NAME

     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, uma_zone_set_warning,
     uma_zone_set_maxaction — zone allocator

SYNOPSIS

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

     uma_zone_t
     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);

     void
     uma_zfree(uma_zone_t zone, void *item);

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

     void
     uma_zdestroy(uma_zone_t zone);

     int
     uma_zone_set_max(uma_zone_t zone, int nitems);

     int
     uma_zone_get_max(uma_zone_t zone);

     int
     uma_zone_get_cur(uma_zone_t zone);

     void
     uma_zone_set_warning(uma_zone_t zone, const char *warning);

     void
     uma_zone_set_maxaction(uma_zone_t zone, void (*maxaction)(uma_zone_t));

     #include <sys/sysctl.h>

     SYSCTL_UMA_MAX(parent, nbr, name, access, zone, descr);

     SYSCTL_ADD_UMA_MAX(ctx, parent, nbr, name, access, zone, descr);

     SYSCTL_UMA_CUR(parent, nbr, name, access, zone, descr);

     SYSCTL_ADD_UMA_CUR(ctx, parent, nbr, name, access, zone, descr);

DESCRIPTION

     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 flags:

     UMA_ZONE_NOFREE
          Slabs of the zone are never returned back to VM.

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

     UMA_ZONE_PCPU
          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, ...);
                 ...
                critical_enter();
                foo_pcpu = (foo_t *)zpcpu_get(foo_base);
                /* do something with foo_pcpu */
                critical_exit();

     UMA_ZONE_OFFPAGE
          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 being freed.  The subsystem may choose to prefer offpage book-keeping for
          certain zones implicitly.

     UMA_ZONE_ZINIT
          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().

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

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

     UMA_ZONE_MALLOC
          The zone is for the malloc(9) subsystem.

     UMA_ZONE_VM
          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.

     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() before.

     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 no more often than every five minutes.  Warnings can be turned off globally by
     setting the vm.zone_warnings sysctl tunable to 0.

     The uma_zone_set_maxaction() function sets a function that will be called when the given
     zone becomes full and fails to allocate an item.  The function will be called with the zone
     locked.  Also, the function that called the allocation function may have held additional
     locks.  Therefore, this function should do very little work (similar to a signal handler).

     The SYSCTL_UMA_MAX(parent, nbr, name, access, zone, descr) macro declares a static sysctl
     oid that exports the effective upper limit number of items for a zone.  The zone argument
     should be a pointer to uma_zone_t.  A read of the oid returns value obtained through
     uma_zone_get_max().  A write to the oid sets new value via uma_zone_set_max().  The
     SYSCTL_ADD_UMA_MAX(ctx, parent, nbr, name, access, zone, descr) macro is provided to create
     this type of oid dynamically.

     The SYSCTL_UMA_CUR(parent, nbr, name, access, zone, descr) macro declares a static read-only
     sysctl oid that exports the approximate current occupancy of the zone.  The zone argument
     should be a pointer to uma_zone_t.  A read of the oid returns value obtained through
     uma_zone_get_cur().  The SYSCTL_ADD_UMA_CUR(ctx, parent, nbr, name, zone, descr) macro is
     provided to create this type of oid dynamically.

RETURN VALUES

     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.

SEE ALSO

     malloc(9)

HISTORY

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

AUTHORS

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

     This manual page was written by Dag-Erling Smørgrav <des@FreeBSD.org>.  Changes for UMA by
     Jeroen Ruigrok van der Werven <asmodai@FreeBSD.org>.