.\"	$NetBSD: pool.9,v 1.39 2005/10/03 16:52:17 wiz Exp $
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.Dd October 2, 2005
.Dt POOL 9
.Os
.Sh NAME
.Nm pool_init ,
.Nm pool_destroy ,
.Nm pool_get ,
.Nm pool_put ,
.Nm pool_prime ,
.Nm pool_sethiwat ,
.Nm pool_setlowat
.Nd resource-pool manager
.Sh SYNOPSIS
.In sys/pool.h
.Ft void
.Fo pool_init
.Fa "struct pool *pp"
.Fa "size_t size"
.Fa "u_int align"
.Fa "u_int align_offset"
.Fa "int flags"
.Fa "char *wchan"
.Fa "struct pool_allocator *palloc"
.Fc
.Ft void
.Fn pool_destroy "struct pool *pp"
.Ft void *
.Fn pool_get "struct pool *pp" "int flags"
.Ft void
.Fn pool_put "struct pool *pp" "void *item"
.Ft int
.Fn pool_prime "struct pool *pp" "int nitems"
.Ft void
.Fn pool_sethiwat "struct pool *pp" "int n"
.Ft void
.Fn pool_setlowat "struct pool *pp" "int n"
.Sh 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
.Pq Xr malloc 9 .
.Ss INITIALIZING A POOL
The function
.Fn pool_init
initializes a resource pool.
The arguments are:
.Pp
.Bl -tag -offset indent -width "align_offset"
.It Fa pp
The handle identifying the pool resource instance.
.It Fa size
Specifies the size of the memory items managed by the pool.
.It Fa align
Specifies the memory address alignment of the items returned by
.Fn pool_get .
This argument must be a power of two.
If zero,
the alignment defaults to an architecture-specific natural alignment.
.It Fa align_offset
The offset within an item to which the
.Fa align
parameter applies.
.It Fa flags
Should be set to zero or
.Dv PR_NOTOUCH .
If
.Dv 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.
.It Fa wchan
The
.Sq wait channel
passed on to
.Xr tsleep 9
if
.Fn pool_get
must wait for items to be returned to the pool.
.It Fa palloc
can be set to
.Dv NULL
or
.Dv pool_allocator_kmem ,
in which case the default kernel memory allocator will be used.
It can also be set to
.Dv pool_allocator_nointr
when the pool will never be accessed from interrupt context.
.El
.Pp
The
.Fn POOL_INIT
macro can be used to both declare and initialize a resource pool.
The
.Fn POOL_INIT
macro has the same arguments as the
.Fn pool_init
function and the resource pool will be initialized automatically
during system startup.
.Ss DESTROYING A POOL
The function
.Fn pool_destroy
destroys a resource pool.
It takes a single argument
.Fa pp
identifying the pool resource instance.
.Ss ALLOCATING ITEMS FROM A POOL
.Fn pool_get
allocates an item from the pool and returns a pointer to it.
The arguments are:
.Bl -tag -offset indent -width "flags"
.It Fa pp
The handle identifying the pool resource instance.
.It Fa flags
The flags can be used to define behaviour in case the pooled resources
are depleted.
If no resources are available and
.Dv PR_NOWAIT
is given,
.Fn pool_get
returns
.Dv NULL .
If
.Dv PR_WAITOK
is given and allocation is attempted with no resources available,
the function will sleep until items are returned to the pool.
.\"Undefined behaviour results if
.\".Dv PR_MALLOCOK
.\"is specified on a pool handle that was created using client-provided
.\"storage.
.\" a bunch of other flags aren't documented.
If both
.Dv PR_LIMITFAIL
and
.Dv PR_WAITOK
are specified, and the pool has reached its hard limit,
.Fn pool_get
will return
.Dv 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.
.El
.Ss RETURNING ITEMS TO A POOL
.Fn pool_put
returns the pool item pointed at by
.Fa item
to the resource pool identified by the pool handle
.Fa pp .
If the number of available items in the pool exceeds the maximum pool
size set by
.Fn pool_sethiwat
and there are no outstanding requests for pool items,
the excess items will be returned to the system.
The arguments to
.Fn pool_put
are:
.Bl -tag -offset indent -width "item"
.It Fa pp
The handle identifying the pool resource instance.
.It Fa item
A pointer to a pool item previously obtained by
.Fn pool_get .
.El
.Ss PRIMING A POOL
.Fn pool_prime
adds items to the pool.
Storage space for the items is allocated by using the page allocation
routine specified to
.Fn pool_create .
.Pp
The arguments to
.Fn pool_prime
are:
.Bl -tag -offset indent -width "storage"
.It Fa pp
The handle identifying the pool resource instance.
.It Fa nitems
The number of items to add to the pool.
.El
.Pp
This function may return
.Dv ENOMEM
in case the requested number of items could not be allocated.
Otherwise,
the return value is 0.
.Ss 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
.Sy watermarks .
The functions
.Fn pool_sethiwat
and
.Fn pool_setlowat
set a pool's high and low watermarks, respectively.
.Pp
.Fn pool_sethiwat
.Bl -tag -offset indent -width "flags"
.It Fa pp
The handle identifying the pool resource instance.
.It Fa 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.
.El
.Pp
.Fn pool_setlowat
.Bl -tag -offset indent -width "flags"
.It Fa pp
The handle identifying the pool resource instance.
.It Fa 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
.Fn pool_prime ,
this function does not allocate the necessary memory up-front.
.El
.Ss POTENTIAL PITFALLS
Note that undefined behaviour results when mixing the storage providing
methods supported by the pool resource routines.
.Pp
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
.Fn pool_init
and
.Fn pool_destroy
should never be called in interrupt context.
.Ss DIAGNOSTICS
Pool usage logs can be enabled by defining the compile-time option
.Dv POOL_DIAGNOSTIC .
.\" .Sh RETURN VALUES
.\" .Sh EXAMPLES
.Sh CODE REFERENCES
The pool manager is implemented in the file
.Pa sys/kern/subr_pool.c .
.\" .Sh AUTHOR
.Sh SEE ALSO
.Xr free 9 ,
.Xr malloc 9 ,
.Xr pool_cache 9 ,
.Xr uvm 9
.Sh HISTORY
The
.Nx
pool manager appeared in
.Nx 1.4 .