UMEM_CACHE_CREATE(3MALLOC) Memory Allocation Library Functions


NAME


umem_cache_create, umem_cache_destroy, umem_cache_alloc, umem_cache_free
- allocation cache manipulation

SYNOPSIS


cc [ flag ... ] file... -lumem [ library ... ]
#include <umem.h>

umem_cache_t *umem_cache_create(char *debug_name, size_t bufsize,
size_t align, umem_constructor_t *constructor,
umem_destructor_t *destructor, umem_reclaim_t *reclaim,
void *callback_data, vmem_t *source, int cflags);


void umem_cache_destroy(umem_cache_t *cache);


void *umem_cache_alloc(umem_cache_t *cache, int flags);


void umem_cache_free(umem_cache_t *cache, void *buffer);


DESCRIPTION


These functions create, destroy, and use an "object cache" An object
cache is a collection of buffers of a single size, with optional content
caching enabled by the use of callbacks (see Cache Callbacks). Object
caches are MT-Safe. Multiple allocations and freeing of memory from
different threads can proceed simultaneously. Object caches are faster
and use less space per buffer than malloc(3MALLOC) and
umem_alloc(3MALLOC). For more information about object caching, see "The
Slab Allocator: An Object-Caching Kernel Memory Allocator" and "Magazines
and vmem: Extending the Slab Allocator to Many CPUs and Arbitrary
Resources".


The umem_cache_create() function creates object caches. Once a cache has
been created, objects can be requested from and returned to the cache
using umem_cache_alloc() and umem_cache_free(), respectively. A cache
with no outstanding buffers can be destroyed with umem_cache_destroy().

Creating and Destroying Caches


The umem_cache_create() function creates a cache of objects and takes as
arguments the following:

debug_name
A human-readable name for debugging purposes.


bufsize
The size, in bytes, of the buffers in this cache.


align
The minimum alignment required for buffers in this
cache. This parameter must be a power of 2. If 0, it is
replaced with the minimum required alignment for the
current architecture.


constructor
The callback to construct an object.


destructor
The callback to destroy an object.


reclaim
The callback to reclaim objects.


callback_data
An opaque pointer passed to the callbacks.


source
This parameter must be NULL.


cflags
This parameter must be either 0 or UMC_NODEBUG. If
UMC_NODEBUG, all debugging features are disabled for
this cache. See umem_debug(3MALLOC).


Each cache can have up to three associated callbacks:

int constructor(void *buffer, void *callback_data, int flags);
void destructor(void *buffer, void *callback_data);
void reclaim(void *callback_data);


The callback_data argument is always equal to the value passed to
umem_cache_create(), thereby allowing a client to use the same callback
functions for multiple caches, but with customized behavior.


The reclaim callback is called when the umem function is requesting more
memory from the operating system. This callback can be used by clients
who retain objects longer than they are strictly needed (for example,
caching non-active state). A typical reclaim callback might return to
the cache ten per cent of the unneeded buffers.


The constructor and destructor callbacks enable the management of buffers
with the constructed state. The constructor takes as arguments a buffer
with undefined contents, some callback data, and the flags to use for any
allocations. This callback should transform the buffer into the
constructed state.


The destructor callback takes as an argument a constructed object and
prepares it for return to the general pool of memory. The destructor
should undo any state that the constructor created. For debugging, the
destructor can also check that the buffer is in the constructed state, to
catch incorrectly freed buffers. See umem_debug(3MALLOC) for further
information on debugging support.


The umem_cache_destroy() function destroys an object cache. If the cache
has any outstanding allocations, the behavior is undefined.

Allocating Objects


The umem_cache_alloc() function takes as arguments:

cache
a cache pointer


flags
flags that determine the behavior if umem_cache_alloc() is
unable to fulfill the allocation request


If successful, umem_cache_alloc() returns a pointer to the beginning of
an object of bufsize length.


There are three cases to consider:

o A new buffer needed to be allocated. If the cache was created
with a constructor, it is applied to the buffer and the
resulting object is returned.

o The object cache was able to use a previously freed buffer.
If the cache was created with a constructor, the object is
returned unchanged from when it was freed.

o The allocation of a new buffer failed. The flags argument
determines the behavior:


UMEM_DEFAULT
The umem_cache_alloc() function returns NULL
if the allocation fails.


