POOL_INIT(9) FreeBSD Kernel Developer's Manual POOL_INIT(9)
NAME
pool_init, pool_destroy, pool_get, pool_put, pool_prime, pool_sethiwat,
pool_setlowat, pool_sethardlimit - resource-pool manager
SYNOPSIS
#include <sys/types.h>
#include <sys/pool.h>
void
pool_init(struct pool *pool, size_t size, u_int align, int ipl,
int flags, const char *wmesg, struct pool_allocator *palloc);
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);
int
pool_sethardlimit(struct pool *pp, unsigned 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)). 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.
ipl The interrupt protection level used to protect the
pool's internals, and at what level the pool can be
safely used. See spl(9) for a list of the IPLs.
flags The bitwise OR of zero or more of the following
values:
PR_WAITOK The pool doesn't need to be
interrupt safe. It is recommended
to specify this flag if the pool
will never be accessed in interrupt
context.
PR_RWLOCK The pool will use an rwlock(9)
instead of a mutex(9) for exclusion.
Requires PR_WAITOK to be specified
as well, both to pool_init() and on
all pool_get() calls on this pool.
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 chooses an appropriate back-end allocator.
DESTROYING A POOL
The pool_destroy() function 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.
pp The handle identifying the pool resource instance.
flags One or more flags. Either PR_WAITOK or PR_NOWAIT must be
specified to define behaviour in case the pooled resources
are depleted. If no resources are available and PR_NOWAIT
was specified, this function returns NULL. If PR_WAITOK was
specified but PR_LIMITFAIL was not, pool_get() will wait
until items are returned to the pool. If both PR_WAITOK and
PR_LIMITFAIL were 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 PR_ZERO was specified and an item has been
successfully allocated, it is zeroed before being returned
to the caller.
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 if possible.
pp The handle identifying the pool resource instance.
item A pointer to a pool item previously obtained by pool_get().
If a non-interrupt safe allocator has been selected by passing the
PR_WAITOK flag to pool_init(), pool_put() may sleep when completely
unused pages are released.
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_init().
pool_prime()
pp The handle identifying the pool resource instance.
nitems The number of items to add to the pool.
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. 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
of pages in the pool will not decrease below the required
value to accommodate the minimum number of items specified
by this function.
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
console, 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
reentered.
CONTEXT
pool_init(), pool_destroy(), pool_prime(), pool_sethiwat(),
pool_setlowat(), and pool_sethardlimit() can be called during autoconf or
from process context.
pool_get() and pool_put() can be called during autoconf or from process
context. If the pool has been initialised with an interrupt safe pool
allocator they can also be called from interrupt context at or below the
interrupt level specified by a call to pool_init().
RETURN VALUES
pool_get() will return a pointer to an item allocated from the pool. If
PR_NOWAIT or PR_LIMITFAIL were passed as flags to the pool it may return
NULL if there are no resources available or if the pool hard limit has
been reached, respectively.
pool_prime() will return ENOMEM if the requested number of items could
not be allocated. Otherwise, the return value is 0.
pool_sethardlimit() will return EINVAL if the current size of the pool
exceeds the requested hard limit. Otherwise, the return value is 0.
CODE REFERENCES
The pool manager is implemented in the file sys/kern/subr_pool.c.
SEE ALSO
free(9), km_alloc(9), malloc(9), mutex(9), rwlock(9), spl(9)
HISTORY
The pool manager first appeared in NetBSD 1.4 and was ported to OpenBSD
by Artur Grabowski <
[email protected]>.
FreeBSD 14.1-RELEASE-p8 May 20, 2024 FreeBSD 14.1-RELEASE-p8