Windows WriteFile() benchmark on writing files in chunks

WriteFile-Chunk-Benchmark.cpp

#include <benchmark/benchmark.h>
#include <string>
#include <sstream>
#include <windows.h>
#include <iostream>

constexpr static unsigned io_chunk_size = 1024 * 1024 * 8;  // 8 MiB

static void BM_write_file_nochunks(benchmark::State& state) {
	state.PauseTiming();

	uint64_t bytes_to_write = state.range(0);
	DWORD total_bytes_written = 0;
	char* buffer = new char[bytes_to_write];

	std::stringstream file_name;
	file_name << "test_nochunks_" << bytes_to_write / 1024 / 1024 << "MB.bin";

	HANDLE hFile = CreateFile(file_name.str().c_str(),// name of the write
	                          GENERIC_WRITE,          // open for writing
	                          0,                      // do not share
	                          NULL,                   // default security
	                          CREATE_ALWAYS,          // create new file only
	                          FILE_ATTRIBUTE_NORMAL,  // normal file
	                          NULL);                  // no attr. template

	state.ResumeTiming();
	BOOL error_flag = WriteFile(hFile, buffer, bytes_to_write, &total_bytes_written, NULL);
	if (!error_flag)
		std::cerr << "ERROR: " << GetLastError() << std::endl;
	state.PauseTiming();

	state.SetBytesProcessed(total_bytes_written);
	CloseHandle(hFile);
}

static void BM_write_file_chunks(benchmark::State& state) {
	state.PauseTiming();

	uint64_t total_bytes_to_write = state.range(0);
	DWORD total_bytes_written = 0;
	char* buffer = new char[total_bytes_to_write];
	uint64_t offset = 0;

	std::stringstream file_name;
	file_name << "test_chunks_" << total_bytes_to_write / 1024 / 1024 << "MB.bin";

	HANDLE hFile = CreateFile(file_name.str().c_str(),// name of the write
		GENERIC_WRITE,          // open for writing
		0,                      // do not share
		NULL,                   // default security
		CREATE_ALWAYS,          // create new file only
		FILE_ATTRIBUTE_NORMAL,  // normal file
		NULL);                  // no attr. template

	state.ResumeTiming();
	while (offset < total_bytes_to_write)
	{
		DWORD bytes_written = 0;
		const auto bytes_to_write = min(io_chunk_size, total_bytes_to_write - total_bytes_written);

		BOOL error_flag = WriteFile(hFile, buffer, bytes_to_write, &bytes_written, NULL);
		if (!error_flag)
			std::cerr << "ERROR: " << GetLastError() << std::endl;

		buffer += bytes_written;
		offset += bytes_written;
		total_bytes_written += bytes_written;
	}
	state.PauseTiming();

	state.SetBytesProcessed(total_bytes_written);
	CloseHandle(hFile);
}

BENCHMARK(BM_write_file_nochunks)->Range(1 << 20, 1 << 29);
BENCHMARK(BM_write_file_chunks)->Range(1 << 20, 1 << 29);

BENCHMARK_MAIN();