|ZONE(9)||Kernel Developer's Manual||ZONE(9)|
int size, uma_ctor ctor,
uma_dtor dtor, uma_init uminit,
uma_fini fini, int align,
zone, void *arg,
zone, void *arg,
zone, void *item,
zone, void *item,
zone, const char
DESCRIPTION¶The zone allocator provides an efficient interface for managing dynamically-sized collections of items of identical 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. The zone allocator provides per-cpu allocation caches with linear scalability on SMP systems as well as round-robin and first-touch policies for NUMA systems.
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.
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
The ctor and dtor
arguments are callback functions that are called by the uma subsystem at the
time of the call to
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
but rather in a batch mode on several objects.
The flags argument of the
uma_zcreate() is a subset of the following
- Slabs of the zone are never returned back to VM.
- Pages belonging to the zone will not be included into mini-dumps.
- 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
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();
- 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
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.
- 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_ZINITflag would not return zeroed memory on every
- The zone should use an internal hash table to find slab book-keeping structure where an allocation being freed belongs to.
- The zone should use special field of vm_page_t to find slab book-keeping structure where an allocation being freed belongs to.
- The zone is for the malloc(9) subsystem.
- The zone is for the VM subsystem.
- The zone should use a first-touch NUMA policy rather than the round-robin default. Callers that do not free memory on the same domain it is allocated from will cause mixing in per-cpu caches. See numa(9) for more details.
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
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
uma_zfree() with a pointer to the zone
and a pointer to the item. If item is
uma_zfree_arg() allow callers to specify an argument
functions, respectively. The
function allows callers to specify a fixed numa(9) domain
to allocate from. This uses a guaranteed but slow path in the allocator
which reduces concurrency. The
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
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_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.
uma_zone_get_max() function returns
the effective upper limit number of items for a zone.
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.
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
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
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
uma_zone_get_max(). A write to the
oid sets new value via
zone, descr) macro is provided
to create this type of oid dynamically.
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
descr) macro is provided to create this type of oid
uma_zalloc() function returns a pointer to an item, or
NULLif the zone ran out of unused items and
IMPLEMENTATION NOTES¶The memory that these allocation calls return is not executable. The
uma_zalloc() function does not support the
M_EXECflag to allocate executable memory. Not all platforms enforce a distinction between executable and non-executable memory.
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.
|June 13, 2018||Linux 4.19.0-6-amd64|