Provided by: freebsd-manpages_11.1-3_all 
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>.
Debian April 26, 2017 ZONE(9)