pool_init, pool_destroy, pool_get, pool_put, pool_prime, pool_sethiwat,
pool_setlowat - resource-pool manager
#include <sys/pool.h>
void
pool_init(struct pool *, size_t size, u_int align, u_int align_offset,
int flags, char *wchan, u_int pagesz,
void *(*palloc)(unsigned long sz, int flags, int tag),
void (*prelease)(void *v, unsigned long sz, int tag), int mtag);
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, caddr_t storage);
void
pool_sethiwat(struct pool *pp, int n);
void
pool_setlowat(struct pool *pp, int n);
POOL_STORAGE_SIZE(size, nitems);
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 can
optionally obtain temporary memory by calling the palloc() function
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.
INITIALIZING A POOL [Toc] [Back]
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 a architecturespecific
natural alignment.
align_offset
The offset within an item to which the align parameter
applies.
flags Should be set to zero.
wchan The `wait channel' passed on to tsleep(9) if pool_get()
must wait for items to be returned to the pool.
pagesz The unit which is used to allocate additional memory to
the pool. It must be a power of two.
palloc is called to add additional memory if the pool is
depleted. It must return pagesz aligned memory. The
argument sz shall be a multiple of pagesz.
prelease is called to release pages back to the system. palloc()
and prelease() may be NULL, in which case the pool manager
uses uvm_km_kmemalloc(9) and uvm_km_free(9) to allocate
and release memory using the kernel_map (see
uvm(9)).
mtag The memory tag passed to palloc() and prelease() when
allocating or releasing memory pages.
DESTROYING A POOL [Toc] [Back]
The function pool_destroy() destroys a resource pool. It takes a single
argument pp identifying the pool resource instance.
ALLOCATING ITEMS FROM A POOL [Toc] [Back]
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 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 is 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 [Toc] [Back]
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(). 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 [Toc] [Back]
pool_prime() adds items to the pool. Storage space for the items is
either allocated by using the page allocation routine specified to
pool_create(), or provided to pool_prime() by the caller through the
storage parameter.
The arguments to pool_prime() are:
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 [Toc] [Back]
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 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.
POTENTIAL PITFALLS [Toc] [Back]
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.
DIAGNOSTICS [Toc] [Back]
Pool usage logs can be enabled by defining the compile-time option
POOL_DIAGNOSTIC.
The pool manager is implemented in the file sys/kern/subr_pool.c.
free(9), malloc(9), uvm(9)
The NetBSD pool manager appeared in NetBSD 1.4.
BSD July 20, 2001 BSD
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