MEMDECOMMIT

MEMDECOMMIT is a term used in Windows memory management to describe the deallocation of physical memory for a range of pages that were previously committed, while keeping the virtual address range reserved. It is performed using the Windows API, typically via VirtualFree with the MEM_DECOMMIT flag.

When a region of memory is allocated with VirtualAlloc and committed (MEM_COMMIT), the system allocates physical storage for those pages. Decommitting reduces the region’s committed size by returning the associated physical memory and pagefile backing to the system. The address space remains reserved for the process, allowing the pages to be re-committed later if needed. Accessing decommitted pages is invalid until they are re-committed, and the exact contents of those pages are not guaranteed to be preserved during decommit.

Decommit should not be confused with releasing the entire region. MEM_DECOMMIT only changes the commit state; to release the address space itself, the MEM_RELEASE option of VirtualFree must be used, and the entire region must be unused.

MEMDECOMMIT is Windows-specific; other operating systems implement memory trimming or deallocation in different ways (for example, Linux uses mechanisms like madvise or munmap to achieve similar effects). In practice, decommitting memory can reduce physical RAM and pagefile usage, but it may incur overhead if memory is later re-committed, so it is typically used under memory pressure or in resource-constrained scenarios.

See also: VirtualAlloc, VirtualFree, MEM_COMMIT, MEM_RELEASE, virtual memory, pagefile.