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I don't understand the difference between MEM_RELEASE and MEM_DECOMMIT in the Windows VirtualFree function.

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VirtualFree function (memoryapi.h) - Win32 apps

Releases, decommits, or releases and decommits a region of pages within the virtual address space of the calling process.

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MEM_RELEASE will free the mapping completely like munmap on Linux, making the address space usable for other things. MEM_DECOMMIT will release the memory but hold onto the reserve address space like clobbering a mapping with mmap + MAP_FIXED + PROT_NONE on Linux.

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and

MEM_DECOMMIT is the same as making a fresh mapping with VirtualAlloc using MEM_RESERVE. You go from a reserved mapping to usable memory with MEM_COMMIT like using mprotect for that purpose on Linux. Windows separates the concept of whether it's committed from protection mode.

11:31 PM · Aug 9, 2021Twitter Web App

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Okay. Great, thank you, that's very helpful.

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So, for example, you may have a case where you need up to 256GB of contiguous memory. It's perfectly fine to reserve that much memory on 64-bit. The only way you could commit that much memory is if you actually had that much physical memory + swap. Windows enforces having enough.

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