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ZONE(9)		       FreeBSD Kernel Developer's Manual	       ZONE(9)

     uma_zcreate, uma_zalloc, uma_zalloc_arg, uma_zfree, uma_zfree_arg,
     uma_zdestroy, uma_zone_set_max, uma_zone_get_max, uma_zone_get_cur	--
     zone allocator

     #include <sys/param.h>
     #include <sys/queue.h>
     #include <vm/uma.h>

     uma_zcreate(char *name, int size, uma_ctor	ctor, uma_dtor dtor,
	 uma_init uminit, uma_fini fini, int align, u_int16_t flags);

     void *
     uma_zalloc(uma_zone_t zone, int flags);

     void *
     uma_zalloc_arg(uma_zone_t zone, void *arg,	int flags);

     uma_zfree(uma_zone_t zone,	void *item);

     uma_zfree_arg(uma_zone_t zone, void *item,	void *arg);

     uma_zdestroy(uma_zone_t zone);

     uma_zone_set_max(uma_zone_t zone, int nitems);

     uma_zone_get_max(uma_zone_t zone);

     uma_zone_get_cur(uma_zone_t zone);

     The zone allocator	provides an efficient interface	for managing dynami-
     cally-sized collections of	items of similar size.	The zone allocator can
     work with preallocated zones as well as with runtime-allocated ones, and
     is	therefore available much earlier in the	boot process than other	memory
     management	routines.

     A zone is an extensible collection	of items of identical size.  The zone
     allocator keeps track of which items are in use and which are not,	and
     provides functions	for allocating items from the zone and for releasing
     them back (which makes them available for later use).

     After the first allocation	of an item, it will have been cleared to
     zeroes, however subsequent	allocations will retain	the contents as	of the
     last free.

     The uma_zcreate() function	creates	a new zone from	which items may	then
     be	allocated from.	 The name argument is a	text name of the zone for
     debugging and stats; this memory should not be freed until	the zone has
     been deallocated.

     The ctor and dtor arguments are callback functions	that are called	by the
     uma subsystem at the time of the call to uma_zalloc() and uma_zfree()
     respectively.  Their purpose is to	provide	hooks for initializing or
     destroying	things that need to be done at the time	of the allocation or
     release of	a resource.  A good usage for the ctor and dtor	callbacks
     might be to adjust	a global count of the number of	objects	allocated.

     The uminit	and fini arguments are used to optimize	the allocation of
     objects from the zone.  They are called by	the uma	subsystem whenever it
     needs to allocate or free several items to	satisfy	requests or memory
     pressure.	A good use for the uminit and fini callbacks might be to ini-
     tialize and destroy mutexes contained within the object.  This would
     allow one to re-use already initialized mutexes when an object is
     returned from the uma subsystem's object cache.  They are not called on
     each call to uma_zalloc() and uma_zfree() but rather in a batch mode on
     several objects.

     To	allocate an item from a	zone, simply call uma_zalloc() with a pointer
     to	that zone and set the flags argument to	selected flags as documented
     in	malloc(9).  It will return a pointer to	an item	if successful, or NULL
     in	the rare case where all	items in the zone are in use and the allocator
     is	unable to grow the zone	or when	M_NOWAIT is specified.

     Items are released	back to	the zone from which they were allocated	by
     calling uma_zfree() with a	pointer	to the zone and	a pointer to the item.
     If	item is	NULL, then uma_zfree() does nothing.

     The variations uma_zalloc_arg() and uma_zfree_arg() allow to specify an
     argument for the ctor and dtor functions, respectively.

     Created zones, which are empty, can be destroyed using uma_zdestroy(),
     freeing all memory	that was allocated for the zone.  All items allocated
     from the zone with	uma_zalloc() must have been freed with uma_zfree()

     The uma_zone_set_max() function limits the	number of items	(and therefore
     memory) that can be allocated to zone.  The nitems	argument specifies the
     requested upper limit number of items.  The effective limit may end up
     being higher than requested, as the implementation	will round up to
     ensure all	memory pages allocated to the zone are utilised	to capacity.
     The limit applies to the total number of items in the zone, which
     includes allocated	items, free items and free items in the	per-cpu
     caches.  On systems with more than	one CPU	it may not be possible to
     allocate the specified number of items even when there is no shortage of
     memory, because all of the	remaining free items may be in the caches of
     the other CPUs when the limit is hit.

     The uma_zone_get_max() function returns the effective upper limit number
     of	items for a zone.

     The uma_zone_get_cur() function returns the approximate current occupancy
     of	the zone.  The returned	value is approximate because appropriate syn-
     chronisation to determine an exact	value is not performend	by the imple-
     mentation.	 This ensures low overhead at the expense of potentially stale
     data being	used in	the calculation.

     The uma_zalloc() function returns a pointer to an item, or	NULL if	the
     zone ran out of unused items and the allocator was	unable to enlarge it.


     The zone allocator	first appeared in FreeBSD 3.0.	It was radically
     changed in	FreeBSD	5.0 to function	as a slab allocator.

     The zone allocator	was written by John S. Dyson.  The zone	allocator was
     rewritten in large	parts by Jeff Roberson <> to function
     as	a slab allocator.

     This manual page was written by Dag-Erling	Smorgrav <>.
     Changes for UMA by	Jeroen Ruigrok van der Werven <>.

FreeBSD	11.1			October	9, 2010			  FreeBSD 11.1


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