man/man9/objcache.9

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.Dd December 22, 2023
.Dt OBJCACHE 9
.Os
.Sh NAME
.Nm objcache_create ,
.Nm objcache_create_mbacked ,
.Nm objcache_create_simple ,
.Nm objcache_destroy ,
.Nm objcache_dtor ,
.Nm objcache_get ,
.Nm objcache_malloc_alloc ,
.Nm objcache_malloc_free ,
.Nm objcache_nop_alloc ,
.Nm objcache_nop_free ,
.\" .Nm objcache_populate_linear ,
.Nm objcache_put ,
.Nm objcache_reclaimlist
.Nd "object caching facility"
.Sh SYNOPSIS
.In sys/objcache.h
.Ft struct objcache *
.Fo objcache_create
.Fa "const char *name"
.Fa "int cluster_limit"
.Fa "int mag_capacity"
.Fa "objcache_ctor_fn *ctor"
.Fa "objcache_dtor_fn *dtor"
.Fa "void *privdata"
.Fa "objcache_alloc_fn *alloc"
.Fa "objcache_free_fn *free"
.Fa "void *allocator_args"
.Fc
.Ft struct objcache *
.Fo objcache_create_mbacked
.Fa "malloc_type_t mtype"
.Fa "size_t objsize"
.Fa "int cluster_limit"
.Fa "int mag_capacity"
.Fa "objcache_ctor_fn *ctor"
.Fa "objcache_dtor_fn *dtor"
.Fa "void *privdata"
.Fc
.Ft struct objcache *
.Fn objcache_create_simple "malloc_type_t mtype" "size_t objsize"
.Ft void
.Fn objcache_destroy "struct objcache *oc"
.Ft void
.Fn objcache_dtor "struct objcache *oc" "void *obj"
.Ft void *
.Fn objcache_get "struct objcache *oc" "int ocflags"
.Ft void *
.Fn objcache_malloc_alloc "void *allocator_args" "int ocflags"
.Ft void
.Fn objcache_malloc_free "void *obj" "void *allocator_args"
.Ft void *
.Fn objcache_nop_alloc "void *allocator_args" "int ocflags"
.Ft void
.Fn objcache_nop_free "void *obj" "void *allocator_args"
.\" .Ft void
.\" .Fo objcache_populate_linear
.\" .Fa "struct objcache *oc"
.\" .Fa "void *elts"
.\" .Fa "int nelts"
.\" .Fa "int size"
.\" .Fc
.Ft void
.Fn objcache_put "struct objcache *oc" "void *obj"
.Ft boolean_t
.Fn objcache_reclaimlist "struct objcache *oc[]" "int nlist"
.Sh DESCRIPTION
Object caching is a technique for manipulating objects that are frequently
allocated and freed.
The idea behind caching is to preserve the invariant portion of an object's
initial state between uses, so it does not have to be destroyed and reborn
every time the object is used.
.Pp
.Fn objcache_create
creates a new object cache.
It is identified by
.Fa name ,
which is used to distinguish the object in diagnostic output.
The
.Fa cluster_limit
determines the number of available magazines in the depot layer.
It must be at least
.Fa mag_capacity *
ncpus * 8.
If 0 is given, then there is no limit to the number of magazines the depot
can have (aside from the inherent limitation imposed by the restricted nature
of the back end allocator).
The
.Fa mag_capacity
describes the capacity of the magazine, that is the largest number of objects
it can hold.
If set to 0, the default value is used as defined in
.Pa sys/kern/kern_objcache.c .
Currently, the default value is 64.
The object caching system itself may adjust the cluster limit and/or
magazines' capacity based on the number of available CPUs.
.Fa ctor
specifies a function that constructs (i.e., performs the one-time
initialization of) an object in the cache.
It is defined as:
.Bd -literal
boolean_t foo_ctor(void *obj, void *privdata, int ocflags);
.Ed
.Pp
If no constructor is needed, it must be set to
.Dv NULL .
.Fa dtor
specifies a deconstructor function that destroys the cached object, before it
is released to the back end that manages the flow of real memory.
It is defined as:
.Bd -literal
void foo_dtor(void *obj, void *privdata);
.Ed
.Pp
If no deconstructor is needed, it must be set to
.Dv NULL .
The interface to underlying allocator is provided by
.Fa alloc ,
.Fa free
and
.Fa allocator_args .
It must adhere to the following form:
.Bd -literal
void *foo_alloc(void *allocator_args, int ocflags);
void foo_free(void *obj, void *allocator_args);
.Ed
.Pp
.Fn objcache_malloc_alloc
and
.Fn objcache_malloc_free
are wrappers for
.Xr kmalloc 9
allocation functions.
Whereas,
.Fn objcache_nop_alloc
and
.Fn objcache_nop_free
are wrappers for allocation policies that pre-allocate at initialization time
instead of doing run-time allocation.
.Pp
.Fn objcache_create_mbacked
creates a new object cache of size
.Fa objsize ,
backed with a
.Vt malloc_type_t
argument.
The latter is used to perform statistics in memory usage and for basic sanity
checks.
For the underlying allocator,
.Xr kmalloc 9
functions are employed.
.Pp
.Fn objcache_create_simple
creates a new object cache of size
.Fa objsize ,
backed with a
.Vt malloc_type_t
argument.
The
.Fa cluster_limit
is set to 0 and the default value for magazines' capacity is used.
.Fa ctor
and
.Fa dtor
are set to
.Dv NULL .
.Fa privdata
is  set to
.Dv NULL
as well.
For the underlying allocator,
.Xr kmalloc 9
functions are employed.
.Pp
.Fn objcache_get
returns an object from the
.Fa oc
object cache.
The object is in its initialized state.
Newly allocated objects are subjected to the object cache's constructor
function, if not
.Dv NULL ,
prior to being returned.
.Fa ocflags
is only used when the depot does not have any non-empty magazines and a new
object needs to be allocated using the back end allocator.
In this case we cannot depend on flags such as
.Dv M_ZERO .
If the back end allocator fails, or if the depot's object limit has been
reached and
.Dv M_WAITOK
is not specified,
.Dv NULL
is returned.
.Pp
.Fn objcache_put
returns
.Fa obj
to the
.Fa oc
object cache.
The object must be in its initialized state prior to this call.
If there is no empty magazine, the object deconstructor is called and
the object is freed.
.Pp
.Fn objcache_dtor
puts
.Fa obj
back into the
.Fa oc
object cache, indicating that the object is not in any shape to be reused and
should be deconstructed and freed immediately.
.Pp
.Fn objcache_reclaimlist
iterates over the
.Fa oclist[]
list with
.Fa nlist
elements and tries to free up some memory.
For each object cache in the reclaim list, the current per-CPU cache is tried
first and then the full magazine depot.
The function returns
.Dv TRUE
as soon as some free memory is found
and
.Dv FALSE
otherwise.
.Pp
.Fn objcache_destroy
destroys the
.Fa oc
object cache.
The object must have no existing references.
.\" .Pp
.\" .Fn objcache_populate_linear
.\" populates the per-cluster depot with elements from a linear block of memory.
.\" Must be called for individually for each cluster.
.\" Populated depots should not be destroyed.
.\" Currently this function is unimplemented.
.Sh IMPLEMENTATION NOTES
.Ss Magazine
A magazine is the very basic functional unit of the object caching scheme.
The number of objects it can hold is fixed and determined by its capacity.
The term magazine is used as an analogy with automatic weapon
(a firearm that can fire several rounds without reloading).
.Ss Per-CPU object cache
The reasoning behind per-CPU caches is to allow CPUs to perform their
transactions (i.e., allocations, frees) in a parallel, yet lockless manner.
.Pp
Each CPU is given two magazines, an active and a backup.
This is done in order to avoid a situation where a tight loop of
two allocations followed by two frees can cause thrashing at the
magazine boundary.
.Pp
If we need to add an object to the cache and the active magazine is full,
room is searched in the backup magazine.
If the backup has room, we swap active with backup and add the object.
If both magazines are full, we get an empty magazine from the depot
and move a fully loaded magazine to the depot.
.Ss Magazine depot
Each object cache manages a global supply of magazines, the depot, that is
available across all CPUs.
The depot maintains two lists of magazines.
One for completely full and one for completely free magazines.
The per-CPU object caches only exchange completely full or
completely empty magazines with the depot layer.
.Sh EXAMPLES
.Bd -literal
/* This is the data structure we are going to cache. */
struct foo {
        int x;
        char str[32];
};

