.\"	$NetBSD: pool.9,v 1.50 2021/12/22 17:28:17 thorpej Exp $
.\"
.\" Copyright (c) 1997, 1998, 2007 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Paul Kranenburg.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
.\" POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd April 12, 2020
.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 ,
.Nm pool_sethardlimit
.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 "const char *wchan"
.Fa "struct pool_allocator *palloc"
.Fa "int ipl"
.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 void
.Fn pool_prime "struct pool *pp" "int n"
.Ft void
.Fn pool_sethiwat "struct pool *pp" "int n"
.Ft void
.Fn pool_setlowat "struct pool *pp" "int n"
.Ft void
.Fn pool_sethardlimit "struct pool *pp" "int n" \
"const char *warnmess" "int ratecap"
.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:
.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,
.Dv PR_NOTOUCH ,
or
.Dv PR_PSERIALIZE .
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.
If
.Dv PR_PSERIALIZE
is given, then the allocator guarantees that a passive serialization
barrier equivalent to
.Dq xc_barrier(0)
will be performed before the object's backing store is returned to
the system.
.Dv PR_PSERIALIZE
implies
.Dv PR_NOTOUCH .
Because of the guarantees provided by
.Dv PR_PSERIALIZE ,
objects muste never be freed to a pool using this option from either
hard or soft interrupt context, as doing so may block.
.It Fa wchan
The
.Sq wait channel
passed on to
.Xr cv_wait 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.
.It Fa ipl
Specifies an interrupt priority level that will block all interrupt
handlers that could potentially access the pool.
.El
.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.
If the
.Dv PR_ZERO
flag is specified, then the memory returned will be zeroed first using
.Xr memset 3 .
.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 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.
.Pp
The targets for the minimum and maximum number of free items which a pool should
try to keep available 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
The limits for the minimum and maximum number of total items (both free and
allocated) that the pool can have at any time are specified by the functions
.Fn pool_prime
and
.Fn pool_sethardlimit ,
respectively.
The defaults for these limits are
.Dv 0
and
.Dv UINT_MAX ,
respectively.
Changing these limits will cause memory to be immediately allocated to the pool
or freed from the pool as needed.
.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 free items to keep in the pool.
As items are returned and the total number of free items 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 free items to keep in the pool.
When the number of free items in the pool drops below this threshold,
a non-blocking attempt is made to allocate memory for more items.
The number of free items is not guaranteed to remain above this threshold.
.El
.Pp
.Fn pool_sethardlimit
.Bl -tag -offset indent -width "flags"
.It Fa pp
The handle identifying the pool resource instance.
.It Fa n
The maximum number of total items in the pool (i.e. the hard limit).
.It Fa warnmess
The warning message that will be logged when the hard limit is reached.
.It Fa ratecap
The minimal interval (in seconds) after which another warning message
is issued when the pool hits its hard limit again.
.El
.Pp
.Fn pool_prime
.Bl -tag -offset indent -width "storage"
.It Fa pp
The handle identifying the pool resource instance.
.It Fa n
The minimum number of total items for the pool.
If the current number of total items is less than the new minimum then new items
are allocated with blocking allocations.
.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 memoryallocators 9 ,
.Xr pool_cache 9 ,
.Xr uvm 9
.Sh HISTORY
The
.Nx
pool manager appeared in
.Nx 1.4 .