NAME
pool_init
,
pool_destroy
, pool_get
,
pool_put
, pool_prime
,
pool_setipl
, 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,
u_int align_offset, 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_setipl
(struct
pool *pp, int
ipl);
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. - 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 an interrupt safe allocator. It is recommended that this be set to pool_allocator_nointr if the pool will never be accessed in an interrupt context.
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 of
PR_URGENT
,PR_WAITOK
,PR_NOWAIT
orPR_LIMITFAIL
, that define behaviour in case the pooled resources are depleted. If no resources are available andPR_WAITOK
is given, this function will wait until items are returned to the pool. Otherwisepool_get
() returnsNULL
.PR_NOWAIT
should be provided in cases wherePR_WAITOK
is not. IfPR_URGENT
is specified and no items are available and palloc cannot allocate a new page, the system will panic (XXX). If bothPR_LIMITFAIL
andPR_WAITOK
are specified, and the pool has reached its hard limit,pool_get
() will returnNULL
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. IfPR_ZERO
is passed 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 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 allocated by using the page allocation
routine specified to pool_init
().
- 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
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.
- 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.
- 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 THE PROTECTION LEVEL
The pool_setipl
() function is used to
specify the interrupt protection level at which the pool can be safely
used.
- pp
- The handle identifying the pool resource instance.
- ipl
- The interrupt protection level used to protect the pool's internals. See spl(9) for a list of the IPLs.
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.
DEBUGGING
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
. Alternatively, the
PR_DEBUGCHK
flag can be passed to enable pool
internal consistency checks before and after each allocation and free.
CODE REFERENCES
The pool manager is implemented in the file sys/kern/subr_pool.c.
SEE ALSO
HISTORY
The pool manager first appeared in NetBSD 1.4 and was ported to OpenBSD by Artur Grabowski ⟨art@openbsd.org⟩.