std.experimental.allocator.building_blocks.bitmapped_block

struct BitmappedBlock(size_t theBlockSize, uint theAlignment = platformAlignment, ParentAllocator = NullAllocator, Flag!"multiblock" f = Yes.multiblock);

BitmappedBlock implements a simple heap consisting of one contiguous area of memory organized in blocks, each of size theBlockSize. 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.

Passing NullAllocator as ParentAllocator (the default) means user code manages allocation of the memory block from the outside; in that case BitmappedBlock must be constructed with a ubyte[] preallocated block and has no responsibility regarding the lifetime of its support underlying storage. If another allocator type is passed, BitmappedBlock defines a destructor that uses the parent allocator to release the memory block. That makes the combination of AllocatorList, BitmappedBlock, and a back-end allocator such as MmapAllocator 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 AffixAllocator 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 BitmappedBlock does not need to maintain the allocated size, freeing memory implicitly coalesces free blocks together. Also, tuning blockSize has a considerable impact on both internal and external fragmentation.

If the last template parameter is set to No.multiblock, the allocator will only serve allocations which require at most theBlockSize. The BitmappedBlock has a specialized implementation for single-block allocations which allows for greater performance, at the cost of not being able to allocate more than one block at a time.

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 blockSize parameter as in BitmappedBlock!(4096). To choose a block size parameter, use BitmappedBlock!(chooseAtRuntime) and pass the block size to the constructor.

Parameters:

theBlockSize	the length of a block, which must be a multiple of theAlignment
theAlignment	alignment of each block
ParentAllocator	allocator from which the BitmappedBlock will draw memory. If set to NullAllocator, the storage must be passed via the constructor
f	Yes.multiblock to support allocations spanning across multiple blocks and No.multiblock to support single block allocations. Although limited by single block allocations, No.multiblock will generally provide higher performance.

Examples:

// Create a block allocator on top of a 10KB stack region.
import std.experimental.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);
writeln(b.length); // 100

Examples:

import std.experimental.allocator.mallocator : Mallocator;
import std.typecons : Flag, Yes;

enum blockSize = 64;
enum numBlocks = 10;

// The 'BitmappedBlock' is implicitly instantiated with Yes.multiblock
auto a = BitmappedBlock!(blockSize, 8, Mallocator, Yes.multiblock)(numBlocks * blockSize);

// Instantiated with Yes.multiblock, can allocate more than one block at a time
void[] buf = a.allocate(2 * blockSize);
writeln(buf.length); // 2 * blockSize
assert(a.deallocate(buf));

// Can also allocate less than one block
buf = a.allocate(blockSize / 2);
writeln(buf.length); // blockSize / 2

// Expands inside the same block
assert(a.expand(buf, blockSize / 2));
writeln(buf.length); // blockSize

// If Yes.multiblock, can expand past the size of a single block
assert(a.expand(buf, 3 * blockSize));
writeln(buf.length); // 4 * blockSize
assert(a.deallocate(buf));

Examples:

import std.experimental.allocator.mallocator : Mallocator;
import std.typecons : Flag, No;

enum blockSize = 64;
auto a = BitmappedBlock!(blockSize, 8, Mallocator, No.multiblock)(1024 * blockSize);

// Since instantiated with No.multiblock, can only allocate at most the block size
void[] buf = a.allocate(blockSize + 1);
assert(buf is null);

buf = a.allocate(blockSize);
writeln(buf.length); // blockSize
assert(a.deallocate(buf));

// This is also fine, because it's less than the block size
buf = a.allocate(blockSize / 2);
writeln(buf.length); // blockSize / 2

// Can expand the buffer until its length is at most 64
assert(a.expand(buf, blockSize / 2));
writeln(buf.length); // blockSize

// Cannot expand anymore
assert(!a.expand(buf, 1));
assert(a.deallocate(buf));

this(ubyte[] data);

this(ubyte[] data, uint blockSize);

this(size_t capacity);

this(size_t capacity, uint blockSize);

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

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

alias blockSize = theBlockSize;

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

alias alignment = theAlignment;

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

ParentAllocator parent;

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

pure nothrow @nogc @safe size_t goodAllocSize(size_t n);

Returns the actual bytes allocated when n bytes are requested, i.e. n.roundUpToMultipleOf(blockSize).

const pure nothrow @nogc @trusted Ternary owns(const 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.)

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

Expands in place a buffer previously allocated by BitmappedBlock. If instantiated with No.multiblock, the expansion fails if the new length exceeds theBlockSize.

nothrow @nogc bool deallocate(void[] b);

Deallocates a block previously allocated with this allocator.

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

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

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 ulong in the metadata. Allocations greater than 64 blocks require a multiword search through the metadata.

If instantiated with No.multiblock, it performs a search for the first zero bit in the bitmap and sets it.

