.\"	$OpenBSD: pool.9,v 1.56 2017/08/14 09:51:35 jmc Exp $
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.Dd $Mdocdate: August 14 2017 $
.Dt POOL_INIT 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/types.h
.In sys/pool.h
.Ft void
.Fo pool_init
.Fa "struct pool *pool"
.Fa "size_t size"
.Fa "u_int align"
.Fa "int ipl"
.Fa "int flags"
.Fa "const char *wmesg"
.Fa "struct pool_allocator *palloc"
.Fc
.Ft void
.Fo pool_destroy
.Fa "struct pool *pp"
.Fc
.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"
.Ft int
.Fo pool_sethardlimit
.Fa "struct pool *pp"
.Fa "unsigned n"
.Fa "const char *warnmess"
.Fa "int ratecap"
.Fc
.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 .
The pool manager obtains memory by using the special-purpose memory
allocator
.Fa palloc
passed to
.Fn 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.
.Ss CREATING A POOL
The function
.Fn pool_init
initializes a resource pool.
The arguments are:
.Bl -tag -offset indent -width "align_offset"
.It Fa pool
Specifies the pool storage to be initialized.
.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 ipl
The interrupt protection level used to protect the pool's internals,
and at what level the pool can be safely used.
See
.Xr spl 9
for a list of the IPLs.
.It Fa flags
The bitwise OR of zero or more of the following values:
.Pp
.Bl -tag -width "PR_WAITOK" -offset indent -compact
.It Dv PR_WAITOK
The pool doesn't need to be interrupt safe.
It is recommended to specify this flag if the pool will never be
accessed in interrupt context.
.It Dv PR_RWLOCK
The pool will use an
.Xr rwlock 9
instead of a
.Xr mutex 9
for exclusion.
Requires
.Dv PR_WAITOK
to be specified as well, both to
.Fn pool_init
and on all
.Fn pool_get
calls on this pool.
.El
.It Fa wmesg
The message passed on to
.Xr tsleep 9
if
.Fn pool_get
must wait for items to be returned to the pool.
.It Fa palloc
The back-end allocator used to manage the memory for the pool.
.Fa palloc
may be
.Dv NULL ,
in which case the pool manager chooses an appropriate back-end allocator.
.El
.Ss DESTROYING A POOL
The
.Fn pool_destroy
function 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.
.Bl -tag -offset indent -width "flags"
.It Fa pp
The handle identifying the pool resource instance.
.It Fa flags
One or more flags.
Either
.Dv PR_WAITOK
or
.Dv PR_NOWAIT
must be specified
to define behaviour in case the pooled resources are depleted.
If no resources are available and
.Dv PR_NOWAIT
was specified,
this function returns
.Dv NULL .
If
.Dv PR_WAITOK
was specified
but
.Dv PR_LIMITFAIL
was not,
.Fn pool_get
will wait until items are returned to the pool.
If both
.Dv PR_WAITOK
and
.Dv PR_LIMITFAIL
were 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
.Dv PR_ZERO
was specified and an item has been successfully allocated, it is zeroed before
being returned to the caller.
.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 if possible.
.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
.Pp
If a non-interrupt safe allocator has been selected by passing the
.Dv PR_WAITOK
flag to
.Fn pmap_init ,
.Fn pool_put
may sleep when completely unused pages are released.
.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_init .
.Pp
.Fn pool_prime
.Bl -tag -offset indent -width "nitems"
.It Fa pp
The handle identifying the pool resource instance.
.It Fa nitems
The number of items to add to the pool.
.El
.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 of 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 SETTING HARD LIMITS
The function
.Fn pool_sethardlimit
sets a hard limit on the pool to
.Fa n
items.
If the hard limit is reached
.Fa warnmess
will be printed to the console, but no more than every
.Fa 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.
.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.
.Sh CONTEXT
.Fn pool_init ,
.Fn pool_destroy ,
.Fn pool_prime ,
.Fn pool_sethiwat ,
.Fn pool_setlowat ,
and
.Fn pool_sethardlimit
can be called during autoconf or from process context.
.Pp
.Fn pool_get
and
.Fn pool_put
can be called during autoconf or from process context.
If the pool has been initialised with an interrupt safe pool allocator
they can also be called from interrupt context at or below the
interrupt level specified by a call to
.Fn pool_init .
.Sh RETURN VALUES
.Fn pool_get
will return a pointer to an item allocated from the pool.
If
.Dv PR_NOWAIT
or
.Dv PR_LIMITFAIL
were passed as flags to the pool it may return
.Dv NULL
if there are no resources available or if the pool hard limit has been reached,
respectively.
.Pp
.Fn pool_prime
will return
.Dv ENOMEM
if the requested number of items could not be allocated.
Otherwise, the return value is 0.
.Pp
.Fn pool_sethardlimit
will return
.Dv EINVAL
if the current size of the pool exceeds the requested hard limit.
Otherwise, the return value is 0.
.Sh CODE REFERENCES
The pool manager is implemented in the file
.Pa sys/kern/subr_pool.c .
.Sh SEE ALSO
.Xr free 9 ,
.Xr malloc 9 ,
.Xr mutex 9 ,
.Xr rwlock 9 ,
.Xr spl 9 ,
.Xr uvm 9
.Sh HISTORY
The pool manager first appeared in
.Nx 1.4
and was ported to
.Ox
by
.An Artur Grabowski Aq Mt art@openbsd.org .