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

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

     #include <sys/pool.h>

     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);

     pool_destroy(struct pool *pp);

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

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

     pool_prime(struct pool *pp, int nitems);

     pool_sethiwat(struct pool *pp, int n);

     pool_setlowat(struct pool *pp, int n);

     pool_sethardlimit(struct pool *pp, int n, const char *warnmess,
         int ratecap);

     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 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

           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
                         parameter 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

           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

           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
     automatically during system startup.

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

     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
                  specified, 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.

     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().

     pool_prime() adds items to the pool.  Storage space for the items is
     allocated by using the page allocation routine specified to

     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.

     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.

     The hard limit represents the maximum number of items a pool is allowed
     to allocate at any given time.  Unless modified via pool_sethardlimit(),
     the hard limit defaults to UINT_MAX.


           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.


           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 up-front.


           pp     The handle identifying the pool resource instance.

           n      The maximum number of items to be allocated from the pool
                  (i.e. the hard limit).

                  The warning message that will be logged when the hard limit
                  is reached.

                  The minimal interval (in seconds) after which another
                  warning message is issued when the pool hits its hard limit

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

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

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

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

     The NetBSD pool manager appeared in NetBSD 1.4.

NetBSD 7.1.2                   November 14, 2011                  NetBSD 7.1.2