c++ - 重新散列时锁定散列映射

问题描述

我正在尝试创建一个线程安全的哈希映射。我为哈希映射中的每个存储桶都有一个互斥体向量，以支持哈希映射中每个条目的读写器锁。

但是，当我想重新散列哈希映射时，我想锁定整个映射，以便在重新散列期间不会发生读/写。

我想我需要一个额外的互斥锁，但是我的 Gets/Puts 方法是否也需要获取这个互斥锁？如何仅在重新散列发生时阻止其他线程执行读/写，而在仅发生写入和读取时不相互阻止？

这是我当前的哈希表类的外观：

template<typename K, typename T>
class HashTable {
    int num_buckets_;
    double threshold_ratio_;
    int num_elements_;
    vector<vector<pair<T, K>>> table_;
    vector<mutex> read_write_locks_;
    mutex mu_;

    int GetHash(const K& key) {
        return hash<K>{}(key) % num_buckets_;
    }

    void Rehash() {
        scoped_lock<mutex> lock(mu_);   // Lock the whole table?
        cout << "Threshold Value has been reached. Rehashing...\n";

        vector<vector<T>> new_table(2 * num_buckets_);
        num_buckets_ = 2 * num_buckets_;

        vector<mutex> new_mutexes(2 * num_buckets_);
        read_write_locks_.swap(new_mutexes);
        // TODO : Implementation
    }

public:
    explicit HashTable(int num_buckets) : num_buckets_(num_buckets), threshold_ratio_(0.75), num_elements_(0) {
        table_.resize(num_buckets);
        vector<mutex> temp(num_buckets);
        read_write_locks_.swap(temp);
    }

    void Put(const K& key, const T& val) {
        ++num_elements_;
        if (static_cast<double>(num_elements_) / num_buckets_ > threshold_ratio_) {
            Rehash();
        }
        int hash_val = GetHash(key);
        scoped_lock<mutex> write_lock(read_write_locks_[hash_val]);
        cout << "Putting Key: " << key << ", Hash: "<< hash_val << "  with Value: " << val << '\n';
        table_[hash_val].push_back({val, key}); //TODO: For existing keys it should replace the value, not create a new entry
    }

    T Get(const K& key) {
        int hash_val = GetHash(key);
        scoped_lock<mutex> read_lock(read_write_locks_[hash_val]);

        if (table_[hash_val].size() >= 1) {
            for (const auto& elem : table_[hash_val]) {
                if (elem.second == key) {
                    cout << "Key: " << key << " gets value: " << elem.first << '\n';
                    return elem.first;
                }
            }
        }
        cerr << "Unable to find key in hash table. Terminating Program. \n";
        exit(EXIT_FAILURE);
    }
};

标签： c++multithreadingc++11hashmapmutex

c++ - 重新散列时锁定散列映射

问题描述

解决方案

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