk usage, and the risk of overcommitment in constrained environments. The effectiveness of preallocation depends on workload characteristics, including throughput and latency requirements, as well as available memory and storage.

Common methods range from high-level APIs that expose preallocation features (for example, reserving capacity in dynamic arrays) to operating system or filesystem calls that allocate space for files (such as fallocate or posix_fallocate). Database and storage engines may preallocate extents or pages to optimize I/O patterns and reduce fragmentation.

See also: memory management, pooling, over-allocation, fragmentation, fallocate, posix_fallocate.