Memory Slices: GLib Reference Manual

Memory Slices

Memory Slices — efficient way to allocate groups of equal-sized chunks of memory

Functions

gpointer	g_slice_alloc ()
gpointer	g_slice_alloc0 ()
gpointer	g_slice_copy ()
void	g_slice_free1 ()
void	g_slice_free_chain_with_offset ()
#define	g_slice_new()
#define	g_slice_new0()
#define	g_slice_dup()
#define	g_slice_free()
#define	g_slice_free_chain()

Includes

#include <gmodule.h>

Description

Memory slices provide a space-efficient and multi-processing scalable way to allocate equal-sized pieces of memory, just like the original GMemChunks (from GLib 2.8), while avoiding their excessive memory-waste, scalability and performance problems.

To achieve these goals, the slice allocator uses a sophisticated, layered design that has been inspired by Bonwick's slab allocator (Bonwick94 Jeff Bonwick, The slab allocator: An object-caching kernel memory allocator. USENIX 1994, and Bonwick01 Bonwick and Jonathan Adams, Magazines and vmem: Extending the slab allocator to many cpu's and arbitrary resources. USENIX 2001)

It uses posix_memalign() to optimize allocations of many equally-sized chunks, and has per-thread free lists (the so-called magazine layer) to quickly satisfy allocation requests of already known structure sizes. This is accompanied by extra caching logic to keep freed memory around for some time before returning it to the system. Memory that is unused due to alignment constraints is used for cache colorization (random distribution of chunk addresses) to improve CPU cache utilization. The caching layer of the slice allocator adapts itself to high lock contention to improve scalability.

The slice allocator can allocate blocks as small as two pointers, and unlike malloc(), it does not reserve extra space per block. For large block sizes, g_slice_new() and g_slice_alloc() will automatically delegate to the system malloc() implementation. For newly written code it is recommended to use the new g_slice API instead of g_malloc() and friends, as long as objects are not resized during their lifetime and the object size used at allocation time is still available when freeing.

Here is an example for using the slice allocator:

gchar *mem[10000];
gint i;

// Allocate 10000 blocks.
for (i = 0; i < 10000; i++)
  {
    mem[i] = g_slice_alloc (50);

    // Fill in the memory with some junk.
    for (j = 0; j < 50; j++)
      mem[i][j] = i * j;
  }

// Now free all of the blocks.
for (i = 0; i < 10000; i++)
  g_slice_free1 (50, mem[i]);

And here is an example for using the using the slice allocator with data structures:

GRealArray *array;

// Allocate one block, using the g_slice_new() macro.
array = g_slice_new (GRealArray);

// We can now use array just like a normal pointer to a structure.
array->data            = NULL;
array->len             = 0;
array->alloc           = 0;
array->zero_terminated = (zero_terminated ? 1 : 0);
array->clear           = (clear ? 1 : 0);
array->elt_size        = elt_size;

// We can free the block, so it can be reused.
g_slice_free (GRealArray, array);

Functions

g_slice_alloc ()

gpointer
g_slice_alloc (gsize block_size);

Allocates a block of memory from the slice allocator.

The block address handed out can be expected to be aligned to at least 1 * sizeof (void*), though in general slices are 2 * sizeof (void*) bytes aligned; if a malloc() fallback implementation is used instead, the alignment may be reduced in a libc dependent fashion.

Note that the underlying slice allocation mechanism can be changed with the G_SLICE=always-malloc environment variable.

Parameters

block_size

the number of bytes to allocate

Returns

a pointer to the allocated memory block, which will be NULL if and only if mem_size is 0

Since: 2.10

g_slice_alloc0 ()

gpointer
g_slice_alloc0 (gsize block_size);

Allocates a block of memory via g_slice_alloc() and initializes the returned memory to 0. Note that the underlying slice allocation mechanism can be changed with the G_SLICE=always-malloc environment variable.

Parameters

block_size

the number of bytes to allocate

Returns

a pointer to the allocated block, which will be NULL if and only if mem_size is 0

Since: 2.10

g_slice_copy ()

gpointer
g_slice_copy (gsize block_size,
              gconstpointer mem_block);

Allocates a block of memory from the slice allocator and copies block_size bytes into it from mem_block .

mem_block must be non-NULL if block_size is non-zero.

