stdx.allocator.building_blocks.bitmapped_block

  • BitmappedBlock

    Declaration

    struct BitmappedBlock(size_t theBlockSize, uint theAlignment = platformAlignment, ParentAllocator = NullAllocator);

    implements a simple heap consisting of one contiguous area of memory organized in blocks, each of size . A block is a unit of allocation. A bitmap serves as bookkeeping data, more precisely one bit per block indicating whether that block is currently allocated or not.

    Discussion

    Passing as (the default) means user code manages allocation of the memory block from the outside; in that case must be constructed with a preallocated block and has no responsibility regarding the lifetime of its support underlying storage. If another allocator type is passed, defines a destructor that uses the parent allocator to release the memory block. That makes the combination of , , and a back-end allocator such as a simple and scalable solution for memory allocation.

    There are advantages to storing bookkeeping data separated from the payload (as opposed to e.g. using to store metadata together with each allocation). The layout is more compact (overhead is one bit per block), searching for a free block during allocation enjoys better cache locality, and deallocation does not touch memory around the payload being deallocated (which is often cold).

    Allocation requests are handled on a first-fit basis. Although linear in complexity, allocation is in practice fast because of the compact bookkeeping representation, use of simple and fast bitwise routines, and caching of the first available block position. A known issue with this general approach is fragmentation, partially mitigated by coalescing. Since does not need to maintain the allocated size, freeing memory implicitly coalesces free blocks together. Also, tuning has a considerable impact on both internal and external fragmentation.

    The size of each block can be selected either during compilation or at run time. Statically-known block sizes are frequent in practice and yield slightly better performance. To choose a block size statically, pass it as the parameter as in . To choose a block size parameter, use and pass the block size to the constructor.

    Examples

    1. // Create a block allocator on top of a 10KB stack region. import stdx.allocator.building_blocks.region : InSituRegion; import std.traits : hasMember; InSituRegion!(10_240, 64) r; auto a = BitmappedBlock!(64, 64)(cast(ubyte[])(r.allocateAll())); static assert(hasMember!(InSituRegion!(10_240, 64), "allocateAll")); const b = a.allocate(100); assert(b.length == 100);

    • blockSize

      Declaration

      alias blockSize = theBlockSize;

      If , offers a read/write property . It must be set before any use of the allocator. Otherwise (i.e. is a legit constant), is an alias for . Whether constant or variable, must also be a multiple of . This constraint is ed statically and dynamically.

    • alignment

      Declaration

      alias alignment = theAlignment;

      The alignment offered is user-configurable statically through parameter , defaulted to .

    • parent

      Declaration

      ParentAllocator parent;

      The parent allocator. Depending on whether holds state or not, this is a member variable or an alias for ParentAllocator.instance.

    • this

      Declaration

      this(ubyte[] data);
      this(size_t capacity);

      Constructs a block allocator given a hunk of memory, or a desired capacity in bytes.

      Discussion

      • If is , only the constructor taking is defined and the user is responsible for freeing if desired.
      • Otherwise, both constructors are defined. The -based constructor assumes memory has been allocated with the parent allocator. The -based constructor uses to allocate an appropriate contiguous hunk of memory. Regardless of the constructor used, the destructor releases the memory by using .

    • goodAllocSize

      Declaration

      size_t goodAllocSize(size_t n);

      Returns the actual bytes allocated when bytes are requested, i.e. .

    • allocate

      Declaration

      @trusted void[] allocate(const size_t s);

      Allocates bytes of memory and returns it, or if memory could not be allocated.

      Discussion

      The following information might be of help with choosing the appropriate block size. Actual allocation occurs in sizes multiple of the block size. Allocating one block is the fastest because only one 0 bit needs to be found in the metadata. Allocating 2 through 64 blocks is the next cheapest because it affects a maximum of two s in the metadata. Allocations greater than 64 blocks require a multiword search through the metadata.

    • alignedAllocate

      Declaration

      void[] alignedAllocate(size_t n, uint a);

      Allocates a block with specified alignment . The alignment must be a power of 2. If , function forwards to . Otherwise, it attempts to overallocate and then adjust the result for proper alignment. In the worst case the slack memory is around two blocks.

    • allocateAll

      Declaration

      void[] allocateAll();

      If the object is empty (has no active allocation), allocates all memory within and returns a slice to it. Otherwise, returns (i.e. no attempt is made to allocate the largest available block).

    • owns

      Declaration

      const Ternary owns(void[] b);

      Returns Ternary.yes if b belongs to the BitmappedBlock object, Ternary.no otherwise. Never returns Ternary.unkown. (This method is somewhat tolerant in that accepts an interior slice.)

    • expand

      Declaration

      @trusted bool expand(ref void[] b, immutable size_t delta);

      Expands an allocated block in place.

    • reallocate

      Declaration

      @system bool reallocate(ref void[] b, size_t newSize);

      Reallocates a previously-allocated block. Contractions occur in place.

    • alignedReallocate

      Declaration

      @system bool alignedReallocate(ref void[] b, size_t newSize, uint a);

      Reallocates a block previously allocated with . Contractions do not occur in place.

    • deallocate

      Declaration

      bool deallocate(void[] b);

      Deallocates a block previously allocated with this allocator.

    • deallocateAll

      Declaration

      bool deallocateAll();

      Forcibly deallocates all memory allocated by this allocator, making it available for further allocations. Does not return memory to .

    • empty

      Declaration

      Ternary empty();

      Returns Ternary.yes if no memory is currently allocated with this allocator, otherwise Ternary.no. This method never returns Ternary.unknown.

  • BitmappedBlockWithInternalPointers

    Declaration

    struct BitmappedBlockWithInternalPointers(size_t theBlockSize, uint theAlignment = platformAlignment, ParentAllocator = NullAllocator);

    A with additional structure for supporting . To that end, adds a bitmap (one bit per block) that marks object starts. The bitmap itself has variable size and is allocated together with regular allocations.

    Discussion

    The time complexity of is , where is the size of the object within which the internal pointer is looked up.

    • this

      Declaration

      this(ubyte[] data);
      this(size_t capacity);

      Constructors accepting desired capacity or a preallocated buffer, similar in semantics to those of .

    • alignment

      Declaration

      alias alignment = theAlignment;
      size_t goodAllocSize(size_t n);
      void[] allocate(size_t bytes);
      void[] allocateAll();
      bool expand(ref void[] b, size_t bytes);
      bool deallocate(void[] b);
      Ternary resolveInternalPointer(const void* p, ref void[] result);
      Ternary empty();

      Allocator primitives.