I'm trying Linux libaio for optimized IO performance in server application. I believe I've done everything necessary (using O_DIRECT, align buffer with memory page...). I'm expecting the call to io_submit returns immediately, but a simple test showing it actually takes something around 80 micro seconds to return on my core i7 laptop. Am I expecting too much or there is something wrong with my test program? (compiled with g++ --std=c++0x -laio )
#include <unistd.h>
#include <fcntl.h>
#include <libaio.h>
#include <errno.h>
#include <cstdlib>
#include <cstdio>
#include <iostream>
#include <chrono>
// Open the file for write, return the file descriptor
int open_write(char const* file)
{
int fd = open(file, O_DIRECT|O_CREAT|O_WRONLY, S_IRWXU|S_IRWXG|S_IROTH);
if (fd < 0) {
perror("open_write");
exit(1);
}
}
// Make a buffer of _size_ byte, fill with 'a', return the buffer, it should be aligned to memory page
void* make_write_buffer(size_t size)
{
void* buf = 0;
int ret = posix_memalign((void**)&buf, sysconf(_SC_PAGESIZE), size);
if (ret < 0 || buf == 0) {
perror("make_write_buffer");
exit(1);
}
memset(buf, 'a', size);
return buf;
}
int main (int argc, char *argv[])
{
static const size_t SIZE = 16 * 1024;
// Prepare file and buffer to write
int write_fd = open_write("test.dat");
void* buf = make_write_buffer(SIZE);
// Prepare aio
io_context_t ctx;
memset(&ctx, 0, sizeof(ctx));
const int maxEvents = 32;
io_setup(maxEvents, &ctx);
iocb *iocbpp = new iocb;
io_prep_pwrite(iocbpp, write_fd, buf, SIZE, 0);
using namespace std::chrono;
// Submit aio task
auto start = monotonic_clock::now();
int status = io_submit(ctx, 1, &iocbpp);
if (status < 0) {
errno = -status;
perror("io_submit");
exit(1);
}
auto dur = duration_cast<microseconds>(monotonic_clock::now() - start);
std::cout << "io_submit takes: " << dur.count() << " microseconds." << std::endl;
io_event events[10];
int n = io_getevents(ctx, 1, 10, events, NULL);
close(write_fd);
io_destroy(ctx);
delete iocbpp;
free(buf);
return 0;
}
