36.13. "resource" — Resource usage information
**********************************************

This module provides basic mechanisms for measuring and controlling
system resources utilized by a program.

Symbolic constants are used to specify particular system resources and
to request usage information about either the current process or its
children.

A single exception is defined for errors:

exception resource.error

   The functions described below may raise this error if the
   underlying system call failures unexpectedly.


36.13.1. 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.RLIM_INFINITY

   Constant used to represent the limit for an unlimited resource.

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 "RLIM_INFINITY" can be used to request a limit
   that is unlimited.

   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. Specifying a limit of "RLIM_INFINITY" when the hard
   or system limit for that resource is not unlimited will result in a
   "ValueError".  A process with the effective UID of super-user can
   request any valid limit value, including unlimited, but
   "ValueError" will still be raised if the requested limit exceeds
   the system imposed limit.

   "setrlimit" may 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.

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.  This only affects the stack of the main thread in a
   multi-threaded 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.


36.13.2. 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.)

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.
