After reading Which is fastest: read, fread, ifstream or mmap?, I try to benchmark C++ ifstream and mmap myself.
The test file is number.txt, and the size is 4GiB:
# ls -alt number.txt
-rw-r--r-- 1 root root 4294967296 Apr 2 13:51 number.txt
The test_ifstream.cpp is like this:
#include <chrono>
#include <fstream>
#include <iostream>
#include <string>
#include <vector>
const std::string FILE_NAME = "number.txt";
const std::string RESULT_FILE_NAME = "result.txt";
char chunk[1048576];
int main(void)
{
std::ifstream ifs(FILE_NAME, std::ios_base::binary);
if (!ifs) {
std::cerr << "Error opeing " << FILE_NAME << std::endl;
exit(1);
}
std::ofstream ofs(RESULT_FILE_NAME, std::ios_base::binary);
if (!ofs) {
std::cerr << "Error opeing " << RESULT_FILE_NAME << std::endl;
exit(1);
}
std::vector<std::chrono::milliseconds> duration_vec(5);
for (std::vector<std::chrono::milliseconds>::size_type i = 0; i < duration_vec.size(); i++) {
unsigned long long res = 0;
ifs.seekg(0);
auto begin = std::chrono::system_clock::now();
for (size_t j = 0; j < 4096; j++) {
ifs.read(chunk, sizeof(chunk));
for (size_t k = 0; k < sizeof(chunk); k++) {
res += chunk[k];
}
}
ofs << res;
duration_vec[i] = std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::system_clock::now() - begin);
std::cout<< duration_vec[i].count() << std::endl;
}
std::chrono::milliseconds total_time{0};
for (auto const& v : duration_vec) {
total_time += v;
}
std::cout << "Average exec time: " << total_time.count() / duration_vec.size() << std::endl;
return 0;
}
The program reads 1MiB(1024 * 1024 = 1048576) every time (the total count is 4096). Use -O2 optimization:
# clang++ -O2 test_ifstream.cpp -o test_ifstream
# ./test_ifstream
57208
57085
57061
57105
57069
Average exec time: 57105
The average execution time is 57105 ms. From the htop output:
We can see test_ifstream occupies very little memory.
The following is test_mmap file:
#include <chrono>
#include <fstream>
#include <iostream>
#include <string>
#include <vector>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
const std::string FILE_NAME = "number.txt";
const std::string RESULT_FILE_NAME = "result.txt";
int main(void)
{
int fd = ::open(FILE_NAME.c_str(), O_RDONLY);
if (fd < 0) {
std::cerr << "Error opeing " << FILE_NAME << std::endl;
exit(1);
}
std::ofstream ofs(RESULT_FILE_NAME, std::ios_base::binary);
if (!ofs) {
std::cerr << "Error opeing " << RESULT_FILE_NAME << std::endl;
exit(1);
}
auto file_size = lseek(fd, 0, SEEK_END);
std::vector<std::chrono::milliseconds> duration_vec(5);
for (std::vector<std::chrono::milliseconds>::size_type i = 0; i < duration_vec.size(); i++) {
lseek(fd, 0, SEEK_SET);
unsigned long long res = 0;
auto begin = std::chrono::system_clock::now();
char *chunk = reinterpret_cast<char*>(mmap(NULL, file_size, PROT_READ, MAP_FILE | MAP_SHARED, fd, 0));
char *addr = chunk;
for (size_t j = 0; j < file_size; j++) {
res += *chunk++;
}
ofs << res;
munmap(addr, file_size);
duration_vec[i] = std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::system_clock::now() - begin);
std::cout<< duration_vec[i].count() << std::endl;
}
std::chrono::milliseconds total_time{0};
for (auto const& v : duration_vec) {
total_time += v;
}
std::cout << "Average exec time: " << total_time.count() / duration_vec.size() << std::endl;
::close(fd);
return 0;
}
Still use -O2 optimization:
# clang++ -O2 test_mmap.cpp -o test_mmap
# ./test_mmap
57241
57095
57038
57008
57175
Average read time: 57111
We can see the execution time of test_mmap is similar as test_ifstream, whereas test_mmap uses more memory:
