POOL(9)

NAME
pool_create, pool_destroy, pool_get, pool_put, pool_prime, pool_sethiwat, pool_setlowat - resource-pool manager

SYNOPSIS
#include <sys/pool.h>
struct pool *
pool_create(size_t size, u_int align, u_int align_offset, int nitems, char *wchan, u_int pagesz, void *(*palloc)(unsigned long sz, int flags, int tag), void (*prelease)(void *v, unsigned long sz, int tag), int mtag);
void *
pool_get(struct pool *pp, int flags);
void
pool_put(struct pool *pp, void *item);
int
pool_prime(struct pool *pp, int nitems, caddr_t storage);
void
pool_sethiwat(struct pool *pp, int n);
void
pool_setlowat(struct pool *pp, int n);
POOL_STORAGE_SIZE(size, nitems);

DESCRIPTION
These utility routines provide management of pools of fixed-sized areas of memory. Resource pools set aside an amount of memory for exclusive use by the resource pool owner. This can be used by applications to guarantee the availability of a minimum amount of memory needed to continue operation independent of the memory resources currently available from the system-wide memory allocator (malloc(9)). The pool manager can optionally obtain temporary memory by calling the palloc() function passed to pool_create(), for extra pool items in case the number of allocations exceeds the nominal number of pool items managed by a pool resource. This temporary memory will be automatically returned to the system at a later time.

  CREATING A POOL
The function pool_create() initializes a resource pool and returns a handle to it. The arguments are:

size
    Specifies the size of the memory items managed by the pool.

align
    Specifies the memory address aligment of the items returned by pool_get(). This argument must be a power of two. If zero, the alignment defaults to a architecture-specific natural aligment.

align_offset
    The offset within an item to which the align parameter applies.

nitems
    Specifies the number of memory items that are allocated to the pool at creation time. This number may be zero, in which case pool_prime() can be used at a later time to add permanent items to the pool.

wchan
    The ‘wait channel’ passed on to tsleep(9) if pool_get() must wait for items to be returned to the pool.

pagesz
    The unit which is used to allocate additional memory to the pool. It must be a power of two.

palloc
    is called to add additional memory if the pool is depleted. It returns pagesz aligned memory. The argument sz will be a multiple of pagesz.

prelease
    is called to release pages back to the system. palloc() and prelease() may be NULL, in which case the pool manager uses uvm_km_kmemalloc(9) and uvm_km_free(9) to allocate and release memory using the kernel_map (see UVM(9)).

mtag
    The memory tag passed to palloc() and prelease() when allocating or releasing memory pages.

storage
    Optional storage provided by the caller to use in lieu of malloc(9) for both the pool handle and all pool items.

If not enough memory is available to create the pool resource, pool_create() returns NULL. If the storage parameter is used, the client is responsible for providing enough storage to accommodate the number of pool items specified by nitems, as well as the space required by the pool's administrative overhead (i.e. the pool handle).

  ALLOCATING ITEMS FROM A POOL
pool_get() allocates an item from the pool and returns a pointer to it.

pp  The handle identifying the pool resource instance.

flags
    One of PR_URGENT or PR_WAITOK, that define behaviour in case the pooled resources are depleted. If no resources are available and PR_WAITOK is given, this function will wait until items are returned to the pool. Otherwise pool_get() returns NULL. If PR_URGENT is specified and no items are available and palloc() cannot allocate a new page, the system will panic (XXX).

  RETURNING ITEMS TO A POOL
pool_put() returns the pool item pointed at by item to the resource pool identified by the pool handle pp. If the number of available items in the pool exceeds the maximum pool size set by pool_sethiwat() and there are no outstanding requests for pool items, the excess items will be returned to the system by calling prelease().

pp  The handle identifying the pool resource instance.

item
    A pointer to a pool item previously obtained by pool_get().

  PRIMING A POOL
pool_prime() adds items to the pool. Storage space for the items is either allocated directly using malloc(9) or given to pool_prime() preallocated by the calling function.

pool_prime()

pp  The handle identifying the pool resource instance.

nitems
    The number of items to add to the pool. Storage for the pool items can be passed in the storage argument. If this parameter is NULL, the items are allocated by using malloc(9). This function may return ENOMEM in case the requested number of items could not be allocated. Otherwise, the return value is 0.

  SETTING POOL RESOURCE WATERMARKS
A pool will attempt to increase its resource usage to keep up with the demand for its items. Conversely, it will return unused memory to the system should the number of accumulated unused items in the pool exceed a programmable limit. The limits for the minimum and maximum number of items which a pool should keep at hand are known as the high and low watermarks. The functions pool_sethiwat() and pool_setlowat() set a pool's high and low watermarks, respectively.

pool_sethiwat()

pp  The handle identifying the pool resource instance.

n   The maximum number of items to keep in the pool. As items are returned and the total number of pages in the pool is larger than the maximum set by this function, any completely unused pages are released immediately (by calling prelease()). If this function is not used to specify a maximum number of items, the pages will remain associated with the pool until the system runs low on memory, at which point the VM system will try to reclaim unused pages.

pool_setlowat()

pp  The handle identifying the pool resource instance.

n   The minimum number of items to keep in the pool. The number pages in the pool will not decrease below the required value to accommodate the minimum number of items specified by this function. Unlike pool_prime(), this function does not allocate the necessary memory upfront.

  POTENTIAL PITFALLS
Note that undefined behaviour results when mixing the storage providing methods supported by the pool resource routines.

The pool resource code uses a per-pool lock to protect its internal state. If any pool functions are called in an interrupt context, the caller must block all interrupts that might cause the code to be reentered.

  DIAGNOSTICS
Pool usage logs can be enabled by defining the compile-time option POOL_DIAGNOSTIC.

CODE REFERENCES
The pool manager is implemented in the file sys/kern/subr_pool.c.

SEE ALSO
malloc(9), free(9), uvm(9)

HISTORY
The NetBSD pool manager appeared in NetBSD 1.4.