UMEM_NOFAIL
The umem_cache_alloc() function cannot return
NULL. A callback is used to determine what
action occurs. See umem_alloc(3MALLOC) for
more information.


Freeing Objects


The umem_cache_free() function takes as arguments:

cache
a cache pointer


buf
a pointer previously returned from umem_cache_alloc(). This
argument must not be NULL.


If the cache was created with a constructor callback, the object must be
returned to the constructed state before it is freed.


Undefined behavior results if an object is freed multiple times, if an
object is modified after it is freed, or if an object is freed to a cache
other than the one from which it was allocated.

Caches with Constructors


When a constructor callback is in use, there is essentially a contract
between the cache and its clients. The cache guarantees that all objects
returned from umem_cache_alloc() will be in the constructed state, and
the client guarantees that it will return the object to the constructed
state before handing it to umem_cache_free().

RETURN VALUES


Upon failure, the umem_cache_create() function returns a null pointer.

ERRORS


The umem_cache_create() function will fail if:

EAGAIN
There is not enough memory available to allocate the cache data
structure.


EINVAL
The debug_name argument is NULL, the align argument is not a
power of two or is larger than the system pagesize, or the
bufsize argument is 0.


ENOMEM
The libumem library could not be initialized, or the bufsize
argument is too large and its use would cause integer overflow
to occur.


EXAMPLES


Example 1: Use a fixed-size structure with no constructor callback.



#include <umem.h>

typedef struct my_obj {
long my_data1;
} my_obj_t;

/*
* my_objs can be freed at any time. The contents of
* my_data1 is undefined at allocation time.
*/

umem_cache_t *my_obj_cache;

...
my_obj_cache = umem_cache_create("my_obj", sizeof (my_obj_t),
0, NULL, NULL, NULL, NULL, NULL, 0);
...
my_obj_t *cur = umem_cache_alloc(my_obj_cache, UMEM_DEFAULT);
...
/* use cur */
...
umem_cache_free(my_obj_cache, cur);
...


Example 2: Use an object with a mutex.



#define _REENTRANT
#include <synch.h>
#include <umem.h>

typedef struct my_obj {
mutex_t my_mutex;
long my_data;
} my_obj_t;

/*
* my_objs can only be freed when my_mutex is unlocked.
*/
int
my_obj_constructor(void *buf, void *ignored, int flags)
{
my_obj_t *myobj = buf;

(void) mutex_init(&my_obj->my_mutex, USYNC_THREAD, NULL);

return (0);
}

void
my_obj_destructor(void *buf, void *ignored)
{
my_obj_t *myobj = buf;

(void) mutex_destroy(&my_obj->my_mutex);
}

umem_cache_t *my_obj_cache;

...
my_obj_cache = umem_cache_create("my_obj", sizeof (my_obj_t),
0, my_obj_constructor, my_obj_destructor, NULL, NULL,
NULL, 0);
...
my_obj_t *cur = umem_cache_alloc(my_obj_cache, UMEM_DEFAULT);
cur->my_data = 0; /* cannot assume anything about my_data */
...
umem_cache_free(my_obj_cache, cur);
...


Example 3: Use a more complex object with a mutex.



#define _REENTRANT
#include <assert.h>
#include <synch.h>
#include <umem.h>

typedef struct my_obj {
mutex_t my_mutex;
cond_t my_cv;
struct bar *my_barlist;
unsigned my_refcount;
} my_obj_t;

/*
* my_objs can only be freed when my_barlist == NULL,
* my_refcount == 0, there are no waiters on my_cv, and
* my_mutex is unlocked.
*/

int
my_obj_constructor(void *buf, void *ignored, int flags)
{
my_obj_t *myobj = buf;

(void) mutex_init(&my_obj->my_mutex, USYNC_THREAD, NULL);
(void) cond_init(&my_obj->my_cv, USYNC_THREAD, NULL);
myobj->my_barlist = NULL;
myobj->my_refcount = 0;

return (0);
}

void
my_obj_destructor(void *buf, void *ignored)
{
my_obj_t *myobj = buf;

assert(myobj->my_refcount == 0);
assert(myobj->my_barlist == NULL);
(void) cond_destroy(&my_obj->my_cv);
(void) mutex_destroy(&my_obj->my_mutex);
}

umem_cache_t *my_obj_cache;

...
my_obj_cache = umem_cache_create("my_obj", sizeof (my_obj_t),
0, my_obj_constructor, my_obj_destructor, NULL, NULL,
NULL, 0);
...
my_obj_t *cur = umem_cache_alloc(my_obj_cache, UMEM_DEFAULT);
...
/* use cur */
...
umem_cache_free(my_obj_cache, cur);
...


