Resource Limits

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NOTE: This page describes Linux rlimit support (an SVr4/BSD4.3/POSIX.1-2001 standard). vServers can also be managed using the Linux Control Group (cgroup) system, for information on Control Group based resource management, see util-vserver - Cgroups.

Most properties related to system resources, might it be the memory consumption, the number of processes or file-handles, qualify for imposing limits on them.

The Linux kernel provides the getrlimit and setrlimit system calls to get and set resource limits per process. Each resource has an associated soft and hard limit. The soft limit is the value that the kernel enforces for the corresponding resource. The hard limit acts as a ceiling for the soft limit: an unprivileged process may only set its soft limit to a value in the range from 0 up to the hard limit, and (irreversibly) lower its hard limit. A privileged process (one with the CAP_SYS_RESOURCE capability) may make arbitrary changes to either limit value.

The Linux-VServer kernel extends this system to provide resource limits for whole contexts, not just single processes. Additionally a few new limit types missing in the vanilla kernel were introduced.

Additionally the context limit system keeps track of observed maxima and resource limit hits, to provide some feedback for the administrator. See Context Accounting for details.


List of Resource Limits

Below is a list of resource limits used for contexts and processes within. The tables contain the following information:

Resource limit ID
Human readable identifier used in userspace utilities
Name used in /proc/virtual/*/limit
command line switch for the ulimit utility
Aprropriate unit for the limit
Special resource limit code to denote if resources are accounted, enforced (see below)
Description of capability/flag effects

Special Resource Limit Codes

The tag column may contain one or more of the following tags:

Tag Description
A Limit will be accounted
E Limit will be enforced

Linux Resource Limits

Below is a list of resource limits available in vanilla Linux (>=2.6.18).

ID Name procfs ulimit Tag Unit Description
0 CPU -- -t ms CPU time in ms
1 FSIZE -- -f KiB Maximum file size
2 DATA -- -d KiB Maximum size of the data segment
3 STACK -- -s KiB Maximum stack size
4 CORE -- -c KiB Maximum core file size
5 RSS RSS -m AE page Maximum resident set size
6 NPROC PROC -u AE 1 Maximum number of processes
7 NOFILE FILES -n AE 1 Maximum number of open files
8 MEMLOCK VML -l AE page Maximum locked-in-memory address space
9 AS VM -v AE page Maximum address space size
10 LOCKS LOCKS -x AE 1 Maximum file locks held
11 SIGPENDING -- -i 1 Maximum number of pending signals
12 MSGQUEUE MSGQ -q AE Byte Maximum bytes in POSIX mqueues
13 NICE -- 1 Maximum nice prio allowed to raise to
14 RTPRIO -- -r 1 Maximum realtime priority

Linux-VServer Resource Limits

Below is a list of additional resource limits available in the Linux-VServer kernel.

ID Name procfs Tag Unit Description
16 NSOCK SOCK AE 1 Maximum number of open sockets
17 OPENFD OFD AE 1 Maximum number of open file descriptors
18 ANON ANON A page Maximum anonymous memory size
19 SHMEM SHM AE page Maximum shared memory size
20 SEMARY SEMA A 1 Maximum number of semaphore arrays
21 NSEMS SEMS A 1 Maximum number of semaphores
22 DENTRY DENT AE 1 Maximum number of dentry structs

Determining the Page Size

You can use the following program to determine the page size for your architecture (if it supports the getpagesize() function)

#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>

int main(int argc, char *argv[])
        int page_size = getpagesize();
        printf("The page size is %d\n", page_size);

Here's how to compile and run it (assuming you save it as pagesize.c):

# gcc pagesize.c -o pagesize
# ./pagesize 
The page size is 4096

Note: Whether getpagesize() is present as a Linux system call depends on the architecture. If it is, it returns the kernel symbol PAGE_SIZE, which is architecture and machine model dependent. Generally, one uses binaries that are architecture but not machine model dependent, in order to have a single binary distribution per architecture. This means that a user program should not find PAGE_SIZE at compile time from a header file, but use an actual system call, at least for those architectures (like sun4) where this dependency exists. Here libc4, libc5, glibc 2.0 fail because their getpagesize() returns a statically derived value, and does not use a system call. Things are OK in glibc 2.1.

If you prefer, you can get the pagesize using python, just start a python console and write:

>>> import resource
>>> resource.getpagesize()

Most Linux operating systems also just support a simple native command

getconf PAGESIZE

Here are some typical page sizes ...

arch pagesize
arm, extensa, s390, sh64, x86, x86_64, v850 4k
alpha, cris 8k
m68k, sparc 4k, 8k
powerpc 4k, 64k
parisc 4k, 16k, 64k
ia64, mips 4k, 8k, 16k, 64k

Configure Resource Limits in util-vserver

For example, if the adress space size (AS) and the resident set size (RSS) should be limited, the appropriate config files should be the following. You might have to create parent directories first.

# ls -al /etc/vservers/myguest/rlimits
total 28
drwxr-xr-x  2 root root 4096 2005-08-24 12:37 .
drwxr-xr-x  5 root root 4096 2005-08-24 00:22 ..
-rw-r--r--  1 root root    6 2005-08-24 12:43 as
-rw-r--r--  1 root root    6 2005-08-24 12:37 rss

# cat /etc/vservers/myguest/rlimits/as

# cat /etc/vservers/myguest/rlimits/rss
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