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0 pointsdietrichepp2y ago0 comments

The way I describe mmap these days is to say it allocates address space. This can sometimes be a clearer way of describing it, since the physical memory will only get allocated once you use the memory (maybe never).

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byteknight2y ago

But is it not still limited by allocating the RAM + Page/Swap size?

aseipp2y ago

Maybe I'm misunderstanding you but: no, you can allocate terabytes of address space on modern 64-bit Linux on a machine with only 8GB of RAM with overcommit. Try it; you can allocate 2^46 bytes of space (~= 100TB) today, with no problem. There is no limit to the allocation space in an overcommit system; there is only a limit to the actual working set, which is very different.

j16sdiz2y ago

You can do it without overcommit -- you can just back the mmap with file

wbkang2y ago

I don't think so, but it's difficult to find an actual reference. For sure it does overcommit like crazy. Here's an output from my mac:

% ps aux | sort -k5 -rh | head -1

xxxxxxxx 88273 1.2 0.9 1597482768 316064 ?? S 4:07PM 35:09.71 /Applications/Slack.app/Contents/Frameworks/Slack Helper (Renderer).app/...

Since ps displays vsz column in KiB, 1597482768 corresponds to 1TB+.

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