/share/man/man9/zone.9

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  2. .\" Copyright (c) 2001 Dag-Erling Coïdan Smørgrav
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  28. .Dd February 25, 2012
  29. .Dt ZONE 9
  30. .Os
  31. .Sh NAME
  32. .Nm uma_zcreate ,
  33. .Nm uma_zalloc ,
  34. .Nm uma_zalloc_arg ,
  35. .Nm uma_zfree ,
  36. .Nm uma_zfree_arg ,
  37. .Nm uma_zdestroy ,
  38. .Nm uma_zone_set_max,
  39. .Nm uma_zone_get_max,
  40. .Nm uma_zone_get_cur
  41. .Nd zone allocator
  42. .Sh SYNOPSIS
  43. .In sys/param.h
  44. .In sys/queue.h
  45. .In vm/uma.h
  46. .Ft uma_zone_t
  47. .Fo uma_zcreate
  48. .Fa "char *name" "int size"
  49. .Fa "uma_ctor ctor" "uma_dtor dtor" "uma_init uminit" "uma_fini fini"
  50. .Fa "int align" "uint16_t flags"
  51. .Fc
  52. .Ft "void *"
  53. .Fn uma_zalloc "uma_zone_t zone" "int flags"
  54. .Ft "void *"
  55. .Fn uma_zalloc_arg "uma_zone_t zone" "void *arg" "int flags"
  56. .Ft void
  57. .Fn uma_zfree "uma_zone_t zone" "void *item"
  58. .Ft void
  59. .Fn uma_zfree_arg "uma_zone_t zone" "void *item" "void *arg"
  60. .Ft void
  61. .Fn uma_zdestroy "uma_zone_t zone"
  62. .Ft int
  63. .Fn uma_zone_set_max "uma_zone_t zone" "int nitems"
  64. .Ft int
  65. .Fn uma_zone_get_max "uma_zone_t zone"
  66. .Ft int
  67. .Fn uma_zone_get_cur "uma_zone_t zone"
  68. .Sh DESCRIPTION
  69. The zone allocator provides an efficient interface for managing
  70. dynamically-sized collections of items of similar size.
  71. The zone allocator can work with preallocated zones as well as with
  72. runtime-allocated ones, and is therefore available much earlier in the
  73. boot process than other memory management routines.
  74. .Pp
  75. A zone is an extensible collection of items of identical size.
  76. The zone allocator keeps track of which items are in use and which
  77. are not, and provides functions for allocating items from the zone and
  78. for releasing them back (which makes them available for later use).
  79. .Pp
  80. After the first allocation of an item,
  81. it will have been cleared to zeroes, however subsequent allocations
  82. will retain the contents as of the last free.
  83. .Pp
  84. The
  85. .Fn uma_zcreate
  86. function creates a new zone from which items may then be allocated from.
  87. The
  88. .Fa name
  89. argument is a text name of the zone for debugging and stats; this memory
  90. should not be freed until the zone has been deallocated.
  91. .Pp
  92. The
  93. .Fa ctor
  94. and
  95. .Fa dtor
  96. arguments are callback functions that are called by
  97. the uma subsystem at the time of the call to
  98. .Fn uma_zalloc
  99. and
  100. .Fn uma_zfree
  101. respectively.
  102. Their purpose is to provide hooks for initializing or
  103. destroying things that need to be done at the time of the allocation
  104. or release of a resource.
  105. A good usage for the
  106. .Fa ctor
  107. and
  108. .Fa dtor
  109. callbacks
  110. might be to adjust a global count of the number of objects allocated.
  111. .Pp
  112. The
  113. .Fa uminit
  114. and
  115. .Fa fini
  116. arguments are used to optimize the allocation of
  117. objects from the zone.
  118. They are called by the uma subsystem whenever
  119. it needs to allocate or free several items to satisfy requests or memory
  120. pressure.
  121. A good use for the
  122. .Fa uminit
  123. and
  124. .Fa fini
  125. callbacks might be to
  126. initialize and destroy mutexes contained within the object.
