POOL(9) BSD 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);
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 con‐
tinue 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 parame‐
ter 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 automati‐
cally 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 speci‐
fied, 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 func‐
tions 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 allo‐
cate 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 warn‐
ing 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 reen‐
tered. 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 ALSOfree(9), malloc(9), memoryallocators(9), pool_cache(9), uvm(9)HISTORY
The NetBSD pool manager appeared in NetBSD 1.4.
BSD November 14, 2011 BSD
