MirOS Manual: pool(9), pool_cache_destroy(9), pool_cache_destruct_object(9), pool_cache_get(9), pool_cache_init(9), pool_cache_invalidate(9), pool_cache_put(9), pool_destroy(9), pool_get(9), pool_init(9), pool_prime(9), pool_put(9), pool_sethiwat(9), pool_setlowat(9)

POOL(9)                       BSD Kernel Manual                        POOL(9)

NAME

     pool_init, pool_destroy, pool_get, pool_put, pool_prime, pool_sethiwat,
     pool_setlowat, pool_cache_init, pool_cache_destroy, pool_cache_get,
     pool_cache_put, pool_cache_destruct_object, pool_cache_invalidate -
     resource-pool manager

SYNOPSIS

     #include <sys/types.h>
     #include <sys/pool.h>

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

     void
     pool_destroy(struct pool *pp);

     void
     pool_set_drain_hook(struct pool *pp, void (*fun)(void *, int),
             void *arg);

     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);

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

     void
     pool_cache_init(struct pool_cache *pc, struct pool *pp,
             int (*ctor)(void *, void *, int), void (*dtor)(void *, void *),
             void *arg);

     void
     pool_cache_destroy(struct pool_cache *pc);

     void *
     pool_cache_get(struct pool_cache *pc, int flags);

     void
     pool_cache_put(struct pool_cache *pc, void *object);

     void
     pool_cache_destruct_object(struct pool_cache *pc, void *object);

     void
     pool_cache_invalidate(struct pool_cache *pc);

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 opera-
     tion independent of the memory resources currently available from the
     system-wide memory allocator (malloc(9)). The pool manager obtains memory
     by using the special-purpose memory allocator palloc() passed to
     pool_init(), 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_init() initializes a resource pool. The arguments are:

           pool          Specifies the pool storage to be initialized.

           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         Specifies various flags set on the pool at creation
                         time.

           wmesg         The message passed on to tsleep(9) if pool_get() must
                         wait for items to be returned to the pool.

           palloc        The back-end allocator used to manage the memory for
                         the pool. palloc() may be NULL, in which case the
                         pool manager uses the pool_allocator_kmem allocator
                         which uses uvm_km_kmemalloc(9) and uvm_km_free(9) to
                         allocate and release memory using the kmem_map
                         (see uvm(9)). It is recommended to set this to
                         pool_allocator_nointr if the pool will never be ac-
                         cessed in interrupt context, since that allocator is
                         much more efficient.

DESTROYING A POOL

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

SETTING DRAIN CALLBACK

     The pool_set_drain_hook() function can be used to specify a function that
     will be called when memory is running low. The callback fun will be
     called with the arguments arg which is the third argument to
     pool_set_drain_hook() and flags which will have PR_WAITOK set if the
     drain hook is allowed to sleep.

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 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 are 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 allo-
     cated by using the page allocation routine specified to pool_init().

     pool_prime()

           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 (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
                  of 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.

SETTING HARD LIMITS

     The function pool_sethardlimit() sets a hard limit on the pool to n
     items. If the hard limit is reached warnmess will be printed to the con-
     sole, but no more than every ratecap seconds. Upon successful completion,
     a value of 0 is returned. The value EINVAL is returned when the current
     size of the pool already exceeds the requested hard limit.

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.

POOL CACHES

     Another set of functions are available as extensions to the pool manager.
     The pool cache functions automatically call constructors and destructors
     when objects are allocated from the pool or returned to it. They have
     similar semantics as the other pool functions. The pp argument to
     pool_cache_init() must already be initialized.

     Objects are not immediately deconstructed when put into the pool cache.
     Instead, they are maintained for future allocations. When the system
     determines that memory needs to be reclaimed, then the deconstructor is
     called on each free object and it is placed back into the pool. The ctor
     and dtor functions are passed arg and a pointer to the object, in that
     order. The ctor is also passed the same flags that are passed to
     pool_cache_get(). The pool_cache_destruct_object() function deconstructs
     and puts an object back into the pool immediately.
     pool_cache_invalidate() deconstructs all cached objects and releases
     their memory.

     Pool caches are also commonly referred to as a slab allocator.

DIAGNOSTICS

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

DEBUGGING

     To debug a misbehaving pool, a kernel can be compiled with the
     MALLOC_DEBUG option and memory debugging on pools can be enabled with the
     PR_DEBUG flag passed in the flags argument in the call to pool_init().
     See malloc(9) for more information about MALLOC_DEBUG.

CODE REFERENCES

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

SEE ALSO

     free(9), malloc(9), uvm(9)

HISTORY

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

MirOS BSD #10-current           July 23, 1998                                3

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