  127. This would
  128. allow one to re-use already initialized mutexes when an object is returned
  129. from the uma subsystem's object cache.
  130. They are not called on each call to
  131. .Fn uma_zalloc
  132. and
  133. .Fn uma_zfree
  134. but rather in a batch mode on several objects.
  135. .Pp
  136. To allocate an item from a zone, simply call
  137. .Fn uma_zalloc
  138. with a pointer to that zone
  139. and set the
  140. .Fa flags
  141. argument to selected flags as documented in
  142. .Xr malloc 9 .
  143. It will return a pointer to an item if successful,
  144. or
  145. .Dv NULL
  146. in the rare case where all items in the zone are in use and the
  147. allocator is unable to grow the zone
  148. or when
  149. .Dv M_NOWAIT
  150. is specified.
  151. .Pp
  152. Items are released back to the zone from which they were allocated by
  153. calling
  154. .Fn uma_zfree
  155. with a pointer to the zone and a pointer to the item.
  156. If
  157. .Fa item
  158. is
  159. .Dv NULL ,
  160. then
  161. .Fn uma_zfree
  162. does nothing.
  163. .Pp
  164. The variations
  165. .Fn uma_zalloc_arg
  166. and
  167. .Fn uma_zfree_arg
  168. allow to
  169. specify an argument for the
  170. .Dv ctor
  171. and
  172. .Dv dtor
  173. functions, respectively.
  174. .Pp
  175. Created zones,
  176. which are empty,
  177. can be destroyed using
  178. .Fn uma_zdestroy ,
  179. freeing all memory that was allocated for the zone.
  180. All items allocated from the zone with
  181. .Fn uma_zalloc
  182. must have been freed with
  183. .Fn uma_zfree
  184. before.
  185. .Pp
  186. The
  187. .Fn uma_zone_set_max
  188. function limits the number of items
  189. .Pq and therefore memory
  190. that can be allocated to
  191. .Fa zone .
  192. The
  193. .Fa nitems
  194. argument specifies the requested upper limit number of items.
  195. The effective limit is returned to the caller, as it may end up being higher
  196. than requested due to the implementation rounding up to ensure all memory pages
  197. allocated to the zone are utilised to capacity.
  198. The limit applies to the total number of items in the zone, which includes
  199. allocated items, free items and free items in the per-cpu caches.
  200. On systems with more than one CPU it may not be possible to allocate
  201. the specified number of items even when there is no shortage of memory,
  202. because all of the remaining free items may be in the caches of the
  203. other CPUs when the limit is hit.
  204. .Pp
  205. The
  206. .Fn uma_zone_get_max
  207. function returns the effective upper limit number of items for a zone.
  208. .Pp
  209. The
  210. .Fn uma_zone_get_cur
  211. function returns the approximate current occupancy of the zone.
  212. The returned value is approximate because appropriate synchronisation to
  213. determine an exact value is not performed by the implementation.
  214. This ensures low overhead at the expense of potentially stale data being used
  215. in the calculation.
  216. .Sh RETURN VALUES
  217. The
  218. .Fn uma_zalloc
  219. function returns a pointer to an item, or
  220. .Dv NULL
  221. if the zone ran out of unused items and the allocator was unable to
  222. enlarge it.
  223. .Sh SEE ALSO
  224. .Xr malloc 9
  225. .Sh HISTORY
  226. The zone allocator first appeared in
  227. .Fx 3.0 .
  228. It was radically changed in
  229. .Fx 5.0
  230. to function as a slab allocator.
  231. .Sh AUTHORS
  232. .An -nosplit
  233. The zone allocator was written by
  234. .An John S. Dyson .
  235. The zone allocator was rewritten in large parts by
  236. .An Jeff Roberson Aq jeff@FreeBSD.org
  237. to function as a slab allocator.
  238. .Pp
  239. This manual page was written by
  240. .An Dag-Erling Sm\(/orgrav Aq des@FreeBSD.org .
  241. Changes for UMA by
  242. .An Jeroen Ruigrok van der Werven Aq asmodai@FreeBSD.org .