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

Allocates s bytes of memory and returns it, or null if memory could not be allocated. allocateFresh behaves just like allocate, the only difference being that this always returns unused(fresh) memory. Although there may still be available space in the BitmappedBlock, allocateFresh could still return null, because all the available blocks have been previously deallocated.

void[] allocateAll();

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

pure nothrow @nogc @safe Ternary empty();

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

pure nothrow @nogc bool deallocateAll();

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

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

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

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

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

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

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

struct SharedBitmappedBlock(size_t theBlockSize, uint theAlignment = platformAlignment, ParentAllocator = NullAllocator, Flag!"multiblock" f = Yes.multiblock);

The threadsafe version of the BitmappedBlock. The semantics of the SharedBitmappedBlock are identical to the regular BitmappedBlock.

Parameters:

theBlockSize	the length of a block, which must be a multiple of theAlignment
theAlignment	alignment of each block
ParentAllocator	allocator from which the BitmappedBlock will draw memory. If set to NullAllocator, the storage must be passed via the constructor
f	Yes.multiblock to support allocations spanning across multiple blocks and No.multiblock to support single block allocations. Although limited by single block allocations, No.multiblock will generally provide higher performance.

Examples:

import std.experimental.allocator.mallocator : Mallocator;
import std.experimental.allocator.common : platformAlignment;
import std.typecons : Flag, Yes, No;

// Create 'numThreads' threads, each allocating in parallel a chunk of memory
static void testAlloc(Allocator)(ref Allocator a, size_t allocSize)
{
    import core.thread : ThreadGroup;
    import std.algorithm.sorting : sort;
    import core.internal.spinlock : SpinLock;

    SpinLock lock = SpinLock(SpinLock.Contention.brief);
    enum numThreads = 10;
    void[][numThreads] buf;
    size_t count = 0;

    // Each threads allocates 'allocSize'
    void fun()
    {
        void[] b = a.allocate(allocSize);
        writeln(b.length); // allocSize

        lock.lock();
        scope(exit) lock.unlock();

        buf[count] = b;
        count++;
    }

    auto tg = new ThreadGroup;
    foreach (i; 0 .. numThreads)
    {
        tg.create(&fun);
    }
    tg.joinAll();

    // Sorting the allocations made by each thread, we expect the buffers to be
    // adjacent inside the SharedBitmappedBlock
    sort!((a, b) => a.ptr < b.ptr)(buf[0 .. numThreads]);
    foreach (i; 0 .. numThreads - 1)
    {
        assert(buf[i].ptr + a.goodAllocSize(buf[i].length) <= buf[i + 1].ptr);
    }

    // Deallocate everything
    foreach (i; 0 .. numThreads)
    {
        assert(a.deallocate(buf[i]));
    }
}

enum blockSize = 64;
auto alloc1 = SharedBitmappedBlock!(blockSize, platformAlignment, Mallocator, Yes.multiblock)(1024 * 1024);
auto alloc2 = SharedBitmappedBlock!(blockSize, platformAlignment, Mallocator, No.multiblock)(1024 * 1024);
testAlloc(alloc1, 2 * blockSize);
testAlloc(alloc2, blockSize);

this(ubyte[] data);

this(ubyte[] data, uint blockSize);

this(size_t capacity);

this(size_t capacity, uint blockSize);

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

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

alias blockSize = theBlockSize;

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

alias alignment = theAlignment;

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

ParentAllocator parent;

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

pure nothrow @nogc @safe size_t goodAllocSize(size_t n);

Returns the actual bytes allocated when n bytes are requested, i.e. n.roundUpToMultipleOf(blockSize).

const pure nothrow @nogc @trusted Ternary owns(const void[] b);

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

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

Expands in place a buffer previously allocated by SharedBitmappedBlock. Expansion fails if the new length exceeds the block size.

nothrow @nogc bool deallocate(void[] b);

Deallocates the given buffer b, by atomically setting the corresponding bit to 0. b must be valid, and cannot contain multiple adjacent blocks.

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

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

The SharedBitmappedBlock cannot allocate more than the given block size. Allocations are satisfied by searching the first unset bit in the bitmap, and atomically setting it. In rare memory pressure scenarios, the allocation could fail.

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

Allocates s bytes of memory and returns it, or null if memory could not be allocated. allocateFresh behaves just like allocate, the only difference being that this always returns unused(fresh) memory. Although there may still be available space in the SharedBitmappedBlock, allocateFresh could still return null, because all the available blocks have been previously deallocated.

void[] allocateAll();

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

nothrow @nogc @safe Ternary empty();

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

nothrow @nogc bool deallocateAll();

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

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

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

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

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

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

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

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

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

The time complexity of resolveInternalPointer is Ο(k), where k is the size of the object within which the internal pointer is looked up.

this(ubyte[] data);

this(size_t capacity);

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

alias alignment = theAlignment;

pure nothrow @nogc @safe 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);

nothrow @nogc @safe Ternary resolveInternalPointer(const void* p, ref void[] result);

Ternary empty();

Allocator primitives.