MALLOC_DEFINE(M_FOOBUF, "foobuf", "Buffer to my little precious data");

struct objcache_malloc_args foo_malloc_args = {
        sizeof(struct foo), M_FOOBUF };

struct objcache *foo_cache;

/*
 * Object cache constructor.
 */
static boolean_t
foo_cache_ctor(void *obj, void *privdata, int ocflags)
{
        struct foo *myfoo = obj;

        /*
         * Do any initialization of the object here. Let's just zero out
         * the data structure for the fun of it.
         */
        bzero(myfoo, sizeof(*myfoo));

        return (TRUE);
}

/*
 * Object cache deconstructor.
 */
static void
foo_cache_dtor(void *obj, void *privdata)
{
        struct foo *myfoo = obj;

        /*
         * Do any clean up here. E.g., if you have kmalloc'ed() inside
         * the constructor, this is the right place and time to kfree().
         */
}

/*
 * Initialize our subsystem.
 */
static void
foo_init(void)
{
        /* Create the object cache. */
        foo_cache = objcache_create("foo",
            0,                          /* infinite depot's capacity */
            0,                          /* default magazine's capacity */
            foo_ctor, foo_dtor, NULL,
            objcache_malloc_alloc,
            objcache_malloc_free,
            &foo_malloc_args);
}

/*
 * Random function.
 */
static void
foo_random(...)
{
        struct foo *myfoo;

        /* Get a `foo' object from the object cache. */
        myfoo = objcache_get(foo_cache, M_WAITOK);

        /* Do stuff with it. */
        /* ... */

        /* We don't need it anymore. Put it back in object cache. */
        objcache_put(foo_cache, myfoo);
}

/*
 * Shutdown our subsystem.
 */
static void
foo_uninit(void)
{
        /* Destroy the object cache. */
        objcache_destroy(foo_cache);
}
.Ed
.Sh SEE ALSO
.Xr memory 9
.Rs
.%A "Jeff Bonwick"
.%T "The Slab Allocator: An Object-Caching Kernel Memory Allocator"
.%R "USENIX Summer 1994 Technical Conference"
.Re
.Rs
.%A "Jeff Bonwick"
.%A "Jonathan Adams"
.%T "Magazines and Vmem: Extending the Slab Allocator to Many CPUs and Arbitrary Resources"
.%R "USENIX 2001 Technical Conference"
.Re
.Sh HISTORY
The object caching system appeared in
.Dx 1.3 .
.Sh AUTHORS
The object caching system was written by
.An -nosplit
.An Jeffrey M. Hsu Aq Mt hsu@freebsd.org .
This manual page was written by
.An Stathis Kamperis Aq Mt ekamperi@gmail.com .