/share/man/man4/man4.i386/pae.4

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  1. .\"
  2. .\" Copyright (c) 2003 Networks Associates Technology, Inc.
  3. .\" All rights reserved.
  4. .\"
  5. .\" This software was developed for the FreeBSD Project by Jake Burkholder,
  6. .\" Safeport Network Services, and Network Associates Laboratories, the
  7. .\" Security Research Division of Network Associates, Inc. under
  8. .\" DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the DARPA
  9. .\" CHATS research program.
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  32. .\" $FreeBSD$
  33. .\"
  34. .Dd April 8, 2003
  35. .Dt PAE 4 i386
  36. .Os
  37. .Sh NAME
  38. .Nm PAE
  39. .Nd Physical Address Extensions
  40. .Sh SYNOPSIS
  41. .Cd "options PAE"
  42. .Sh DESCRIPTION
  43. The
  44. .Dv PAE
  45. option provides support for the physical address extensions capability
  46. of the
  47. .Tn Intel
  48. .Tn Pentium Pro
  49. and above CPUs,
  50. and allows for up to 64 gigabytes of memory to be used in systems capable
  51. of supporting it.
  52. With the
  53. .Dv PAE
  54. option, memory above 4 gigabytes is simply added to the general page pool.
  55. The system makes no distinction between memory above or below 4 gigabytes,
  56. and no specific facility is provided for a process or the kernel to access
  57. more memory than they would otherwise be able to access, through a sliding
  58. window or otherwise.
  59. .Sh SEE ALSO
  60. .Xr smp 4 ,
  61. .Xr tuning 7 ,
  62. .Xr config 8 ,
  63. .Xr bus_dma 9
  64. .Sh HISTORY
  65. The
  66. .Dv PAE
  67. option first appeared in
  68. .Fx 4.9
  69. and
  70. .Fx 5.1 .
  71. .Sh AUTHORS
  72. .An Jake Burkholder Aq jake@FreeBSD.org
  73. .Sh BUGS
  74. Since KLD modules are not compiled with the same options headers that
  75. the kernel is compiled with,
  76. they must not be loaded into a kernel compiled with the
  77. .Dv PAE
  78. option.
  79. .Pp
  80. Many devices or their device drivers are not capable of direct memory access
  81. to physical addresses above 4 gigabytes.
  82. In order to make use of direct memory access IO in a system with more than
  83. 4 gigabytes of memory when the
  84. .Dv PAE
  85. option is used,
  86. these drivers must use a facility for remapping or substituting physical
  87. memory which is not accessible to the device.
  88. One such facility is provided by the
  89. .Nm busdma
  90. interface.
  91. Device drivers which do not account for such devices will not work reliably
  92. in a system with more than 4 gigabytes of memory when the
  93. .Dv PAE
  94. option is used,
  95. and may cause data corruption.
  96. The
  97. .Pa PAE
  98. kernel configuration file includes the
  99. .Dv PAE
  100. option, and explicitly excludes all device drivers which are known to not work
  101. or have not been tested in a system with the
  102. .Dv PAE
  103. option and more than 4 gigabytes of memory.
  104. .Pp
  105. Many parameters which determine how memory is used in the kernel are based on
  106. the amount of physical memory.
  107. The formulas used to determine the values of these parameters for specific
  108. memory configurations may not take into account the fact there may be more
  109. than 4 gigabytes of memory, and may not scale well to these memory
  110. configurations.
  111. In particular,
  112. it may be necessary to increase the amount of virtual address space available
  113. to the kernel,
  114. or to reduce the amount of a specific resource that is heavily used,
  115. in order to avoid running out of virtual address space.
  116. The
  117. .Dv KVA_PAGES
  118. option may be used to increase the kernel virtual address space,
  119. and the
  120. .Va kern.maxvnodes
  121. .Xr sysctl 8
  122. may be used to decrease the number of vnodes allowed,
  123. an example of a resource that the kernel is likely to overallocate in
  124. large memory configurations.
  125. For optimal performance and stability it may be necessary to consult the
  126. .Xr tuning 7
  127. manual page, and make adjustments to the parameters documented there.