Example 4: Use objects with a subordinate buffer while reusing callbacks.



#include assert.h>
#include umem.h>

typedef struct my_obj {
char *my_buffer;
size_t my_size;
} my_obj_t;

/*
* my_size and the my_buffer pointer should never be changed
*/

int
my_obj_constructor(void *buf, void *arg, int flags)
{
size_t sz = (size_t)arg;

my_obj_t *myobj = buf;

if ((myobj->my_buffer = umem_alloc(sz, flags)) == NULL)
return (1);

my_size = sz;

return (0);
}

void
my_obj_destructor(void *buf, void *arg)
{
size_t sz = (size_t)arg;

my_obj_t *myobj = buf;

assert(sz == buf->my_size);
umem_free(myobj->my_buffer, sz);
}

...
umem_cache_t *my_obj_4k_cache;
umem_cache_t *my_obj_8k_cache;
...
my_obj_cache_4k = umem_cache_create("my_obj_4k", sizeof (my_obj_t),
0, my_obj_constructor, my_obj_destructor, NULL,
(void *)4096, NULL, 0);

my_obj_cache_8k = umem_cache_create("my_obj_8k", sizeof (my_obj_t),
0, my_obj_constructor, my_obj_destructor, NULL,
(void *)8192, NULL, 0);
...
my_obj_t *my_obj_4k = umem_cache_alloc(my_obj_4k_cache,
UMEM_DEFAULT);
my_obj_t *my_obj_8k = umem_cache_alloc(my_obj_8k_cache,
UMEM_DEFAULT);
/* no assumptions should be made about the contents
of the buffers */
...
/* make sure to return them to the correct cache */
umem_cache_free(my_obj_4k_cache, my_obj_4k);
umem_cache_free(my_obj_8k_cache, my_obj_8k);
...


See the EXAMPLES section of umem_alloc(3MALLOC) for examples involving
the UMEM_NOFAIL flag.

ATTRIBUTES


See attributes(5) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Committed |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO


setcontext(2), atexit(3C), libumem(3LIB), longjmp(3C), swapcontext(3C),
thr_exit(3C), umem_alloc(3MALLOC), umem_debug(3MALLOC), attributes(5)


Bonwick, Jeff, "The Slab Allocator: An Object-Caching Kernel Memory
Allocator", Proceedings of the Summer 1994 Usenix Conference.


Bonwick, Jeff and Jonathan Adams, "Magazines and vmem: Extending the Slab
Allocator to Many CPUs and Arbitrary Resources", Proceedings of the
Summer 2001 Usenix Conference.

WARNINGS


Any of the following can cause undefined results:

o Destroying a cache that has outstanding allocated buffers.

o Using a cache after it has been destroyed.

o Calling umem_cache_free() on the same buffer multiple times.

o Passing a NULL pointer to umem_cache_free().

o Writing past the end of a buffer.

o Reading from or writing to a buffer after it has been freed.

o Performing UMEM_NOFAIL allocations from an atexit(3C) handler.


Per-cache callbacks can be called from a variety of contexts. The use of
functions that modify the active context, such as setcontext(2),
swapcontext(3C), and thr_exit(3C), or functions that are unsafe for use
in multithreaded applications, such as longjmp(3C) and siglongjmp(3C),
result in undefined behavior.


A constructor callback that performs allocations must pass its flags
argument unchanged to umem_alloc(3MALLOC) and umem_cache_alloc(). Any
allocations made with a different flags argument results in undefined
behavior. The constructor must correctly handle the failure of any
allocations it makes.

NOTES


Object caches make the following guarantees about objects:

o If the cache has a constructor callback, it is applied to
every object before it is returned from umem_cache_alloc() for
the first time.

o If the cache has a constructor callback, an object passed to
umem_cache_free() and later returned from umem_cache_alloc()
is not modified between the two events.

o If the cache has a destructor, it is applied to all objects
before their underlying storage is returned.


No other guarantees are made. In particular, even if there are buffers
recently freed to the cache, umem_cache_alloc() can fail.


March 24, 2008 UMEM_CACHE_CREATE(3MALLOC)