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uma_zfree.9freebsd

BSD mandoc

NAME

uma_zcreate uma_zalloc uma_zalloc_arg uma_zfree uma_zfree_arg uma_zdestroy uma_zone_set_max - zone allocator

SYNOPSIS

In sys/param.h In sys/queue.h In vm/uma.h Ft uma_zone_t Fo uma_zcreate Fa char *name int size Fa uma_ctor ctor uma_dtor dtor uma_init uminit uma_fini fini Fa int align u_int16_t flags Fc Ft void * Fn uma_zalloc uma_zone_t zone int flags Ft void * Fn uma_zalloc_arg uma_zone_t zone void *arg int flags Ft void Fn uma_zfree uma_zone_t zone void *item Ft void Fn uma_zfree_arg uma_zone_t zone void *item void *arg Ft void Fn uma_zdestroy uma_zone_t zone Ft void Fn 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 Fn uma_zcreate function creates a new zone from which items may then be allocated from. The Fa 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 Fa ctor and Fa dtor arguments are callback functions that are called by the uma subsystem at the time of the call to Fn uma_zalloc and Fn 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 Fa ctor and Fa dtor callbacks might be to adjust a global count of the number of objects allocated.

The Fa uminit and Fa 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 Fa uminit and Fa 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 Fn uma_zalloc and Fn uma_zfree but rather in a batch mode on several objects.

To allocate an item from a zone, simply call Fn uma_zalloc with a pointer to that zone and set the Fa 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 Fn uma_zfree with a pointer to the zone and a pointer to the item.

The variations Fn uma_zalloc_arg and Fn uma_zfree_arg allow to specify an argument for the ctor and dtor functions, respectively.

Created zones, which are empty, can be destroyed using Fn uma_zdestroy , freeing all memory that was allocated for the zone. All items allocated from the zone with Fn uma_zalloc must have been freed with Fn uma_zfree before.

The purpose of Fn uma_zone_set_max is to limit the maximum amount of memory that the system can dedicated toward the zone specified by the Fa zone argument. The Fa 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 Fn 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 Fx 3.0 . It was radically changed in Fx 5.0 to function as a slab allocator.

AUTHORS

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

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