Resource Limits

Resources usage can be limited using the setrlimit() function described below. Each resource is controlled by a pair of limits: a soft limit and a hard limit. The soft limit is the current limit, and may be lowered or raised by a process over time. The soft limit can never exceed the hard limit. The hard limit can be lowered to any value greater than the soft limit, but not raised. (Only processes with the effective UID of the super-user can raise a hard limit.)

The specific resources that can be limited are system dependent. They are described in the getrlimit(2) man page. The resources listed below are supported when the underlying operating system supports them; resources which cannot be checked or controlled by the operating system are not defined in this module for those platforms.

resource.getrlimit(resource)

Returns a tuple (soft, hard) with the current soft and hard limits of resource. Raises ValueError if an invalid resource is specified, or error if the underlying system call fails unexpectedly.

resource.setrlimit(resource, limits)

Sets new limits of consumption of resource. The limits argument must be a tuple (soft, hard) of two integers describing the new limits. A value of -1 can be used to specify the maximum possible upper limit.

Raises ValueError if an invalid resource is specified, if the new soft limit exceeds the hard limit, or if a process tries to raise its hard limit (unless the process has an effective UID of super-user). Can also raise error if the underlying system call fails.

These symbols define resources whose consumption can be controlled using the setrlimit() and getrlimit() functions described below. The values of these symbols are exactly the constants used by C programs.

The Unix man page for getrlimit(2) lists the available resources. Note that not all systems use the same symbol or same value to denote the same resource. This module does not attempt to mask platform differences — symbols not defined for a platform will not be available from this module on that platform.

resource.RLIMIT_CORE

The maximum size (in bytes) of a core file that the current process can create. This may result in the creation of a partial core file if a larger core would be required to contain the entire process image.

resource.RLIMIT_CPU

The maximum amount of processor time (in seconds) that a process can use. If this limit is exceeded, a SIGXCPU signal is sent to the process. (See the signal module documentation for information about how to catch this signal and do something useful, e.g. flush open files to disk.)

resource.RLIMIT_FSIZE

The maximum size of a file which the process may create. This only affects the stack of the main thread in a multi-threaded process.

resource.RLIMIT_DATA

The maximum size (in bytes) of the process’s heap.

resource.RLIMIT_STACK

The maximum size (in bytes) of the call stack for the current process.

resource.RLIMIT_RSS

The maximum resident set size that should be made available to the process.

resource.RLIMIT_NPROC

The maximum number of processes the current process may create.

resource.RLIMIT_NOFILE

The maximum number of open file descriptors for the current process.

resource.RLIMIT_OFILE

The BSD name for RLIMIT_NOFILE.

resource.RLIMIT_MEMLOCK

The maximum address space which may be locked in memory.

resource.RLIMIT_VMEM

The largest area of mapped memory which the process may occupy.

resource.RLIMIT_AS

The maximum area (in bytes) of address space which may be taken by the process.

Resource Usage

These functions are used to retrieve resource usage information:

resource.getrusage(who)

This function returns an object that describes the resources consumed by either the current process or its children, as specified by the who parameter. The who parameter should be specified using one of the RUSAGE_* constants described below.

The fields of the return value each describe how a particular system resource has been used, e.g. amount of time spent running is user mode or number of times the process was swapped out of main memory. Some values are dependent on the clock tick internal, e.g. the amount of memory the process is using.

For backward compatibility, the return value is also accessible as a tuple of 16 elements.

The fields ru_utime and ru_stime of the return value are floating point values representing the amount of time spent executing in user mode and the amount of time spent executing in system mode, respectively. The remaining values are integers. Consult the getrusage(2) man page for detailed information about these values. A brief summary is presented here:

Index	Field	Resource
0	ru_utime	time in user mode (float)
1	ru_stime	time in system mode (float)
2	ru_maxrss	maximum resident set size
3	ru_ixrss	shared memory size
4	ru_idrss	unshared memory size
5	ru_isrss	unshared stack size
6	ru_minflt	page faults not requiring I/O
7	ru_majflt	page faults requiring I/O
8	ru_nswap	number of swap outs
9	ru_inblock	block input operations
10	ru_oublock	block output operations
11	ru_msgsnd	messages sent
12	ru_msgrcv	messages received
13	ru_nsignals	signals received
14	ru_nvcsw	voluntary context switches
15	ru_nivcsw	involuntary context switches

This function will raise a ValueError if an invalid who parameter is specified. It may also raise error exception in unusual circumstances.

Changed in version 2.3: Added access to values as attributes of the returned object.

resource.getpagesize()

Returns the number of bytes in a system page. (This need not be the same as the hardware page size.) This function is useful for determining the number of bytes of memory a process is using. The third element of the tuple returned by getrusage() describes memory usage in pages; multiplying by page size produces number of bytes.

The following RUSAGE_* symbols are passed to the getrusage() function to specify which processes information should be provided for.

resource.RUSAGE_SELF

RUSAGE_SELF should be used to request information pertaining only to the process itself.

resource.RUSAGE_CHILDREN

Pass to getrusage() to request resource information for child processes of the calling process.

resource.RUSAGE_BOTH

Pass to getrusage() to request resources consumed by both the current process and child processes. May not be available on all systems.