POOL(9) OpenBSD Kernel Manual POOL(9)
NAME
pool_init, pool_destroy, pool_get, pool_put, pool_prime, pool_sethiwat,
pool_setlowat - resource-pool manager
SYNOPSIS
#include <sys/pool.h>
void
pool_init(struct pool *pool, size_t size, u_int align,
u_int align_offset, int nitems, 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, caddr_t storage);
void
pool_sethiwat(struct pool *pp, int n);
void
pool_setlowat(struct pool *pp, int n);
int
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)). The pool manager ob-
tains memory by using the special-purpose memory allocator palloc()
passed to pool_init(), for extra pool items in case the number of alloca-
tions exceeds the nominal number of pool items managed by a pool re-
source. This temporary memory will be automatically returned to the sys-
tem 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 pow-
er of two. If zero, the alignment defaults to an ar-
chitecture-specific natural alignment.
align_offset The offset within an item to which the align parame-
ter applies.
nitems Specifies the number of memory items that are allo-
cated to the pool at creation time. This number may
be zero, in which case pool_prime() can be used at a
later time to add permanent items to the 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 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
(seeuvm(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.
DESTROYING A POOL
The pool_destroy() function destroys a resource pool. It takes a single
argument pp identifying the pool resource instance.
SETTING DRAIN CALLBACK
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.
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 of 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.
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 by calling prelease().
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 ei-
ther allocated by using the page allocation routine specified to
pool_init(), or provided to pool_prime() by the caller through the
storage parameter.
pool_prime()
pp The handle identifying the pool resource instance.
nitems The number of items to add to the pool.
storage
Optional pre-allocated storage.
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
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
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.
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 con-
sole, but no more than every ratecap seconds. Upon successful comple-
tion, 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 reen-
tered.
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), uvm(9)
HISTORY
The pool manager first appeared in NetBSD 1.4 and was ported to OpenBSD
by Artur Grabowski <art@openbsd.org>.
OpenBSD 3.2 July 23, 1998 4