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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_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
     dynamically-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
     initialize 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.

     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
          associated 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
          using 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()

     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
     synchronisation to determine an exact value is not performed by the
     implementation.  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
     allocate an item.  The warning will be printed not often than every five
     minutes.  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 <>.

FreeBSD 11.0-PRERELEASE         March 21, 2013         FreeBSD 11.0-PRERELEASE


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