Question

Most C++ users that learned C prefer to use the printf / scanf family of functions even when they’re coding in C++.

Although I admit that I find the interface way better (especially POSIX-like format and localization), it seems that an overwhelming concern is performance.

Taking at look at this question:

How can I speed up line by line reading of a file

It seems that the best answer is to use fscanf and that the C++ ifstream is consistently 2-3 times slower.

I thought it would be great if we could compile a repository of “tips” to improve IOStreams performance, what works, what does not.

Points to consider

• buffering (rdbuf()->pubsetbuf(buffer, size))
• synchronization (std::ios_base::sync_with_stdio)
• locale handling (Could we use a trimmed-down locale, or remove it altogether ?)

Of course, other approaches are welcome.

Note: a “new” implementation, by Dietmar Kuhl, was mentioned, but I was unable to locate many details about it. Previous references seem to be dead links.

Answer 1

Here is what I have gathered so far:

Buffering:

If by default the buffer is very small, increasing the buffer size can definitely improve the performance:

• it reduces the number of HDD hits
• it reduces the number of system calls

Buffer can be set by accessing the underlying streambuf implementation.

char Buffer[N];

std::ifstream file("file.txt");

file.rdbuf()->pubsetbuf(Buffer, N);
// the pointer reader by rdbuf is guaranteed
// to be non-null after successful constructor

Warning courtesy of @iavr: according to cppreference it is best to call pubsetbuf before opening the file. Various standard library implementations otherwise have different behaviors.

Locale Handling:

Locale can perform character conversion, filtering, and more clever tricks where numbers or dates are involved. They go through a complex system of dynamic dispatch and virtual calls, so removing them can help trimming down the penalty hit.

The default C locale is meant not to perform any conversion as well as being uniform across machines. It’s a good default to use.

Synchronization:

I could not see any performance improvement using this facility.

One can access a global setting (static member of std::ios_base) using the sync_with_stdio static function.

Measurements:

Playing with this, I have toyed with a simple program, compiled using gcc 3.4.2 on SUSE 10p3 with -O2.

C : 7.76532e+06
C++: 1.0874e+07

Which represents a slowdown of about 20%… for the default code. Indeed tampering with the buffer (in either C or C++) or the synchronization parameters (C++) did not yield any improvement.

Results by others:

@Irfy on g++ 4.7.2-2ubuntu1, -O3, virtualized Ubuntu 11.10, 3.5.0-25-generic, x86_64, enough ram/cpu, 196MB of several “find / >> largefile.txt” runs

C : 634572
C++: 473222

C++ 25% faster

@Matteo Italia on g++ 4.4.5, -O3, Ubuntu Linux 10.10 x86_64 with a random 180 MB file

C : 910390
C++: 776016

C++ 17% faster

@Bogatyr on g++ i686-apple-darwin10-g++-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5664), mac mini, 4GB ram, idle except for this test with a 168MB datafile

C : 4.34151e+06
C++: 9.14476e+06

C++ 111% slower

@Asu on clang++ 3.8.0-2ubuntu4, Kubuntu 16.04 Linux 4.8-rc3, 8GB ram, i5 Haswell, Crucial SSD, 88MB datafile (tar.xz archive)

C : 270895
C++: 162799

C++ 66% faster

So the answer is: it’s a quality of implementation issue, and really depends on the platform :/

The code in full here for those interested in benchmarking:

#include <fstream>
#include <iostream>
#include <iomanip>

#include <cmath>
#include <cstdio>

#include <sys/time.h>

template <typename Func>
double benchmark(Func f, size_t iterations)
{
  f();

  timeval a, b;
  gettimeofday(&a, 0);
  for (; iterations --> 0;)
  {
    f();
  }
  gettimeofday(&b, 0);
  return (b.tv_sec * (unsigned int)1e6 + b.tv_usec) -
         (a.tv_sec * (unsigned int)1e6 + a.tv_usec);
}


struct CRead
{
  CRead(char const* filename): _filename(filename) {}

  void operator()() {
    FILE* file = fopen(_filename, "r");

    int count = 0;
    while ( fscanf(file,"%s", _buffer) == 1 ) { ++count; }

    fclose(file);
  }

  char const* _filename;
  char _buffer[1024];
};

struct CppRead
{
  CppRead(char const* filename): _filename(filename), _buffer() {}

  enum { BufferSize = 16184 };

  void operator()() {
    std::ifstream file(_filename, std::ifstream::in);

    // comment to remove extended buffer
    file.rdbuf()->pubsetbuf(_buffer, BufferSize);

    int count = 0;
    std::string s;
    while ( file >> s ) { ++count; }
  }

  char const* _filename;
  char _buffer[BufferSize];
};


int main(int argc, char* argv[])
{
  size_t iterations = 1;
  if (argc > 1) { iterations = atoi(argv[1]); }

  char const* oldLocale = setlocale(LC_ALL,"C");
  if (strcmp(oldLocale, "C") != 0) {
    std::cout << "Replaced old locale '" << oldLocale << "' by 'C'\n";
  }

  char const* filename = "largefile.txt";

  CRead cread(filename);
  CppRead cppread(filename);

  // comment to use the default setting
  bool oldSyncSetting = std::ios_base::sync_with_stdio(false);

  double ctime = benchmark(cread, iterations);
  double cpptime = benchmark(cppread, iterations);

  // comment if oldSyncSetting's declaration is commented
  std::ios_base::sync_with_stdio(oldSyncSetting);

  std::cout << "C  : " << ctime << "\n"
               "C++: " << cpptime << "\n";

  return 0;
}