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

void
`pool_prime`(struct pool *pp, int n);

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, `PR_NOTOUCH`, or `PR_PSERIALIZE`. 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. If `PR_PSERIALIZE` is given, then the allocator guarantees that a passive serialization barrier equivalent to “xc_barrier(0)” will be performed before the object's backing store is returned to the system. `PR_PSERIALIZE` implies `PR_NOTOUCH`. Because of the guarantees provided by `PR_PSERIALIZE`, objects muste never be freed to a pool using this option from either hard or soft interrupt context, as doing so may block.
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.

## 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. If the `PR_ZERO` flag is specified, then the memory returned will be zeroed first using memset(3).

## 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`().

## 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 targets for the minimum and maximum number of free items which a pool should try to keep available 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 limits for the minimum and maximum number of total items (both free and allocated) that the pool can have at any time are specified by the functions `pool_prime`() and `pool_sethardlimit`(), respectively. The defaults for these limits are `0` and `UINT_MAX`, respectively. Changing these limits will cause memory to be immediately allocated to the pool or freed from the pool as needed.

`pool_sethiwat`()

pp
  The handle identifying the pool resource instance.
n
  The maximum number of free items to keep in the pool. As items are returned and the total number of free items 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 free items to keep in the pool. When the number of free items in the pool drops below this threshold, a non-blocking attempt is made to allocate memory for more items. The number of free items is not guaranteed to remain above this threshold.

`pool_sethardlimit`()

pp
  The handle identifying the pool resource instance.
n
  The maximum number of total items in 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.

`pool_prime`()

pp
  The handle identifying the pool resource instance.
n
  The minimum number of total items for the pool. If the current number of total items is less than the new minimum then new items are allocated with blocking allocations.

## 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.

April 12, 2020 | NetBSD 11.0