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#pragma once
#include <Common/ICachePolicy.h>
#include <list>
#include <unordered_map>
namespace DB
{
/// Cache policy LRU evicts entries which are not used for a long time. Also see cache policy SLRU for reference.
/// WeightFunction is a functor that takes Mapped as a parameter and returns "weight" (approximate size) of that value.
/// Cache starts to evict entries when their total weight exceeds max_size_in_bytes.
/// Value weight should not change after insertion.
/// To work with the thread-safe implementation of this class use a class "CacheBase" with first parameter "LRU"
/// and next parameters in the same order as in the constructor of the current class.
template <typename Key, typename Mapped, typename HashFunction = std::hash<Key>, typename WeightFunction = EqualWeightFunction<Mapped>>
class LRUCachePolicy : public ICachePolicy<Key, Mapped, HashFunction, WeightFunction>
{
public:
using Base = ICachePolicy<Key, Mapped, HashFunction, WeightFunction>;
using typename Base::MappedPtr;
using typename Base::KeyMapped;
using typename Base::OnWeightLossFunction;
/** Initialize LRUCachePolicy with max_size_in_bytes and max_count.
* max_size_in_bytes == 0 means the cache accepts no entries.
* max_count == 0 means no elements size restrictions.
*/
LRUCachePolicy(size_t max_size_in_bytes_, size_t max_count_, OnWeightLossFunction on_weight_loss_function_)
: Base(std::make_unique<NoCachePolicyUserQuota>())
, max_size_in_bytes(max_size_in_bytes_)
, max_count(max_count_)
, on_weight_loss_function(on_weight_loss_function_)
{
}
size_t sizeInBytes() const override
{
return current_size_in_bytes;
}
size_t count() const override
{
return cells.size();
}
size_t maxSizeInBytes() const override
{
return max_size_in_bytes;
}
void setMaxCount(size_t max_count_) override
{
max_count = max_count_;
removeOverflow();
}
void setMaxSizeInBytes(size_t max_size_in_bytes_) override
{
max_size_in_bytes = max_size_in_bytes_;
removeOverflow();
}
void clear() override
{
queue.clear();
cells.clear();
current_size_in_bytes = 0;
}
void remove(const Key & key) override
{
auto it = cells.find(key);
if (it == cells.end())
return;
auto & cell = it->second;
current_size_in_bytes -= cell.size;
queue.erase(cell.queue_iterator);
cells.erase(it);
}
MappedPtr get(const Key & key) override
{
auto it = cells.find(key);
if (it == cells.end())
return {};
Cell & cell = it->second;
/// Move the key to the end of the queue. The iterator remains valid.
queue.splice(queue.end(), queue, cell.queue_iterator);
return cell.value;
}
std::optional<KeyMapped> getWithKey(const Key & key) override
{
auto it = cells.find(key);
if (it == cells.end())
return std::nullopt;
Cell & cell = it->second;
/// Move the key to the end of the queue. The iterator remains valid.
queue.splice(queue.end(), queue, cell.queue_iterator);
return std::make_optional<KeyMapped>({it->first, cell.value});
}
void set(const Key & key, const MappedPtr & mapped) override
{
auto [it, inserted] = cells.emplace(std::piecewise_construct,
std::forward_as_tuple(key),
std::forward_as_tuple());
Cell & cell = it->second;
if (inserted)
{
try
{
cell.queue_iterator = queue.insert(queue.end(), key);
}
catch (...)
{
cells.erase(it);
throw;
}
}
else
{
current_size_in_bytes -= cell.size;
queue.splice(queue.end(), queue, cell.queue_iterator);
}
cell.value = mapped;
cell.size = cell.value ? weight_function(*cell.value) : 0;
current_size_in_bytes += cell.size;
removeOverflow();
}
std::vector<KeyMapped> dump() const override
{
std::vector<KeyMapped> res;
for (const auto & [key, cell] : cells)
res.push_back({key, cell.value});
return res;
}
private:
using LRUQueue = std::list<Key>;
using LRUQueueIterator = typename LRUQueue::iterator;
LRUQueue queue;
struct Cell
{
MappedPtr value;
size_t size;
LRUQueueIterator queue_iterator;
};
using Cells = std::unordered_map<Key, Cell, HashFunction>;
Cells cells;
/// Total weight of values.
size_t current_size_in_bytes = 0;
size_t max_size_in_bytes;
size_t max_count;
WeightFunction weight_function;
OnWeightLossFunction on_weight_loss_function;
void removeOverflow()
{
size_t current_weight_lost = 0;
size_t queue_size = cells.size();
while ((current_size_in_bytes > max_size_in_bytes || (max_count != 0 && queue_size > max_count)) && (queue_size > 0))
{
const Key & key = queue.front();
auto it = cells.find(key);
if (it == cells.end())
std::terminate(); // Queue became inconsistent
const auto & cell = it->second;
current_size_in_bytes -= cell.size;
current_weight_lost += cell.size;
cells.erase(it);
queue.pop_front();
--queue_size;
}
on_weight_loss_function(current_weight_lost);
if (current_size_in_bytes > (1ull << 63))
std::terminate(); // Queue became inconsistent
}
};
}
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