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d452094f43
Formerly, _next was always updated to the last free'd allocation, which left large gaps on alloc/free bursts. Now, we try keep allocation density high from the start of the array and ensure that allocations happen at the lowest available index in the bit array. Fixes #3679
130 lines
2.8 KiB
C++
130 lines
2.8 KiB
C++
/*
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* \brief Allocator using bitmaps
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* \author Alexander Boettcher
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* \author Stefan Kalkowski
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* \date 2012-06-14
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*/
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/*
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* Copyright (C) 2012-2017 Genode Labs GmbH
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*
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* This file is part of the Genode OS framework, which is distributed
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* under the terms of the GNU Affero General Public License version 3.
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*/
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#ifndef _INCLUDE__UTIL__BIT_ALLOCATOR_H_
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#define _INCLUDE__UTIL__BIT_ALLOCATOR_H_
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#include <util/bit_array.h>
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namespace Genode { template<unsigned> class Bit_allocator; }
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template<unsigned BITS>
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class Genode::Bit_allocator
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{
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protected:
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enum {
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BITS_PER_BYTE = 8UL,
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BITS_PER_WORD = sizeof(addr_t) * BITS_PER_BYTE,
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BITS_ALIGNED = (BITS + BITS_PER_WORD - 1UL)
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& ~(BITS_PER_WORD - 1UL),
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};
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using Array = Bit_array<BITS_ALIGNED>;
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addr_t _next = 0;
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Array _array { };
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/**
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* Reserve consecutive number of bits
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*
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* \noapi
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*/
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void _reserve(addr_t bit_start, size_t const num)
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{
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if (!num) return;
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_array.set(bit_start, num);
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}
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public:
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class Out_of_indices : Exception {};
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class Range_conflict : Exception {};
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Bit_allocator() { _reserve(BITS, BITS_ALIGNED - BITS); }
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Bit_allocator(const Bit_allocator & o) : _array(o._array) { }
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/**
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* Allocate block of bits
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*
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* \param num_log2 2-based logarithm of size of block
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*
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* The requested block is allocated at the lowest available index in
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* the bit array.
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*
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* \throw Array::Out_of_indices
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*/
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addr_t alloc(size_t const num_log2 = 0)
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{
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addr_t const step = 1UL << num_log2;
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addr_t max = ~0UL;
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do {
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try {
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/* throws exception if array is accessed outside bounds */
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for (addr_t i = _next & ~(step - 1); i < max; i += step) {
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if (_array.get(i, step))
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continue;
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_array.set(i, step);
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_next = i + step;
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return i;
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}
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} catch (typename Array::Invalid_index_access) { }
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max = _next;
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_next = 0;
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} while (max != 0);
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throw Out_of_indices();
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}
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/**
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* Allocate specific block of bits
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*
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* \param first_bit desired address of block
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* \param num_log2 2-based logarithm of size of block
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*
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* \throw Range_conflict
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* \throw Array::Invalid_index_access
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*/
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void alloc_addr(addr_t const bit_start, size_t const num_log2 = 0)
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{
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addr_t const step = 1UL << num_log2;
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if (_array.get(bit_start, step))
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throw Range_conflict();
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_array.set(bit_start, step);
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_next = bit_start + step;
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return;
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}
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void free(addr_t const bit_start, size_t const num_log2 = 0)
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{
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_array.clear(bit_start, 1UL << num_log2);
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/*
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* We only rewind the _next pointer (if needed) to densely allocate
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* from the start of the array and avoid gaps.
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*/
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if (bit_start < _next)
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_next = bit_start;
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}
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};
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#endif /* _INCLUDE__UTIL__BIT_ALLOCATOR_H_ */
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