Linux-insides: Limits on resources in Linux | Hacker News Reader

Linux-insides: Limits on resources in Linux

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73 points0xAX7 comments

Seems odd that it doesn't mention cgroups or the CLONE_* flags used to create namespaces for containers.

Yes, cgroups can be used to impose limits on resources; and from the outside looking in, can look very similar to setrlimit(). But from the inside of the kernel looking out (generally, the perspective taken by the linux-insides book), they are very different subsystems; I expect that cgroups will come in quite a bit later. If the book were discussing how to limit resources, sure it should mention cgroups; but it is discussing how things work in the kernel, not how to get the kernel to do certain things.

> $ strace ulimit -s 2>&1 | grep rl

Does it actually work? On my system ulimit is a shell builtin, not a separate command:

  $ strace ulimit -s
  strace: Can't stat 'ulimit': No such file or directory

Bash has a few builtin that are clones of commands (mostly from coreutils). To execute the command instead of the builtin, just give the full path to the command (not sure and cannot verify, right now but this might mean either /bin/ulimit or /use/bin/ulimit ). There are other notable ones like echo, and time where the builtins have slightly different behaviour than the commands.

While what you said is true in general; ulimit is usually not a separate program. It modifies the state of the current process (similarly to `cd`), so it has to be built-in.

Now, there's no solid reason why it couldn't work like `unshare` or `chroot` (which also wrap operations that modify the current process), and spawn a second child shell with the modified properties. But, that isn't how it works on any system I have ever used, isn't what users expect, and isn't in conformance with POSIX.

To quote POSIX: Since `ulimit` affects the current shell execution environment, it is always provided as a shell regular built-in.

> so it has to be built-in.

We have fork() and exec(), and resource limits carry across exec(), so it does not /have/ to be a built-in.

strace -e getrlimit,setrlimit bash -c 'ulimit -s 2 && echo foo'

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