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NAME
UMA — general-purpose kernel object allocator
SYNOPSIS
#include <sys/param.h>
#include <sys/queue.h>
#include <vm/uma.h>
options UMA_FIRSTTOUCH
options UMA_XDOMAIN
typedef int (*uma_ctor)(void *mem, int size, void *arg, int flags);
typedef void (*uma_dtor)(void *mem, int size, void *arg);
typedef int (*uma_init)(void *mem, int size, int flags);
typedef void (*uma_fini)(void *mem, int size);
typedef int (*uma_import)(void *arg, void **store, int count, int domain,
int flags);
typedef void (*uma_release)(void *arg, void **store, int count);
typedef void *(*uma_alloc)(uma_zone_t zone, vm_size_t size, int domain,
uint8_t *pflag, int wait);
typedef void (*uma_free)(void *item, vm_size_t size, uint8_t pflag);
uma_zone_t
uma_zcreate(char *name, int size, uma_ctor ctor, uma_dtor dtor, uma_init zinit, uma_fini zfini,
int align, uint16_t flags);
uma_zone_t
uma_zcache_create(char *name, int size, uma_ctor ctor, uma_dtor dtor, uma_init zinit, uma_fini zfini,
uma_import zimport, uma_release zrelease, void *arg, int flags);
uma_zone_t
uma_zsecond_create(char *name, uma_ctor ctor, uma_dtor dtor, uma_init zinit, uma_fini zfini,
uma_zone_t master);
void
uma_zdestroy(uma_zone_t zone);
void *
uma_zalloc(uma_zone_t zone, int flags);
void *
uma_zalloc_arg(uma_zone_t zone, void *arg, int flags);
void *
uma_zalloc_domain(uma_zone_t zone, void *arg, int domain, int flags);
void *
uma_zalloc_pcpu(uma_zone_t zone, int flags);
void *
uma_zalloc_pcpu_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_zfree_domain(uma_zone_t zone, void *item, void *arg);
void
uma_zfree_pcpu(uma_zone_t zone, void *item);
void
uma_zfree_pcpu_arg(uma_zone_t zone, void *item, void *arg);
void
uma_prealloc(uma_zone_t zone, int nitems);
void
uma_zone_reserve(uma_zone_t zone, int nitems);
void
uma_zone_reserve_kva(uma_zone_t zone, int nitems);
void
uma_zone_set_allocf(uma_zone_t zone, uma_alloc allocf);
void
uma_zone_set_freef(uma_zone_t zone, uma_free freef);
int
uma_zone_set_max(uma_zone_t zone, int nitems);
int
uma_zone_set_maxcache(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));
void
uma_reclaim();
#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
UMA (Universal Memory Allocator) provides an efficient interface for managing dynamically-sized
collections of items of identical size, referred to as zones. Zones keep track of which items are in use
and which are not, and UMA provides functions for allocating items from a zone and for releasing them
back, making them available for subsequent allocation requests. Zones maintain per-CPU caches with
linear scalability on SMP systems as well as round-robin and first-touch policies for NUMA systems. The
number of items cached per CPU is bounded, and each zone additionally maintains an unbounded cache of
items that is used to quickly satisfy per-CPU cache allocation misses.
Two types of zones exist: regular zones and cache zones. In a regular zone, items are allocated from a
slab, which is one or more virtually contiguous memory pages that have been allocated from the kernel's
page allocator. Internally, slabs are managed by a UMA keg, which is responsible for allocating slabs
and keeping track of their usage by one or more zones. In typical usage, there is one keg per zone, so
slabs are not shared among multiple zones.
Normal zones import items from a keg, and release items back to that keg if requested. Cache zones do
not have a keg, and instead use custom import and release methods. For example, some collections of
kernel objects are statically allocated at boot-time, and the size of the collection does not change. A
cache zone can be used to implement an efficient allocator for the objects in such a collection.
The uma_zcreate() and uma_zcache_create() functions create a new regular zone and cache zone,
respectively. The uma_zsecond_create() function creates a regular zone which shares the keg of the zone
specified by the master argument. 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 initialize a data structure embedded
in the item, such as a queue(3) head.
The zinit and zfini arguments are used to optimize the allocation of items from the zone. They are
called by the UMA subsystem whenever it needs to allocate or free items to satisfy requests or memory
pressure. A good use for the zinit and zfini callbacks might be to initialize and destroy a mutex
contained within an item. This would allow one to avoid destroying and re-initializing the mutex each
time the item is freed and re-allocated. They are not called on each call to uma_zalloc() and
uma_zfree() but rather when an item is imported into a zone's cache, and when a zone releases an item to
the slab allocator, typically as a response to memory pressure.
