kmem_cache_create

, kmem_cache_alloc

, kmem_cache_free

, kmem_cache_destroy

- kernel memory cache allocator operations

Synopsis

#include <sys/types.h>
#include <sys/kmem.h>

kmem_cache_t *kmem_cache_create(char *name, size_t bufsize, 
     size_t align, int (*constructor)(void *, void *, int), 
     void (*destructor)(void *, void *), void (*reclaim)(void *), 
     void *private, void *vmp, int cflags);

void kmem_cache_destroy(kmem_cache_t *cp);

void *kmem_cache_alloc(kmem_cache_t *cp, intkmflag);

void kmem_cache_free(kmem_cache_t *cp, void *obj);

 [Synopsis for callback functions:]

int (*constructor)(void *buf, void *un, int kmflags);

void (*destructor)(void *buf, void *un);

Interface Level

Solaris DDI specific (Solaris DDI)

Parameters

The parameters for the kmem_cache_* functions are as follows:

name

Descriptive name of a kstat(9S) structure of class kmem_cache. Only alphanumeric characters can be used in name.

bufsize

Size of the objects it manages.

align

Required object alignment.

constructor

Pointer to an object constructor function. Parameters are defined below.

destructor

Pointer to an object destructor function. Parameters are defined below.

reclaim

Drivers should pass NULL.

private

Pass-through argument for constructor/destructor.

vmp

Drivers should pass NULL.

cflags

Drivers must pass 0.

kmflag

Possible flags are:

KM_SLEEP

Allow sleeping (blocking) until memory is available.

KM_NOSLEEP

Return NULL immediately if memory is not available.

KM_PUSHPAGE

Allow the allocation to use reserved memory.

*obj

Pointer to the object allocated by kmem_cache_alloc().

The parameters for the callback constructor function are as follows:

void *buf

Pointer to the object to be constructed.

void *un

The private parameter from the call to kmem_cache_create(); it is typically a pointer to the soft-state structure.

int kmflags

Propagated kmflag values.

The parameters for the callback destructor function are as follows:

void *buf

Pointer to the object to be deconstructed.

void *un

The private parameter from the call to kmem_cache_create(); it is typically a pointer to the soft-state structure.

Description

In many cases, the cost of initializing and destroying an object exceeds the cost of allocating and freeing memory for it. The functions described here address this condition.

Object caching is a technique for dealing with objects that are:

• frequently allocated and freed, and

• have setup and initialization costs.

The idea is to allow the allocator and its clients to cooperate to preserve the invariant portion of an object's initial state, or constructed state, between uses, so it does not have to be destroyed and re-created every time the object is used. For example, an object containing a mutex only needs to have mutex_init() applied once, the first time the object is allocated. The object can then be freed and reallocated many times without incurring the expense of mutex_destroy() and mutex_init() each time. An object's embedded locks, condition variables, reference counts, lists of other objects, and read-only data all generally qualify as constructed state. The essential requirement is that the client must free the object (using kmem_cache_free()) in its constructed state. The allocator cannot enforce this, so programming errors will lead to hard-to-find bugs.

A driver should call kmem_cache_create() at the time of _fini(9E) or attach(9E), and call the corresponding kmem_cache_destroy() at the time of _fini(9E) or detach(9E).

kmem_cache_create() creates a cache of objects, each of size size bytes, aligned on an align boundary. Drivers not requiring a specific alignment can pass 0. name identifies the cache for statistics and debugging. constructor and destructor convert plain memory into objects and back again; constructor can fail if it needs to allocate memory but cannot. private is a parameter passed to the constructor and destructor callbacks to support parameterized caches (for example, a pointer to an instance of the driver's soft-state structure). To facilitate debugging, kmem_cache_create() creates a kstat(9S) structure of class kmem_cache and name name. It returns an opaque pointer to the object cache.

kmem_cache_alloc() gets an object from the cache. The object will be in its constructed state. kmflag has either KM_SLEEP or KM_NOSLEEP set, indicating whether it is acceptable to wait for memory if none is currently available.

