tbb::concurrent_hash_map的Find()与迭代并行，迭代会丢失数据吗？

Question

我有两个线程，一个在 tbb::concurrent_hash_map 上执行 find()，另一个在不执行任何插入或删除的情况下遍历此映射。奇怪的是，如果不执行find()，迭代遍历是正常的，但是当执行find()时，使用HashMap::iterator迭代得到的数据却少了。为什么会这样？

为什么执行find()时通过HashMap::iterator迭代得到的数据少了？

Answer 1

我添加了一个测试示例，包括两段代码。一个是我封装tbb::concurrent_hash_map的地方，另一个是测试示例。结果如下：

writeTime=0.370875
------------finish write---------------
------ hashMap size = 3174014
 cnt size=3210251
 error
-----readTime=2.22228

代码：

#include<map>
#include<string>
#include<vector>
#include<fstream>
#include<iostream>
#include <omp.h>
#include <tbb/concurrent_hash_map.h>

template <typename KeyType, typename ValueType>
class ConcurrentHashMap {
  private:
    typedef tbb::concurrent_hash_map<KeyType, ValueType> HashMap;
    typedef typename HashMap::const_accessor HashMapConstAccessor;
    typedef typename HashMap::accessor HashMapAccessor;
    typedef typename HashMap::iterator HashMapIterator;
    typedef typename HashMap::value_type HashMapValuePair;

  public:
    ConcurrentHashMap (ValueType default_value) : DEFAULT_VALUE(default_value) {
    }

    size_t size() {
      return hashMap.size();
    }

    bool insert(const KeyType& key, const ValueType& value) {
      HashMapAccessor accessor;
      bool inserted = hashMap.insert(accessor, key);
      if (inserted) {
        accessor->second = value;
      }
      return inserted;
    }

    bool erase(const KeyType& key) {
      return hashMap.erase(key);
    }

    bool find(const KeyType& key, ValueType& value) {
      HashMapConstAccessor accessor;
      if (hashMap.find(accessor, key)) {
        value = accessor->second;
        return true;
      }
      value = DEFAULT_VALUE;
      return false;
    }

    void clear() {
      hashMap.clear();
    }

    const ValueType& operator[](const KeyType& key) const {
      HashMapConstAccessor accessor;
      hashMap.find(accessor, key);
      if (hashMap.find(accessor, key)) {
        return accessor->second;
      }
      // accessor->second = DEFAULT_VALUE;
      return DEFAULT_VALUE;
    }

    HashMapIterator begin() {
      return hashMap.begin();
    }

    HashMapIterator end() {
      return hashMap.end();
    }

  private:
    HashMap hashMap;
    ValueType DEFAULT_VALUE;
};

#include<map>
#include<string>
#include<vector>
#include<fstream>
#include<iostream>
#include <omp.h>
#include <tbb/tbb.h>

#include "./ConcurrentHashMap.h"
#include "./ConcurrentUnorderMap.h"

using namespace std;

class A;

using HashMap = ConcurrentHashMap<int, A*>;
// using HashMap = ConcurrentUnorderedMap<int, A*>;

class A {
public:
  A(int _a, int _b): a(_a), b(_b) {

  }
  void sub () {

  }
  int a = 1;
  int b = 0;;
};

void test(int N, HashMap& hashMap) {
  int thread_num = 16;

  std::thread writer(
    [&] () {
      auto writeStartTime = std::chrono::high_resolution_clock::now();
      #pragma omp parallel for num_threads(thread_num)
      for (int i = 0; i < N; i++) {
        hashMap.insert(i, new A(1, i));
      }
      auto writeEndTime = std::chrono::high_resolution_clock::now();
      double writeTime = std::chrono::duration<double>(writeEndTime 
                                                      - writeStartTime).count();
      std::cout << "writeTime=" << writeTime << std::endl;
    }
  );

  writer.join();
}

int main () {
  // cmd: g++ test_con_hashmap.cpp -fopenmp -ltbb  && ./a.out
  int N = 3174014;
  std::nullptr_t NULLPOINTER = nullptr;
  HashMap hashMap(NULLPOINTER);

  test(N, hashMap);

  cout << "------------finish write---------------" << std::endl;

  size_t hashmap_size = hashMap.size();
  cout << "\n------ hashMap size = " << hashmap_size << endl;

{ 
  int thread_num = 32;
  std::thread reader(
    [&] () {
      std::this_thread::sleep_for(std::chrono::milliseconds(1));
      auto readStartTime = std::chrono::high_resolution_clock::now();

      std::random_device rd;
      std::mt19937 gen(rd());
      std::uniform_int_distribution<int> dis(0, N - 1);


      for (int i = 0; i < 10; i++) {
        #pragma omp parallel for num_threads(thread_num)
        for (int i = 0; i < N; i++) {
          A* value;
          int randomKey = dis(gen);
          if(hashMap.find(randomKey, value)){
            
          }
        }
      }
      auto readEndTime = std::chrono::high_resolution_clock::now();
      double readTime = std::chrono::duration<double>(readEndTime 
                                               - readStartTime).count();
      std::cout << "-----readTime=" << readTime << std::endl;
    }
  );

  size_t cnt = 0;
  for(auto iterator1 = hashMap.begin(); 
      iterator1 != hashMap.end(); ++iterator1 ){
    cnt++;
  }
  std::cout << " cnt=" << cnt << std::endl;


  if (cnt != hashmap_size) {
    std::cout << " error" << std::endl;
  }


  reader.join();
}

  hashMap.clear();

  return 0;
}