Parameters

block_size	the number of bytes to allocate
mem_block	the memory to copy

Returns

a pointer to the allocated memory block, which will be NULL if and only if mem_size is 0

Since: 2.14

g_slice_free1 ()

void
g_slice_free1 (gsize block_size,
               gpointer mem_block);

Frees a block of memory.

The memory must have been allocated via g_slice_alloc() or g_slice_alloc0() and the block_size has to match the size specified upon allocation. Note that the exact release behaviour can be changed with the G_DEBUG=gc-friendly environment variable, also see G_SLICE for related debugging options.

If mem_block is NULL, this function does nothing.

Parameters

block_size	the size of the block
mem_block	a pointer to the block to free

Since: 2.10

g_slice_free_chain_with_offset ()

void
g_slice_free_chain_with_offset (gsize block_size,
                                gpointer mem_chain,
                                gsize next_offset);

Frees a linked list of memory blocks of structure type type .

The memory blocks must be equal-sized, allocated via g_slice_alloc() or g_slice_alloc0() and linked together by a next pointer (similar to GSList). The offset of the next field in each block is passed as third argument. Note that the exact release behaviour can be changed with the G_DEBUG=gc-friendly environment variable, also see G_SLICE for related debugging options.

If mem_chain is NULL, this function does nothing.

Parameters

block_size	the size of the blocks
mem_chain	a pointer to the first block of the chain
next_offset	the offset of the `next` field in the blocks

Since: 2.10

g_slice_new()

#define             g_slice_new(type)

A convenience macro to allocate a block of memory from the slice allocator.

It calls g_slice_alloc() with sizeof (@type) and casts the returned pointer to a pointer of the given type, avoiding a type cast in the source code. Note that the underlying slice allocation mechanism can be changed with the G_SLICE=always-malloc environment variable.

This can never return NULL as the minimum allocation size from sizeof (@type) is 1 byte.

Parameters

type

the type to allocate, typically a structure name

Returns

a pointer to the allocated block, cast to a pointer to type .

[not nullable]

Since: 2.10

g_slice_new0()

#define             g_slice_new0(type)

A convenience macro to allocate a block of memory from the slice allocator and set the memory to 0.

It calls g_slice_alloc0() with sizeof (@type) and casts the returned pointer to a pointer of the given type, avoiding a type cast in the source code. Note that the underlying slice allocation mechanism can be changed with the G_SLICE=always-malloc environment variable.

This can never return NULL as the minimum allocation size from sizeof (@type) is 1 byte.

Parameters

type

the type to allocate, typically a structure name

Returns

a pointer to the allocated block, cast to a pointer to type .

[not nullable]

Since: 2.10

g_slice_dup()

#define             g_slice_dup(type, mem)

A convenience macro to duplicate a block of memory using the slice allocator.

It calls g_slice_copy() with sizeof (@type) and casts the returned pointer to a pointer of the given type, avoiding a type cast in the source code. Note that the underlying slice allocation mechanism can be changed with the G_SLICE=always-malloc environment variable.

This can never return NULL.

Parameters

type	the type to duplicate, typically a structure name
mem	the memory to copy into the allocated block.	[not nullable]

Returns

a pointer to the allocated block, cast to a pointer to type .

[not nullable]

Since: 2.14

g_slice_free()

#define             g_slice_free(type, mem)

A convenience macro to free a block of memory that has been allocated from the slice allocator.

It calls g_slice_free1() using sizeof (type) as the block size. Note that the exact release behaviour can be changed with the G_DEBUG=gc-friendly environment variable, also see G_SLICE for related debugging options.

If mem is NULL, this macro does nothing.

Parameters

type	the type of the block to free, typically a structure name
mem	a pointer to the block to free

Since: 2.10

g_slice_free_chain()

#define             g_slice_free_chain(type, mem_chain, next)

Frees a linked list of memory blocks of structure type type .

The memory blocks must be equal-sized, allocated via g_slice_alloc() or g_slice_alloc0() and linked together by a next pointer (similar to GSList). The name of the next field in type is passed as third argument. Note that the exact release behaviour can be changed with the G_DEBUG=gc-friendly environment variable, also see G_SLICE for related debugging options.

If mem_chain is NULL, this function does nothing.

Parameters

type	the type of the `mem_chain` blocks
mem_chain	a pointer to the first block of the chain
next	the field name of the next pointer in `type`

Since: 2.10