POOL(9)                      OpenBSD Kernel Manual                     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);

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 con-
     tinue 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 allo-
     cations exceeds the nominal number of pool items managed by a pool re-
     source.  This temporary memory will be automatically returned to the sys-
     tem at a later time.

   CREATING A POOL
     The function pool_create() initializes a resource pool and returns a han-
     dle 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 re-
                     turned 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 deplet-
                     ed.  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 manag-
                     er uses uvm_km_kmemalloc(9) and uvm_km_free(9) to allo-
                     cate and release memory using the kernel_map (seeUVM(9)).

           mtag      The memory tag passed to palloc() and prelease() when al-
                     locating 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 or more of of PR_URGENT, PR_WAITOK or PR_LIMITFAIL, 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). If both PR_LIMITFAIL
                  and PR_WAITOK is specified, and the pool has reached its
                  hard limit, pool_get() will return NULL without waiting, al-
                  lowing the caller to do its own garbage collection; however,
                  it will still wait if the pool is not yet at its hard limit.

   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 ei-
     ther allocated directly using malloc(9) or given to pool_prime() preallo-
     cated 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 re-
                   quested 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 max-
                  imum 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 allo-
                  cate 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 reen-
     tered.

   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)

BUGS
     Pool will not work with old VM and is only used by UVM.

HISTORY
     The pool manager first appeared in NetBSD 1.4 and was ported to OpenBSD
     by Artur Grabowski <art@openbsd.org>.

OpenBSD 2.8                      July 23, 1998                               3
