Wandering Thoughts: Recent Entries For 2007/09/13

Limiting a process's memory usage on Linux

Due to recent events I have become interested in this issue, so I have been poking around and doing some experiments. Unfortunately, while Linux has a bewildering variety of memory related per-process resource limits that you can set, most of them don't work or don't do you any good.

What you have, in theory and practice:

• ulimit -m, the maximum RSS, doesn't do anything; the kernel maintains the number but never seems to use it for anything.

• ulimit -d, the maximum data segment size, is effectively useless since it only affects memory that the program obtains through brk(2)/sbrk(2). These days, these aren't used very much; GNU libc does most of its memory allocation using mmap(), especially for big blocks of memory.

• ulimit -v, the maximum size of the address space, works but affects all mmap()s, even of things that will never require swap space, such as mmap()ing a big file.

What I really want is something that can effectively limit a process's 'committed address space' (to use the term that /proc/meminfo and the kernel documentation on swap overcommit use). I don't care if a process wants to mmap() a 50 gigabyte file, but I care a lot if it wants 50G of anonymous, unbacked address space, because the latter is what will drive the system into out-of-memory.

Unfortunately I can imagine entirely legitimate reasons to want to mmap() huge files (especially huge sparse files) on a 64-bit machine, so any limit on the total process address space on our compute servers will have to be a soft limit.

Since the Linux kernel already tracks committed address space information for the whole system, it's possible that it would not be too much work to extend it to a per-process limit. (The likely fly in the ointment is that memory regions can be shared between processes, which complicates the accounting and raises questions about what you do when a process modifies a virtual memory region in a way that is legal for it but pushes another process sharing the VMA over its limit.)