NAME
pool_init,
pool_destroy,
pool_get,
pool_put,
pool_prime,
pool_sethiwat,
pool_setlowat,
pool_sethardlimit —
resource-pool manager
SYNOPSIS
#include <sys/pool.h>
void
pool_init(
struct pool *pp,
size_t size,
u_int align,
u_int align_offset,
int flags,
const char *wchan,
struct pool_allocator
*palloc,
int ipl);
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);
void
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 continue operation
independent of the memory resources currently available from the system-wide
memory allocator (
malloc(9)).
INITIALIZING A POOL
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 an architecture-specific natural
alignment.
-
-
- align_offset
- The offset within an item to which the
align parameter applies.
-
-
- flags
- Should be set to zero or
PR_NOTOUCH
. If PR_NOTOUCH
is given, free items are never used to keep internal state so that the
pool can be used for non memory backed objects.
-
-
- wchan
- The ‘wait channel’ passed on to
cv_wait(9) if
pool_get() must wait for items to be returned to the
pool.
-
-
- palloc
- Can be set to
NULL
or
pool_allocator_kmem
, in which case the default
kernel memory allocator will be used. It can also be set to
pool_allocator_nointr
when the pool will never be
accessed from interrupt context.
-
-
- ipl
- Specifies an interrupt priority level that will block all
interrupt handlers that could potentially access the pool.
DESTROYING A POOL
The function
pool_destroy() 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. The arguments are:
-
-
- pp
- The handle identifying the pool resource instance.
-
-
- flags
- The flags can be used to define behaviour in case the
pooled resources are depleted. If no resources are available and
PR_NOWAIT
is given, pool_get()
returns NULL
. If PR_WAITOK
is given and allocation is attempted with no resources available, the
function will sleep until items are returned to the pool. 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
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. 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
pool_prime() adds items to the pool. Storage space for the
items is allocated by using the page allocation routine specified to
pool_create().
The arguments to
pool_prime() are:
-
-
- 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 AND LIMITS
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.
The hard limit represents the maximum number of items a pool is allowed to
allocate at any given time. Unless modified via
pool_sethardlimit(), the hard limit defaults to
UINT_MAX
.
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
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.
pool_sethardlimit()
-
-
- pp
- The handle identifying the pool resource instance.
-
-
- n
- The maximum number of items to be allocated from the pool
(i.e. the hard limit).
-
-
- warnmess
- The warning message that will be logged when the hard limit
is reached.
-
-
- ratecap
- The minimal interval (in seconds) after which another
warning message is issued when the pool hits its hard limit again.
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. Additionally, the
functions
pool_init() and
pool_destroy()
should never be called in interrupt context.
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),
memoryallocators(9),
pool_cache(9),
uvm(9)
HISTORY
The
NetBSD pool manager appeared in
NetBSD 1.4.