For uma_zcache_create(), the zimport and zrelease functions are called to import items into the zone and
to release items from the zone, respectively. The zimport function should store pointers to items in the
store array, which contains a maximum of count entries. The function must return the number of imported
items, which may be less than the maximum. Similarly, the store parameter to the zrelease function
contains an array of count pointers to items. The arg parameter passed to uma_zcache_create() is
provided to the import and release functions. The domain parameter to zimport specifies the requested
numa(4) domain for the allocation. It is either a NUMA domain number or the special value UMA_ANYDOMAIN.
The flags argument of uma_zcreate() and uma_zcache_create() is a subset of the following flags:
UMA_ZONE_NOFREE
Slabs allocated to the zone's keg are never freed.
UMA_ZONE_NODUMP
Pages belonging to the zone will not be included in minidumps.
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 the allocation address plus a multiple of the 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();
Note that M_ZERO cannot be used when allocating items from a PCPU zone. To obtain zeroed memory
from a PCPU zone, use the uma_zalloc_pcpu() function and its variants instead, and pass M_ZERO.
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.
UMA_ZONE_NUMA
The zone should use a first-touch NUMA policy rather than the round-robin default. If the
UMA_FIRSTTOUCH kernel option is configured, all zones implicitly use a first-touch policy, and the
UMA_ZONE_NUMA flag has no effect. The UMA_XDOMAIN kernel option, when configured, causes UMA to do
the extra tracking to ensure that allocations from first-touch zones are always local. Otherwise,
consumers that do not free memory on the same domain from which it was allocated will cause mixing
in per-CPU caches. See numa(4) for more details.
Zones can be destroyed using uma_zdestroy(), freeing all memory that is cached in the zone. All items
allocated from the zone must be freed to the zone before the zone may be safely destroyed.
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 variants uma_zalloc_arg() and uma_zfree_arg() allow callers to specify an argument for the ctor and
dtor functions of the zone, respectively. The uma_zalloc_domain() function allows callers to specify a
fixed numa(4) domain to allocate from. This uses a guaranteed but slow path in the allocator which
reduces concurrency. The uma_zfree_domain() function should be used to return memory allocated in this
fashion. This function infers the domain from the pointer and does not require it as an argument.
The uma_prealloc() function allocates slabs for the requested number of items, typically following the
initial creation of a zone. Subsequent allocations from the zone will be satisfied using the pre-
allocated slabs. Note that slab allocation is performed with the M_WAITOK flag, so uma_prealloc() may
sleep.
The uma_zone_reserve() function sets the number of reserved items for the zone. uma_zalloc() and
variants will ensure that the zone contains at least the reserved number of free items. Reserved items
may be allocated by specifying M_USE_RESERVE in the allocation request flags. uma_zone_reserve() does
not perform any pre-allocation by itself.
The uma_zone_reserve_kva() function pre-allocates kernel virtual address space for the requested number
of items. Subsequent allocations from the zone will be satisfied using the pre-allocated address space.
Note that unlike uma_zone_reserve(), uma_zone_reserve_kva() does not restrict the use of the pre-
allocation to M_USE_RESERVE requests.
The uma_zone_set_allocf() and uma_zone_set_freef() functions allow a zone's default slab allocation and
free functions to be overridden. This is useful if the zone's items have special memory allocation
constraints. For example, if multi-page objects are required to be physically contiguous, an allocf
function which requests contiguous memory from the kernel's page allocator may be used.
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_set_maxcache() function limits the number of free items which may be cached in the zone,
excluding the per-CPU caches, which are bounded in size. For example, to implement a ‘pure’ per-CPU
cache, a cache zone may be configured with a maximum cache size of 0.
The uma_zone_get_max() function returns the effective upper limit number of items for a zone.
The uma_zone_get_cur() function returns an approximation of the number of items currently allocated from
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(9) 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(9)
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.
IMPLEMENTATION NOTES
The memory that these allocation calls return is not executable. The uma_zalloc() function does not
support the M_EXEC flag to allocate executable memory. Not all platforms enforce a distinction between
executable and non-executable memory.
SEE ALSO
numa(4), vmstat(8), malloc(9)
Jeff Bonwick, The Slab Allocator: An Object-Caching Kernel Memory Allocator, 1994.
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 August 20, 2020 UMA(9)