POOL(9) BSD Kernel Manual POOL(9)
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
#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);
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.
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.
The pool_destroy() function destroys a resource pool. It takes a single argument pp identifying the pool resource instance.
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.
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.
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().
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.
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.
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.
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.
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.
Pool usage logs can be enabled by defining the compile-time option POOL_DIAGNOSTIC.
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.
The pool manager is implemented in the file sys/kern/subr_pool.c.
free(9), malloc(9), uvm(9)
The pool manager first appeared in NetBSD 1.4 and was ported to OpenBSD by Artur Grabowski <email@example.com>. MirOS BSD #10-current July 23, 1998 3
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