## 8.15.2 Resource Usage

These functiona are used to retrieve resource usage information:

`getrusage`(who)
  This function returns a large tuple 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 elements 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.

  The first two elements 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:

  Offset | Resource
  --- | ---
  `0` | time in user mode (float)
  `1` | time in system mode (float)
  `2` | maximum resident set size
  `3` | shared memory size
  `4` | unshared memory size
  `5` | unshared stack size
  `6` | page faults not requiring I/O
  `7` | page faults requiring I/O
  `8` | number of swap outs
  `9` | block input operations
  `10` | block output operations
  `11` | messages sent
  `12` | messages received
  `13` | signals received
  `14` | voluntary context switches
  `15` | 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.

`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.

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

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

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