POOL(9)

NAME
pool_init, pool_destroy, pool_get, pool_put, pool_prime, pool_sethiwat, pool_setlowat, pool_sethardlimit - 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);
void
pool_sethardlimit(struct pool *pp, int n, const char *warnmess, int ratecap);

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 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 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 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 automatically 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 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.

  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 AND LIMITS
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.

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 allocate the necessary memory up-front.

pool_sethardlimit()

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).

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

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

  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 reentered. 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.