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Only rewind next-free pointer in Bit_allocator

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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
This commit is contained in:
Christian Helmuth 2020-03-04 07:40:27 +01:00 committed by Norman Feske
parent 139f390317
commit d452094f43
1 changed files with 17 additions and 1 deletions
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18
repos/base/include/util/bit_allocator.h
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@ -57,6 +57,16 @@ class Genode::Bit_allocator
Bit_allocator() { _reserve(BITS, BITS_ALIGNED - BITS); } Bit_allocator() { _reserve(BITS, BITS_ALIGNED - BITS); }
Bit_allocator(const Bit_allocator & o) : _array(o._array) { } Bit_allocator(const Bit_allocator & o) : _array(o._array) { }
/**
* Allocate block of bits
*
* \param num_log2 2-based logarithm of size of block
*
* The requested block is allocated at the lowest available index in
* the bit array.
*
* \throw Array::Out_of_indices
*/
addr_t alloc(size_t const num_log2 = 0) addr_t alloc(size_t const num_log2 = 0)
{ {
addr_t const step = 1UL << num_log2; addr_t const step = 1UL << num_log2;
@ -106,7 +116,13 @@ class Genode::Bit_allocator
void free(addr_t const bit_start, size_t const num_log2 = 0) void free(addr_t const bit_start, size_t const num_log2 = 0)
{ {
_array.clear(bit_start, 1UL << num_log2); _array.clear(bit_start, 1UL << num_log2);
_next = bit_start;
/*
* We only rewind the _next pointer (if needed) to densely allocate
* from the start of the array and avoid gaps.
*/
if (bit_start < _next)
_next = bit_start;
} }
}; };