Difference between revisions of "Resource Limits"

Latest revision as of 15:06, 16 April 2013

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.

[edit] List of Resource Limits

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

ID 

Resource limit ID

Name 

Human readable identifier used in userspace utilities

procfs 

Name used in /proc/virtual/*/limit

ulimit 

command line switch for the ulimit utility

Unit 

Aprropriate unit for the limit

Tag 

Special resource limit code to denote if resources are accounted, enforced (see below)

Description 

Description of capability/flag effects

[edit] Special Resource Limit Codes

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

[edit] Linux Resource Limits

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

[edit] Linux-VServer Resource Limits

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

[edit] 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);
        exit(0);
}

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()
4096

Most Linux operating systems also just support a simple native command

getconf PAGESIZE

Here are some typical page sizes ...

[edit] 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
90000

# cat /etc/vservers/myguest/rlimits/rss
10000