Manual Pages  — UMA

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.

Jeff Bonwick, The Slab Allocator: An Object-Caching Kernel Memory Allocator, 1994.