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Comment by AceJohnny2 | Hacker News Reader

> > > memory mapping means that the model will stay behind and eat your memory even after the process is closed

> > I don't think I'm unterstanding this right: You're saying that memory will not be freed by the OS after the process terminates?

> You're understanding it perfectly. The whole raison d'etre for mmap() is the ability to leave stuff in RAM (or swap, albeit if that happens it's completely detrimental to this use case) unlinked from the process itself. Basically it's just storing a file/memory block in RAM which can be accessed from multiple processes.

Wow, does this developer not understand the nuances of mmap().

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The fundamental operation of mmap is to add new entries to the page table of a process, and the precise properties of those entries are heavily dependent on what the arguments to mmap are.

When you mmap a regular file, you're essentially adding an entry to the page table that shares the data with the kernel's filesystem cache. I think he was trying to explain the implications of this fact, but doing so in an incredibly garbled manner, and getting his conclusions wrong.

There are performance implications to using mmap (not always good, not always bad), but both sides of the discussion here immediately dug their heels in on their conclusion without anyone trying to do any analysis to see what the actual implications were, and why.

> I think he was trying to explain the implications of this fact, but doing so in an incredibly garbled manner, and getting his conclusions wrong.

Yeah. I like using the expression "knows just enough to be dangerous" (usually applied in humility to myself), and this situation is such a perfect example. Someone who seems to know just enough about the advanced workings under mmap() to completely misunderstand the implications.

If you squint maybe you can argue that mmap will leave things in the page cache. But, you know, it doesn’t matter and not even munmap will save you there so I have no idea what they’re getting at.

Rather than just pointing and laughing, your post would have more substance if you could correct the flawed assumptions.

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