POOL(9)                NetBSD Kernel Developer's Manual                POOL(9)

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

SYNOPSIS
     #include <sys/pool.h>

     void
     pool_init(struct pool *pp, size_t size, u_int align, u_int align_offset,
         int flags, const char *wchan, struct pool_allocator *palloc,
         int ipl);

     void
     pool_destroy(struct pool *pp);

     void *
     pool_get(struct pool *pp, int flags);

     void
     pool_put(struct pool *pp, void *item);

     int
     pool_prime(struct pool *pp, int nitems);

     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)).

   INITIALIZING A POOL
     The function pool_init() initializes a resource pool.  The arguments are:

           pp            The handle identifying the pool resource instance.

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

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

           align_offset  The offset within an item to which the align parame-
                         ter applies.

           flags         Should be set to zero or PR_NOTOUCH.  If PR_NOTOUCH
                         is given, free items are never used to keep internal
                         state so that the pool can be used for non memory
                         backed objects.

           wchan         The `wait channel' passed on to cv_wait(9) if
                         pool_get() must wait for items to be returned to the
                         pool.

           palloc        Can be set to NULL or pool_allocator_kmem, in which
                         case the default kernel memory allocator will be
                         used.  It can also be set to pool_allocator_nointr
                         when the pool will never be accessed from interrupt
                         context.

           ipl           Specifies an interrupt priority level that will block
                         all interrupt handlers that could potentially access
                         the pool.

     The POOL_INIT() macro can be used to both declare and initialize a
     resource pool.  The POOL_INIT() macro has the same arguments as the
     pool_init() function and the resource pool will be initialized automati-
     cally during system startup.

   DESTROYING A POOL
     The function pool_destroy() destroys a resource pool.  It takes a single
     argument pp identifying the pool resource instance.

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

           pp     The handle identifying the pool resource instance.

           flags  The flags can be used to define behaviour in case the pooled
                  resources are depleted.  If no resources are available and
                  PR_NOWAIT is given, pool_get() returns NULL.  If PR_WAITOK
                  is given and allocation is attempted with no resources
                  available, the function will sleep until items are returned
                  to the pool.  If both PR_LIMITFAIL and PR_WAITOK are speci-
                  fied, and the pool has reached its hard limit, pool_get()
                  will return NULL without waiting, allowing 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.  The arguments to pool_put() are:

           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
     allocated by using the page allocation routine specified to
     pool_create().

     The arguments to pool_prime() are:

           pp       The handle identifying the pool resource instance.

           nitems   The number of items to add to the pool.

     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.  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 allo-
                  cate the necessary memory up-front.

   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.  Additionally, the functions pool_init() and pool_destroy() should
     never be called in interrupt context.

   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
     free(9), malloc(9), memoryallocators(9), pool_cache(9), uvm(9)

HISTORY
     The NetBSD pool manager appeared in NetBSD 1.4.

NetBSD 5.0_STABLE                July 25, 2007               NetBSD 5.0_STABLE
