zone(9)

NAME

uma_zcreate, uma_zalloc, uma_zfree, uma_zdestroy,

uma_zone_set_max - 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, u_int16_t
flags);
void *
uma_zalloc(uma_zone_t zone, int flags);
void
uma_zfree(uma_zone_t zone, void *item);
void
uma_zdestroy(uma_zone_t zone);
void
uma_zone_set_max(uma_zone_t zone, int nitems);

DESCRIPTION

The zone allocator provides an efficient interface for man

aging dynamically-sized collections of items of similar size. The zone

allocator can
work with preallocated zones as well as with runtime-allo

cated 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).

The zone allocator stores state information inside the items

proper while
they are not allocated, so structures that will be managed

by the zone
allocator and wish to use the type stable property of zones

by leaving
some fields pre-filled between allocations, must reserve two

pointers at
the very beginning for internal use by the zone allocator,

as follows:

struct my_item {

struct my_item *z_rsvd1;
struct my_item *z_rsvd2;
/* rest of structure */

};

Alternatively they should assume those entries corrupted af

ter each allocation. 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, with the exception of the fields mentioned

above.

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 ini

tializing 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 allo

cation 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 ob

ject 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 suc

cessful, 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 al

located by
calling uma_zfree() with a pointer to the zone and a pointer

to the item.

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 purpose of uma_zone_set_max() is to limit the maximum

amount of memory that the system can dedicated toward the zone specified

by the zone
argument. The nitems argument gives the upper limit of

items in the
zone. This limits 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 al

locate 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.

RETURN VALUES

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.

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 Smorgrav

<des@FreeBSD.org>.
Changes for UMA by Jeroen Ruigrok van der Werven <as

modai@FreeBSD.org>.

BSD July 21, 2003

BSD