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Comment by dupontcyborg | Hacker News Reader

i may be missing context, but shared memory across processes, without ipc?

There's nothing special about threads vs processes in Linux. mmap works the same, the challenge is to map the same file. You can share a path, pass a file descriptor via fork or unix domain socket, among other techniques.

That induces disk I/O overhead (even if it somehow doesn't impact IPC performance)

The file doesn’t have to be disk-backed.

Don't you need something mounted for that?

No, you can use a memfd.

And `/dev/shm/` (which postgres uses by default on most Unix platforms)

It doesnt. Processes can share memory

Being really pedantic here, shared memory is considered IPC, but not the kind you're thinking of. Shared address space, no overhead.

As long as we're pedantic ... the subject is shared memory. Unless you specify the same, non-null, target address in the call to mmap (and the kernel happens to grant you that mapping on all calling sites), the addresses will be different; the address space is not shared (each mapping might also have different access permissions).

That distinction is important as pointers generally cannot be shared (a problem which can of course be solved with one more indirection ;-) .

"Shared address space, no overhead"

But concurrent access, so synchronization is required (lock or whatever), so overhead :)

Yeah that's true, and I'd say threads need synchronization for concurrent access too, but supposedly the options for doing that are faster than what you need to use across processes.

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