Conversation

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

learn.microsoft.com

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.

Replying to

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.

Replying to

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.

Replying to

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

Daniel Micay

@DanielMicay

Replying to

@mcclure111

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.

11:36 PM · Aug 9, 2021Twitter Web App

Replying to

and

It works the same way if you set /proc/sys/vm/overcommit_memory to 2 on Linux, i.e. memory accounting mode. There are a lot of common reasons to need to reserve contiguous memory for later use like GC collected heaps or just for huge dynamic arrays avoiding copying on growth.