libcryfs/implementations/caching/cache/Cache.h

#pragma once
#ifndef MESSMER_BLOCKSTORE_IMPLEMENTATIONS_CACHING_CACHE_H_
#define MESSMER_BLOCKSTORE_IMPLEMENTATIONS_CACHING_CACHE_H_

#include "CacheEntry.h"
#include "QueueMap.h"
#include "PeriodicTask.h"
#include <memory>
#include <mutex>
#include <boost/optional.hpp>
#include <future>

namespace blockstore {
namespace caching {

template<class Key, class Value>
class Cache {
public:
  static constexpr uint32_t MAX_ENTRIES = 1000;
  //TODO Experiment with good values
  static constexpr double PURGE_LIFETIME_SEC = 0.5; //When an entry has this age, it will be purged from the cache
  static constexpr double PURGE_INTERVAL = 0.5; // With this interval, we check for entries to purge
  static constexpr double MAX_LIFETIME_SEC = PURGE_LIFETIME_SEC + PURGE_INTERVAL; // This is the oldest age an entry can reach (given purging works in an ideal world, i.e. with the ideal interval and in zero time)

  Cache();
  virtual ~Cache();

  void push(const Key &key, Value value);
  boost::optional<Value> pop(const Key &key);

private:
  void _popOldEntries();
  static void _destructElementsInParallel(std::vector<CacheEntry<Key, Value>> *list);

  mutable std::mutex _mutex;
  QueueMap<Key, CacheEntry<Key, Value>> _cachedBlocks;
  std::unique_ptr<PeriodicTask> _timeoutFlusher;
};

template<class Key, class Value> constexpr uint32_t Cache<Key, Value>::MAX_ENTRIES;
template<class Key, class Value> constexpr double Cache<Key, Value>::PURGE_LIFETIME_SEC;
template<class Key, class Value> constexpr double Cache<Key, Value>::PURGE_INTERVAL;
template<class Key, class Value> constexpr double Cache<Key, Value>::MAX_LIFETIME_SEC;

template<class Key, class Value>
Cache<Key, Value>::Cache(): _cachedBlocks(), _timeoutFlusher(nullptr) {
  //Don't initialize timeoutFlusher in the initializer list,
  //because it then might already call Cache::popOldEntries() before Cache is done constructing
  _timeoutFlusher = std::make_unique<PeriodicTask>(std::bind(&Cache::_popOldEntries, this), PURGE_INTERVAL);
}

template<class Key, class Value>
Cache<Key, Value>::~Cache() {
}

template<class Key, class Value>
boost::optional<Value> Cache<Key, Value>::pop(const Key &key) {
  std::lock_guard<std::mutex> lock(_mutex);
  auto found = _cachedBlocks.pop(key);
  if (!found) {
    return boost::none;
  }
  return found->releaseValue();
}

template<class Key, class Value>
void Cache<Key, Value>::push(const Key &key, Value value) {
  std::lock_guard<std::mutex> lock(_mutex);
  assert(_cachedBlocks.size() <= MAX_ENTRIES);
  if (_cachedBlocks.size() == MAX_ENTRIES) {
    _cachedBlocks.pop();
    assert(_cachedBlocks.size() == MAX_ENTRIES-1);
  }
  _cachedBlocks.push(key, CacheEntry<Key, Value>(std::move(value)));
}

template<class Key, class Value>
void Cache<Key, Value>::_popOldEntries() {
  std::lock_guard<std::mutex> lock(_mutex);
  std::vector<CacheEntry<Key, Value>> entriesToDelete;
  while(_cachedBlocks.size() > 0 && _cachedBlocks.peek()->ageSeconds() > PURGE_LIFETIME_SEC) {
	entriesToDelete.push_back(*_cachedBlocks.pop());
  }
  _destructElementsInParallel(&entriesToDelete);
}

template<class Key, class Value>
void Cache<Key, Value>::_destructElementsInParallel(std::vector<CacheEntry<Key, Value>> *list) {
  //TODO Check whether this parallel destruction below works (just comment it in but keep the list->clear()) and check performance impacts. Is it better to have a lower parallelity level, i.e. #core threads?
  /*
  std::vector<std::future<void>> waitHandles;
  for (auto & entry : *list) {
	waitHandles.push_back(std::async(std::launch::async, [&entry] {
	  entry.releaseValue();
	}));
  }
  for (auto & waitHandle : waitHandles) {
	waitHandle.wait();
  }
  */
  list->clear();
}

}
}

#endif