pool_init, pool_destroy, pool_get, pool_put, pool_prime,
pool_sethiwat,
pool_setlowat, pool_cache_init, pool_cache_destroy,
pool_cache_get,
pool_cache_put, pool_cache_destruct_object,
pool_cache_invalidate - resource-pool
manager
#include <sys/types.h>
#include <sys/pool.h>
void
pool_init(struct pool *pool, size_t size, u_int align,
u_int align_offset, int flags, const 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);
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);
void
pool_cache_init(struct pool_cache *pc, struct pool *pp,
int (*ctor)(void *, void *, int), void (*dtor)(void
*, void *),
void *arg);
void
pool_cache_destroy(struct pool_cache *pc);
void *
pool_cache_get(struct pool_cache *pc, int flags);
void
pool_cache_put(struct pool_cache *pc, void *object);
void
pool_cache_destruct_object(struct pool_cache *pc, void
*object);
void
pool_cache_invalidate(struct pool_cache *pc);
These utility routines provide management of pools of fixedsized 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 [Toc] [Back]
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.
align_offset The offset within an item to which the
align parameter
applies.
flags Specifies various flags set on the pool
at creation
time.
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 (see
uvm(9)). It is recommended to set this
to
pool_allocator_nointr if the pool will
never be accessed
in interrupt context, since that
allocator is
much more efficient.
DESTROYING A POOL [Toc] [Back]
The pool_destroy() function destroys a resource pool. It
takes a single
argument pp identifying the pool resource instance.
SETTING DRAIN CALLBACK [Toc] [Back]
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 [Toc] [Back]
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 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, 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().
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 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.
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
of 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.
SETTING HARD LIMITS [Toc] [Back]
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 [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.
POOL CACHES [Toc] [Back]
Another set of functions are available as extensions to the
pool manager.
The pool cache functions automatically call constructors and
destructors
when objects are allocated from the pool or returned to it.
They have
similar semantics as the other pool functions. The pp argument to
pool_cache_init() must already be initialized.
Objects are not immediately deconstructed when put into the
pool cache.
Instead, they are maintained for future allocations. When
the system determines
that memory needs to be reclaimed, then the deconstructor is
called on each free object and it is placed back into the
pool. The ctor
and dtor functions are passed arg and a pointer to the object, in that
order. The ctor is also passed the same flags that are
passed to
pool_cache_get(). The pool_cache_destruct_object() function
deconstructs
and puts an object back into the pool immediately.
pool_cache_invalidate() deconstructs all cached objects and
releases
their memory.
Pool caches are also commonly referred to as a slab allocator.
DIAGNOSTICS [Toc] [Back]
Pool usage logs can be enabled by defining the compile-time
option
POOL_DIAGNOSTIC.
DEBUGGING [Toc] [Back]
To debug a misbehaving pool, a kernel can be compiled with
the
MALLOC_DEBUG option and memory debugging on pools can be enabled with the
PR_DEBUG flag passed in the flags argument in the call to
pool_init().
See malloc(9) for more information about MALLOC_DEBUG.
The pool manager is implemented in the file
sys/kern/subr_pool.c.
free(9), malloc(9), uvm(9)
The pool manager first appeared in NetBSD 1.4 and was ported
to OpenBSD
by Artur Grabowski <[email protected]>.
OpenBSD 3.6 July 23, 1998
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