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DESCRIPTION

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.

The flags argument of the uma_zcreate() is a subset of the following flags:

UMA_ZONE_NOFREE

Slabs of the zone are never returned back to VM.

UMA_ZONE_NODUMP

Pages belonging to the zone will not be included into mini-dumps.

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 allocation address plus multiple of 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();

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. 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 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 variations uma_zalloc_arg() and uma_zfree_arg() allow callers to specify an argument for the ctor and dtor functions, respectively. The uma_zalloc_domain() 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 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.

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 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_get_max() function returns the effective upper limit number of items for a zone.

The 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.

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 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 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.