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

     uma_zcreate, uma_zalloc, uma_zalloc_arg, uma_zfree, uma_zfree_arg,
     uma_find_refcnt, uma_zdestroy, uma_zone_set_max, uma_zone_get_max,
     uma_zone_get_cur, uma_zone_set_warning -- 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, uint16_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);

     uint32_t *
     uma_find_refcnt(uma_zone_t	zone, void *item);

     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);

     uma_zone_set_warning(uma_zone_t zone, const char *warning);

     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 ze-
     roes, 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 de-
     bugging 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() re-
     spectively.  Their	purpose	is to provide hooks for	initializing or	de-
     stroying things that need to be done at the time of the allocation	or re-
     lease 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 ob-
     jects 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 al-
     low 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 ob-

     The flags argument	of the uma_zcreate() is	a subset of the	following

	  Slabs	of the zone are	never returned back to VM.

	  Each item in the zone	would have internal reference counter associ-
	  ated with it.	 See uma_find_refcnt().

	  Pages	belonging to the zone will not be included into	mini-dumps.

	  An allocation	from zone would	have mp_ncpu shadow copies, that are
	  privately assigned to	CPUs.  A CPU can address its private copy us-
	  ing base allocation address plus multiple of current CPU id and
	  sizeof(struct	pcpu):

		foo_zone = uma_zcreate(..., UMA_ZONE_PCPU);
		foo_base = uma_zalloc(foo_zone,	...);
		foo_pcpu = (foo_t *)zpcpu_get(foo_base);
		/* do something	with foo_pcpu */

	  By default book-keeping of items within a slab is done in the	slab
	  page itself.	This flag explicitly tells subsystem that book-keeping
	  structure should be allocated	separately from	special	internal zone.
	  This flag requires either UMA_ZONE_VTOSLAB or	UMA_ZONE_HASH, since
	  subsystem requires a mechanism to find a book-keeping	structure to
	  an item beeing freed.	 The subsystem may choose to prefer offpage
	  book-keeping for certain zones implicitly.

	  The zone will	have its uma_init method set to	internal method	that
	  initializes a	new allocated slab to all zeros.  Do not mistake
	  uma_init method with uma_ctor.  A zone with UMA_ZONE_ZINIT flag
	  would	not return zeroed memory on every uma_zalloc().

	  The zone should use an internal hash table to	find slab book-keeping
	  structure where an allocation	being freed belongs to.

	  The zone should use special field of vm_page_t to find slab book-
	  keeping structure where an allocation	being freed belongs to.

	  The zone is for the malloc(9)	subsystem.

	  The zone is for the VM subsystem.

     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	and 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.

     If	zone was created with UMA_ZONE_REFCNT flag, then pointer to reference
     counter for an item can be	retrieved with help of the uma_find_refcnt()

     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() be-

     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 is returned
     to	the caller, as it may end up being higher than requested due to	the
     implementation rounding 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 performed by the imple-
     mentation.	 This ensures low overhead at the expense of potentially stale
     data being	used in	the calculation.

     The uma_zone_set_warning()	function sets a	warning	that will be printed
     on	the system console when	the given zone becomes full and	fails to allo-
     cate an item.  The	warning	will be	printed	not often than every five min-
     utes.  Warnings can be turned off globally	by setting the
     vm.zone_warnings sysctl tunable to	0.

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


     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 <>.

BSD				March 21, 2013				   BSD


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