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From Alexander Rojas <alexan...@mesosphere.io>
Subject Re: Review Request 52877: Fixed wrong float serialization in JSON in locales different from C.
Date Tue, 01 Nov 2016 12:27:05 GMT

-----------------------------------------------------------
This is an automatically generated e-mail. To reply, visit:
https://reviews.apache.org/r/52877/
-----------------------------------------------------------

(Updated Nov. 1, 2016, 1:27 p.m.)


Review request for mesos, Adam B, Benjamin Bannier, Joris Van Remoortere, and Michael Park.


Changes
-------

Add local thread variables.


Bugs: MESOS-6349
    https://issues.apache.org/jira/browse/MESOS-6349


Repository: mesos


Description
-------

Resolves an isse with the JSON serialization functions where floats
would use the process active locale as a decimal separator instead
of the JSON conformant period (`.`).


Diffs (updated)
-----

  3rdparty/stout/include/stout/json.hpp 9734561549eea867d57f14a0ab71febeb9dddc06 
  3rdparty/stout/include/stout/jsonify.hpp 8220d001001e8b9f247c05be58534f1174611aad 

Diff: https://reviews.apache.org/r/52877/diff/


Testing (updated)
-------

In order to test the different options the following benchmarking program was
written using [Google benchmark](https://github.com/google/benchmark):

```c++
#include <benchmark/benchmark_api.h>
#include <stout/json.hpp>

#include <iostream>
#include <random>
#include <string>
#include <sstream>

static void BM_Jsonify(benchmark::State& state) {
  while (state.KeepRunning()) {
    state.PauseTiming();
    JSON::Object object;
    object.values["float00"] = 1234567890.12345;
    object.values["float01"] = 123456789.012345;
    object.values["float02"] = 12345678.9012345;
    object.values["float03"] = 1234567.89012345;
    object.values["float04"] = 123456.789012345;
    object.values["float05"] = 12345.6789012345;
    object.values["float06"] = 1234.56789012345;
    object.values["float07"] = 123.456789012345;
    object.values["float08"] = 12.3456789012345;
    object.values["float09"] = 1.23456789012345;
    object.values["float10"] = 1234567890.12345;
    object.values["float11"] = 123456789.012345;
    object.values["float12"] = 12345678.9012345;
    object.values["float13"] = 1234567.89012345;
    object.values["float14"] = 123456.789012345;
    object.values["float15"] = 12345.6789012345;
    object.values["float16"] = 1234.56789012345;
    object.values["float17"] = 123.456789012345;
    object.values["float18"] = 12.3456789012345;
    object.values["float19"] = 1.23456789012345;
    object.values["float20"] = 1234567890.12345;
    object.values["float21"] = 123456789.012345;
    object.values["float22"] = 12345678.9012345;
    object.values["float23"] = 1234567.89012345;
    object.values["float24"] = 123456.789012345;
    object.values["float25"] = 12345.6789012345;
    object.values["float26"] = 1234.56789012345;
    object.values["float27"] = 123.456789012345;
    object.values["float28"] = 12.3456789012345;
    object.values["float29"] = 1.23456789012345;
    object.values["float30"] = 1234567890.12345;
    object.values["float31"] = 123456789.012345;
    object.values["float32"] = 12345678.9012345;
    object.values["float33"] = 1234567.89012345;
    object.values["float34"] = 123456.789012345;
    object.values["float35"] = 12345.6789012345;
    object.values["float36"] = 1234.56789012345;
    object.values["float37"] = 123.456789012345;
    object.values["float38"] = 12.3456789012345;
    object.values["float39"] = 1.23456789012345;
    object.values["float40"] = 1234567890.12345;
    object.values["float41"] = 123456789.012345;
    object.values["float42"] = 12345678.9012345;
    object.values["float43"] = 1234567.89012345;
    object.values["float44"] = 123456.789012345;
    object.values["float45"] = 12345.6789012345;
    object.values["float46"] = 1234.56789012345;
    object.values["float47"] = 123.456789012345;
    object.values["float48"] = 12.3456789012345;
    object.values["float49"] = 1.23456789012345;

    state.ResumeTiming();

    benchmark::DoNotOptimize(stringify(object));
  }
}

BENCHMARK(BM_Jsonify);

BENCHMARK_MAIN();
```

The program creates a JSON object with 50 floats in it, and then it creates
its string representation. Since the algorithms in `json.hpp` and `jsonify.hpp`
are similar but not the same, they were benchmarked as different. While the
original solutions are non solutions since they produce erroneous results if
localization is active, they constitute a baseline for comparison with the
alternatives. These are the benchmarks for the original algorithms and their
alternatives:

1. `json.hpp` default:

```c++
char buffer[50] {};
snprintf(
    buffer,
    sizeof(buffer),
    "%#.*g",
    std::numeric_limits<double>::digits10,
    number.value);

std::string trimmed = strings::trim(buffer, strings::SUFFIX, "0");
return out << trimmed << (trimmed.back() == '.' ? "0" : "");
```

   Perfomance

```
Run on (8 X 1000 MHz CPU s)
2016-11-01 11:08:49
Benchmark    Time(ns)    CPU(ns) Iterations
-------------------------------------------
BM_Jsonify      62934      62927      11653
```

2. `jsonify.hpp` default (it changes the call to `strings::trim` in order to
   avoid string copies:

```c++
char buffer[50] {};
const int size = snprintf(
    buffer,
    sizeof(buffer),
    "%#.*g",
    std::numeric_limits<double>::digits10,
    number.value);

int back = size - 1;
for (; back > 0; --back) {
  if (buffer[back] != '0') {
    break;
  }
  buffer[back] = '\0';
}
return out << buffer << (buffer[back] == '.' ? "0" : "");
```

   Performance

```
Run on (8 X 1000 MHz CPU s)
2016-11-01 11:10:21
Benchmark    Time(ns)    CPU(ns) Iterations
-------------------------------------------
BM_Jsonify      58679      58658      11894
```

3. Proposal 1, manually searching for a character which is not expected (and it
   is assumed to be the decimal separator) and replacing it:

```c++
char buffer[50] {};
snprintf(
    buffer,
    sizeof(buffer),
    "%#.*g",
    std::numeric_limits<double>::digits10,
    number.value);

for (char &ch : buffer) {
  if (!isdigit(ch) && ch != '-' && ch != '.' && ch!= '\0') {
    ch = '.';
    break;
  }
}

std::string trimmed = strings::trim(buffer, strings::SUFFIX, "0");
return out << trimmed << (trimmed.back() == '.' ? "0" : "");
```

   Performance

```
Run on (8 X 1000 MHz CPU s)
2016-11-01 11:11:18
Benchmark    Time(ns)    CPU(ns) Iterations
-------------------------------------------
BM_Jsonify      63559      63535      10825
```

4. Proposal 2, the C++ approach, use streams and stream manipulators:

```c++
std::stringstream ss;
ss.imbue(std::locale::classic());
ss << std::setprecision(std::numeric_limits<double>::digits10)
   << std::showpoint << number.value;

std::string trimmed = strings::trim(ss.str(), strings::SUFFIX, "0");
return out << trimmed << (trimmed.back() == '.' ? "0" : "");
```

   Performance

```
Run on (8 X 1000 MHz CPU s)
2016-11-01 11:12:35
Benchmark    Time(ns)    CPU(ns) Iterations
-------------------------------------------
BM_Jsonify      74237      74242       8678
```

5. Proposal 3, try to avoid copies (there are still some being created):

```c++
std::stringstream ss;
ss.imbue(std::locale::classic());
ss << std::setprecision(std::numeric_limits<double>::digits10)
   << std::showpoint << number.value;

std::string representation = ss.str();
int back = representation.size() - 1;
for (; back > 0; --back) {
  if (representation[back] != '0') {
    break;
  }
}
representation.resize(back + 1);
out << representation << (representation[back] == '.' ? "0" : "");
```

   Performance

```
Run on (8 X 1000 MHz CPU s)
2016-11-01 11:13:40
Benchmark    Time(ns)    CPU(ns) Iterations
-------------------------------------------
BM_Jsonify      73785      73757       9534
```

6. Using thread local variables:

```c++
static THREAD_LOCAL std::stringstream* ss;
static THREAD_LOCAL std::once_flag once_flag;
std::call_once(once_flag, [=]() {
  ss = new std::stringstream;
  ss->imbue(std::locale::classic());
});
*ss << std::setprecision(std::numeric_limits<double>::digits10)
    << std::showpoint << number.value;.
      std::string trimmed = strings::trim(ss->str(), strings::SUFFIX, "0");
out << trimmed << (trimmed.back() == '.' ? "0" : "");
ss->str("");
return out;
```

   Performance

```
Run on (8 X 1000 MHz CPU s)
2016-11-01 11:14:45
Benchmark    Time(ns)    CPU(ns) Iterations
-------------------------------------------
BM_Jsonify      68407      68415       9716
```


Thanks,

Alexander Rojas


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