Improve performance of accessing elements in a map and writing to a file

前端未结

关注

 1  1138

一向 2021-01-26 04:11

I have a map structure in C++ which holds about 60 million pairs. After I populate the map, I would like to write it to a plain file.

At the moment this code is quite

1条回答

忘掉有多难 (楼主)

2021-01-26 04:29

On my machine (i7/16gb/win7/vs2013), the example below runs in about:

6s (c stream/binary);
22s (c stream/text);
9s (c++ stream/binary) and
56s (c++ stream/text).

On a different machine (PentiumD/4gb/win10/vs2015), the longest time was about 4 minutes.

#include 
#include 
#include 
#include 

int main()
{
  typedef std::pair C_Value;
  typedef std::map< uint32_t, C_Value > C_Map;

  //
  C_Map m;
  const uint32_t maxsize = 50000000;
  try
  {
    for ( int i = 0; i < maxsize; ++i )
      m[i] = C_Value( i, i );
  }
  catch ( ... )
  {
    std::cout << "too many elements\n";
    return -1;
  }

  //
  std::cout << "writing " << m.size() << " elements... ";
  auto t_start = std::chrono::high_resolution_clock::now();

#if 1
  //
  FILE* f = fopen( "test.bin", "wb" );
  if ( ! f )
  {
    std::cout << "could not open file\n";
    return -2;
  }

  //
  for ( auto e : m )
  {
    fwrite( &e.second.first, sizeof e.second.first, 1, f );
    fwrite( &e.second.second, sizeof e.second.second, 1, f );
  }

  //
  fclose( f );
#endif
#if 0
  //
  FILE* f = fopen( "test.bin", "w" );
  if ( ! f )
  {
    std::cout << "could not open file\n";
    return -2;
  }

  //
  for ( auto e : m )
  {
    fprintf( f, "%d, %d\n", e.second.first, e.second.second );
  }

  //
  fclose( f );
#endif
#if 0
  std::ofstream os( "test.bin", std::ios::binary );
  if ( ! os )
  {
    std::cout << "could not open file\n";
    return -2;
  }

  //
  for ( auto e : m )
  {
    os.write( (const char*)&e.second.first, sizeof e.second.first );
    os.write( (const char*)&e.second.second, sizeof e.second.second );
  }

  //
  os.close();
#endif
#if 0
  std::ofstream os( "test.bin" );
  if ( ! os )
  {
    std::cout << "could not open file\n";
    return -2;
  }

  //
  for ( auto e : m )
  {
    os << e.second.first << ',' << e.second.second << '\n';
  }

  //
  os.close();
#endif

  //
  auto t_end = std::chrono::high_resolution_clock::now();
  std::cout << "done in [ms]: " << std::chrono::duration( t_end-t_start ).count() << std::endl;

  return 0;
}

0 讨论(0)