A small pool of reserved memory is available to allow the system to progress toward the goal of freeing additional memory while in a low memory situation. The KM_PUSHPAGE flag enables use of this reserved memory pool on an allocation. This flag can be used by drivers that implement strategy(9E) on memory allocations associated with a single I/O operation. The driver guarantees that the I/O operation will complete (or timeout) and, on completion, that the memory will be returned. The KM_PUSHPAGE flag should be used only in kmem_cache_alloc() calls. All allocations from a given cache should be consistent in their use of the flag. A driver that adheres to these restrictions can guarantee progress in a low memory situation without resorting to complex private allocation and queuing schemes. If KM_PUSHPAGE is specified, KM_SLEEP can also be used without causing deadlock.

kmem_cache_free() returns an object to the cache. The object must be in its constructed state.

kmem_cache_destroy() destroys the cache and releases all associated resources. All allocated objects must have been previously freed.

Context

Constructors can be invoked during any call to kmem_cache_alloc(), and will run in that context. Similarly, destructors can be invoked during any call to kmem_cache_free(), and can also be invoked during kmem_cache_destroy(). Therefore, the functions that a constructor or destructor invokes must be appropriate in that context.

kmem_cache_create() and kmem_cache_destroy() must not be called from interrupt context.

kmem_cache_alloc() can be called from interrupt context only if the KM_NOSLEEP flag is set. It can be called from user or kernel context with any valid flag.

kmem_cache_free() can be called from user, kernel, or interrupt context.

Examples

Example 1 Object Caching

Consider the following data structure:

struct foo {
    kmutex_t foo_lock;
    kcondvar_t foo_cv;
    struct bar *foo_barlist;
    int foo_refcnt;
    };

Assume that a foo structure cannot be freed until there are no outstanding references to it (foo_refcnt == 0) and all of its pending bar events (whatever they are) have completed (foo_barlist == NULL). The life cycle of a dynamically allocated foo would be something like this:

foo = kmem_alloc(sizeof (struct foo), KM_SLEEP);
mutex_init(&foo->foo_lock, ...);
cv_init(&foo->foo_cv, ...);
foo->foo_refcnt = 0;
foo->foo_barlist = NULL;
    use foo;
ASSERT(foo->foo_barlist == NULL);
ASSERT(foo->foo_refcnt == 0);
cv_destroy(&foo->foo_cv);
mutex_destroy(&foo->foo_lock);
kmem_free(foo);

Notice that between each use of a foo object we perform a sequence of operations that constitutes nothing but expensive overhead. All of this overhead (that is, everything other than use foo above) can be eliminated by object caching.

int
foo_constructor(void *buf, void *arg, int tags)
{
    struct foo *foo = buf;
    mutex_init(&foo->foo_lock, ...);
    cv_init(&foo->foo_cv, ...);
    foo->foo_refcnt = 0;
    foo->foo_barlist = NULL;
    return (0);
}

void
foo_destructor(void *buf, void *arg)
{
    struct foo *foo = buf;
    ASSERT(foo->foo_barlist == NULL);
    ASSERT(foo->foo_refcnt == 0);
    cv_destroy(&foo->foo_cv);
    mutex_destroy(&foo->foo_lock);
}

un = ddi_get_soft_state(foo_softc, instance);
(void) snprintf(buf, KSTAT_STRLEN, "foo%d_cache",
        ddi_get_instance(dip));
foo_cache = kmem_cache_create(buf,
        sizeof (struct foo), 0,
        foo_constructor, foo_destructor,
        NULL, un, 0);

To allocate, use, and free a foo object:

foo = kmem_cache_alloc(foo_cache, KM_SLEEP);
    use foo;
kmem_cache_free(foo_cache, foo);

This makes foo allocation fast, because the allocator will usually do nothing more than fetch an already-constructed foo from the cache. foo_constructor and foo_destructor will be invoked only to populate and drain the cache, respectively.

Return Values

If successful, the constructor function must return 0. If KM_NOSLEEP is set and memory cannot be allocated without sleeping, the constructor must return -1.

kmem_cache_create() returns a pointer to the allocated cache. If the name parameter contains non-alphanumeric characters, kmem_cache_create() returns NULL.

If successful, kmem_cache_alloc() returns a pointer to the allocated object. If KM_NOSLEEP is set and memory cannot be allocated without sleeping, kmem_cache_alloc() returns NULL.

Attributes

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

See Also

condvar(9F), kmem_alloc(9F), mutex(9F), kstat(9S)

Writing Device Drivers

The Slab Allocator: An Object-Caching Kernel Memory Allocator, Bonwick, J.; USENIX Summer 1994 Technical Conference (1994).