ZONE(9) MidnightBSD Kernel Developer’s Manual 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 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.

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 or when 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.

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 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.

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 Smørgrav 〈des@FreeBSD.org〉. Changes for UMA by Jeroen Ruigrok van der Werven 〈asmodai@FreeBSD.org〉.

MidnightBSD 0.3 October 2, 2006 MidnightBSD 0.3