ClickHouse/dbms/include/DB/Common/SmallObjectPool.h

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#pragma once
#include <DB/Common/Arena.h>
#include <common/likely.h>
#include <ext/range.hpp>
#include <ext/size.hpp>
#include <ext/bit_cast.hpp>
#include <cstdlib>
#include <memory>
namespace DB
{
/** Can allocate memory objects of fixed size with deletion support.
* For small `object_size`s allocated no less than getMinAllocationSize() bytes. */
class SmallObjectPool
{
private:
struct Block { Block * next; };
static constexpr auto getMinAllocationSize() { return sizeof(Block); }
const std::size_t object_size;
Arena pool;
Block * free_list{};
public:
SmallObjectPool(
const std::size_t object_size_, const std::size_t initial_size = 4096, const std::size_t growth_factor = 2,
const std::size_t linear_growth_threshold = 128 * 1024 * 1024)
: object_size{std::max(object_size_, getMinAllocationSize())},
pool{initial_size, growth_factor, linear_growth_threshold}
{
if (pool.size() < object_size)
return;
const auto num_objects = pool.size() / object_size;
auto head = free_list = ext::bit_cast<Block *>(pool.alloc(num_objects * object_size));
for (const auto i : ext::range(0, num_objects - 1))
{
(void) i;
head->next = ext::bit_cast<Block *>(ext::bit_cast<char *>(head) + object_size);
head = head->next;
}
head->next = nullptr;
}
char * alloc()
{
if (free_list)
{
const auto res = reinterpret_cast<char *>(free_list);
free_list = free_list->next;
return res;
}
return pool.alloc(object_size);
}
void free(const void * ptr)
{
union {
const void * p_v;
Block * block;
};
p_v = ptr;
block->next = free_list;
free_list = block;
}
/// Размер выделенного пула в байтах
size_t size() const
{
return pool.size();
}
};
}