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annotate Doc/libresource.tex @ 5817:48b94de85bf3 legacy-trunk

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author Guido van Rossum <guido@python.org>
date Thu, 17 Jul 1997 16:29:42 +0000
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1 \section{Built-in Module \sectcode{resource}}
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2
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3 \bimodindex{resource}
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4 This module provides basic mechanisms for measuring and controlling
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5 system resources utilized by a program.
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6
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7 Symbolic constants are used to specify particular system resources and
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8 to request usage information about either the current process or its
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9 children.
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10
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11 Resources usage can be limited using the \code{setrlimit} function
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12 described below. Each resource is controlled by a pair of limits: a
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13 soft limit and a hard limit. The soft limit is the current limit, and
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14 may be lowered or raised by a process over time. The soft limit can
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15 never exceed the hard limit. The hard limit can be lowered to any
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16 value greater than the soft limit, but not raised. (Only process with
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17 the effective UID of the super-user can raise a hard limit).
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18
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19 The specific resources that can be limited are system dependent. They
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20 are described in the \code{getrlimit} man page. Typical resources
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21 include:
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22
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23 \begin{description}
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24
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25 \item[RLIMIT_CORE]
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26 The maximum size (in bytes) of a core file that the current process
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27 can create.
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28
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29 \item[RLIMIT_CPU]
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30 The maximum amount of CPU time (in seconds) that a process can use. If
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31 this limit is exceeded, a \code{SIGXCPU} signal is sent to the
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32 process. (See the \code{signal} module documentation for information
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33 about how to catch this signal and do something useful, e.g. flush
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34 open files to disk.)
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35
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36 \end{description}
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37
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38 \begin{datadesc}{RLIMIT_*}
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39 These symbols define resources whose consumption can be controlled
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40 using the \code{setrlimit} and \code{getrlimit} functions defined
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41 below. The values of these symbols are exactly the constants used
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42 by C programs.
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43
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44 The \UNIX{} man page for \file{getrlimit} lists the available
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45 resources. Note that not all systems use the same symbol or same
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46 value to denote the same resource.
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47 \end{datadesc}
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48
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49 \begin{datadesc}{RUSAGE_*}
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50 These symbols are passed to the \code{getrusage} function to specify
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51 whether usage information is being request for the current process,
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52 \code{RUSAGE_SELF} or its child processes \code{RUSAGE_CHILDREN}. On
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53 some system, \code{RUSAGE_BOTH} requests information for both.
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54 \end{datadesc}
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55
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56 \begin{datadesc}{error}
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57 The functions described below may raise this error if the underlying
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58 system call failures unexpectedly.
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59 \end{datadesc}
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60
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61 The resource module defines the following functions:
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62
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63 \begin{funcdesc}{getrusage}{who}
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64 This function returns a large tuple that describes the resources
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65 consumed by either the current process or its children, as specified
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66 by the \var{who} parameter. The elements of the return value each
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67 describe how a particular system resource has been used, e.g. amount
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68 of time spent running is user mode or number of times the process was
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69 swapped out of main memory. Some values are dependent on the clock
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70 tick internal, e.g. the amount of memory the process is using.
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71
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72 The first two elements of the return value are floating point values
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73 representing the amount of time spent executing in user mode and the
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74 amount of time spent executing in system mode, respectively. The
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75 remaining values are integers. Consult the \code{getrusage} man page
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76 for detailed information about these values. A brief summary is
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77 presented here:
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78
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79 \begin{tabular}{rl}
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80 \emph{offset} & \emph{resource} \\
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81 0 & time in user mode (float) \\
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82 1 & time in system mode (float) \\
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83 2 & maximum resident set size \\
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84 3 & shared memory size \\
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85 4 & unshared memory size \\
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86 5 & unshared stack size \\
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87 6 & page faults not requiring I/O \\
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88 7 & page faults requiring I/O \\
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89 8 & number of swap outs \\
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90 9 & block input operations \\
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91 10 & block output operations \\
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92 11 & messages sent \\
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93 12 & messages received \\
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94 13 & signals received \\
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95 14 & voluntary context switches \\
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96 15 & involuntary context switches \\
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97 \end{tabular}
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98
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99 This function will raise a ValueError if an invalid \var{who}
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100 parameter is specified. It may also raise a \code{resource.error}
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101 exception in unusual circumstances.
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102 \end{funcdesc}
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103
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104 \begin{funcdesc}{getpagesize}{}
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105 Returns the number of bytes in a system page. (This need not be the
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106 same as the hardware page size.) This function is useful for
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107 determining the number of bytes of memory a process is using. The
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108 third element of the tuple returned by \code{getrusage} describes
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109 memory usage in pages; multiplying by page size produces number of
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110 bytes.
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111 \end{funcdesc}
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112
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113 \begin{funcdesc}{getrlimit}{resource}
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114 Returns a tuple \code{(\var{soft}, \var{hard})} with the current
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115 soft and hard limits of \var{resource}. Raises ValueError if
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116 an invalid resource is specified, or \code{resource.error} if the
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117 underyling system call fails unexpectedly.
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118 \end{funcdesc}
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119
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120 \begin{funcdesc}{setrlimit}{resource\, limits}
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121 Sets new limits of consumption of \var{resource}. The \var{limits}
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122 argument must be a tuple \code{(\var{soft}, \var{hard})} of two
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123 integers describing the new limits. A value of -1 can be used to
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124 specify the maximum possible upper limit.
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125
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126 Raises ValueError if an invalid resource is specified, if the new
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127 soft limit exceeds the hard limit, or if a process tries to raise its
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128 hard limit (unless the process has an effective UID of
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129 super-user). Can also raise a \code{resource.error} if the
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130 underyling system call fails.
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131 \end{funcdesc}