在 C++ 中读取镶木地板文件比在 python 中慢 [英] Reading parquet file is slower in c++ than in python

问题描述

我已经编写了使用 c++ 和 python 读取相同镶木地板文件的代码.python读取文件的时间比c++少得多，但众所周知，c++的执行速度比python快.我在这里附上了代码 -

I have written code to read the same parquet file using c++ and using python. The time taken to read the file is much less for python than in c++, but as generally we know, execution in c++ is faster than in python. I have attached the code here -

#include <arrow/api.h>
#include <parquet/arrow/reader.h>
#include <arrow/filesystem/localfs.h>
#include <chrono>
#include <iostream>

int main(){
   // ...
   arrow::Status st;
   arrow::MemoryPool* pool = arrow::default_memory_pool();
   arrow::fs::LocalFileSystem file_system;
   std::shared_ptr<arrow::io::RandomAccessFile> input = file_system.OpenInputFile("data.parquet").ValueOrDie();

   // Open Parquet file reader
   std::unique_ptr<parquet::arrow::FileReader> arrow_reader;
   st = parquet::arrow::OpenFile(input, pool, &arrow_reader);
   if (!st.ok()) {
      // Handle error instantiating file reader...
   }

   // Read entire file as a single Arrow table
   std::shared_ptr<arrow::Table> table;
   auto t1 = std::chrono::high_resolution_clock::now();
   st = arrow_reader->ReadTable(&table);
   auto t2 = std::chrono::high_resolution_clock::now();
   if (!st.ok()) {
      // Handle error reading Parquet data...
   }
   else{
       auto ms_int = std::chrono::duration_cast<std::chrono::milliseconds> (t2 - t1);
       std::cout << "Time taken to read parquet file is : " << ms_int.count() << "ms\n";
   }
}

我在python中使用的代码是-

The code i used in python is -

#!/usr/bin/env python3
import pandas as pd
import pyarrow as pa
import pyarrow.parquet as pq
import time

start_time = time.time()

table = pq.read_table('data.parquet')

end_time = time.time()

print("Time taken to read parquet is : ",(end_time - start_time)*1000, "ms")

在为大约 87mb 的文件运行 c++ 代码时，c++ 的输出是 -

On running the c++ code for a file of size about 87mb, the output for c++ is -

读取 parquet 文件所需的时间为:186 毫秒

Time taken to read parquet file is : 186ms

虽然对于 python 输出是 -

While for python the output is -

读取 parquet 所需的时间为:108.66141319274902 毫秒

Time taken to read parquet is : 108.66141319274902 ms

为什么c++和python中read_table函数的执行时间相差这么大?

Why there is such a difference in time of execution for the function read_table in c++ and python ?

推荐答案

如果你想进行比较，试试这个 CPP 代码:

If you want a comparison try this CPP code:

#include <cassert>
#include <chrono>
#include <cstdlib>
#include <iostream>

using namespace std::chrono;

#include <arrow/api.h>
#include <arrow/filesystem/api.h>
#include <parquet/arrow/reader.h>

using arrow::Result;
using arrow::Status;

namespace {

Result<std::unique_ptr<parquet::arrow::FileReader>> OpenReader() {
  arrow::fs::LocalFileSystem file_system;
  ARROW_ASSIGN_OR_RAISE(auto input, file_system.OpenInputFile("data.parquet"));

  parquet::ArrowReaderProperties arrow_reader_properties =
      parquet::default_arrow_reader_properties();

  arrow_reader_properties.set_pre_buffer(true);
  arrow_reader_properties.set_use_threads(true);

  parquet::ReaderProperties reader_properties =
      parquet::default_reader_properties();

  // Open Parquet file reader
  std::unique_ptr<parquet::arrow::FileReader> arrow_reader;
  auto reader_builder = parquet::arrow::FileReaderBuilder();
  reader_builder.properties(arrow_reader_properties);
  ARROW_RETURN_NOT_OK(reader_builder.Open(std::move(input), reader_properties));
  ARROW_RETURN_NOT_OK(reader_builder.Build(&arrow_reader));

  return arrow_reader;
}

Status RunMain(int argc, char **argv) {
  // Read entire file as a single Arrow table
  std::shared_ptr<arrow::Table> table;
  for (auto i = 0; i < 10; i++) {
    ARROW_ASSIGN_OR_RAISE(auto arrow_reader, OpenReader());
    auto t1 = std::chrono::high_resolution_clock::now();
    ARROW_RETURN_NOT_OK(arrow_reader->ReadTable(&table));
    std::cout << table->num_rows() << "," << table->num_columns() << std::endl;
    auto t2 = std::chrono::high_resolution_clock::now();
    auto ms_int =
        std::chrono::duration_cast<std::chrono::milliseconds>(t2 - t1);
    std::cout << "Time taken to read parquet file is : " << ms_int.count()
              << "ms\n";
  }

  return Status::OK();
}

} // namespace

int main(int argc, char **argv) {
  Status st = RunMain(argc, argv);
  if (!st.ok()) {
    std::cerr << st << std::endl;
    return 1;
  }
  return 0;
}

然后与此python代码进行比较:

Then compare with this python code:

#!/usr/bin/env python3                                                                                                                                                                                     
import pandas as pd
import pyarrow as pa
import pyarrow.parquet as pq
import time

for i in range(10):
    parquet_file = pq.ParquetFile('/home/pace/experiments/so4/data.parquet', pre_buffer=True)
    start_time = time.time()
    table = parquet_file.read()
    end_time = time.time()
    print("Time taken to read parquet is : ",(end_time - start_time)*1000, "ms")

在我的系统上运行 10 次后，t 检验无法区分这两种分布 (p=0.64).

On my system after 10 runs a t-test fails to distinguish the two distributions (p=0.64).

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