Memory Slices

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 (Jeff Bonwick, The slab allocator: An object-caching kernel memory allocator. USENIX 1994, and 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);