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Comment by quotemstr | Hacker News Reader

> committed memory is that in use + memory that's being paged out

That's not actually true though. You can see an increase in a process's commit charge without anything new being written to the pagefile. Commit is a check that the kernel writes to applications; you're confusing that check for cash in a wallet. You can also have commit without any virtual address space to blame for it through section handles or other tricks. It's complicated.

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I'm not going to dispute that but just want to highlight it doesn't change the fact that committed RAM can show as extremely high just by working with memory mapped files.

Memory mapping a GB log file for example will absolutely show GB's of committed memory but in reality you'll only have the last page in actual physical RAM.

Right. When you memory-map a file, what you've essentially done is add a temporary new pagefile to the system (your mapped file), and when you work with memory backed by that file, it's no different from working with "anonymous" memory backed by the system-wide pagefile.

Aside: I have to say this is a genius perspective on mmap, and I now finally understand why the same syscall is used for both. I never understood the link.

Thank you.

I understand commit can happen without physical page file being written to. But you say commit can happen even without any increase in virtual address space usage? That seems strange, could you explain how it can happen?

Change a PAGE_READONLY mapping to a PAGE_WRITECOPY one. The commit charge is billed at VirtualProtect time, and the protection can fail if you run out of commit. The kernel doesn't have to commit anything for a PAGE_READONLY mapping because all pages in such a mapping are guaranteed to be clean and trivially evictable. Not so once you introduce the possibility of COW faults. Reserving a big range of address space and committing it a little bit at a time is a very common pattern.

Another thing: create a 1GB section. Map it, and fill it up. Unmap it. Map it again. What you wrote is still there. Between the map and remap, you have commit without corresponding address space.

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