linux kernel aio functionality - c

Question

I am testing kernel asynchronous io functions (not posix aio) and am trying to figure out how it works. The code below is a complete program where I simply write an array repeatedly to a file opened using O_DIRECT. I get an error in the callback function "write missed bytes expect 1024 got 0" (see the fprintf statement in work_done()).
For those not familiar with kernel aio, the code below does the following:
Init some structs
Prepare aio (io_prep_pwrite)
Submit io requests (io_submit)
Check for event completion (io_getevents)
Call a callback function to see if everything went ok.
I get an error at step 5. If I do not open the file using O_DIRECT, things work fine, but it beats the purpose of having async writes.
Can someone tell me what I am doing wrong? Is this the correct usage of kernel aio, for example, is my use of callbacks correct? Are there any restrictions on the usage of O_DIRECT?
I compile using 'gcc -Wall test.c -laio'
Thanks in advance.
/*
* File: myaiocp.c
* Author: kmehta
*
* Created on July 11, 2011, 12:50 PM
*
*
* Testing kernel aio.
* Program creates a 2D matrix and writes it multiple times to create a file of desired size.
* Writes are performed using kernel aio functions (io_prep_pwrite, io_submit, etc.)
*/
#define _GNU_SOURCE
#define _XOPEN_SOURCE 600
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
#include <pthread.h>
#include <fcntl.h>
#include <string.h>
#include <sys/uio.h>
#include <sys/time.h>
#include <omp.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <libaio.h>
char ** buf;
long seg_size;
int seg_rows;
double total_size;
char * filename;
static int wait_count = 0;
void io_task();
void cleanup();
void allocate_2D_matrix(int[]);
int file_open(char *);
void wr_done(io_context_t ctx, struct iocb* iocb, long res, long res2);
int main(int argc, char **argv) {
total_size = 1048576; //1MB
seg_size = 1024; //1kB
seg_rows = 1024;
filename = "aio.out";
int dims[] = {seg_rows, seg_size};
allocate_2D_matrix(dims); //Creates 2D matrix
io_task();
cleanup();
return 0;
}
/*
* Create a 2D matrix
*/
void allocate_2D_matrix(int dims[2]) {
int i;
char *data;
//create the matrix
data = (char *) calloc(1, dims[0] * dims[1] * sizeof (char));
if (data == NULL) {
printf("\nCould not allocate memory for matrix.\n");
exit(1);
}
buf = (char **) malloc(dims[0] * sizeof (char *));
if (buf == NULL) {
printf("\nCould not allocate memory for matrix.\n");
exit(1);
}
for (i = 0; i < dims[0]; i++) {
buf[i] = &(data[i * dims[1]]);
}
}
static void io_error(const char *func, int rc)
{
if (rc == -ENOSYS)
fprintf(stderr, "AIO not in this kernel\n");
else if (rc < 0)
fprintf(stderr, "%s: %s\n", func, strerror(-rc));
else
fprintf(stderr, "%s: error %d\n", func, rc);
exit(1);
}
/*
* Callback function
*/
static void work_done(io_context_t ctx, struct iocb *iocb, long res, long res2)
{
if (res2 != 0) {
io_error("aio write", res2);
}
if (res != iocb->u.c.nbytes) {
fprintf(stderr, "write missed bytes expect %lu got %ld\n",
iocb->u.c.nbytes, res2);
exit(1);
}
wait_count --;
printf("%d ", wait_count);
}
/*
* Wait routine. Get events and call the callback function work_done()
*/
int io_wait_run(io_context_t ctx, long iter)
{
struct io_event events[iter];
struct io_event *ep;
int ret, n;
/*
* get up to aio_maxio events at a time.
*/
ret = n = io_getevents(ctx, iter, iter, events, NULL);
printf("got %d events\n", n);
/*
* Call the callback functions for each event.
*/
for (ep = events ; n-- > 0 ; ep++) {
io_callback_t cb = (io_callback_t)ep->data ; struct iocb *iocb = ep->obj ; cb(ctx, iocb, ep->res, ep->res2);
}
return ret;
}
void io_task() {
long offset = 0;
int bufIndex = 0;
//Open file
int fd = file_open(filename);
//Initialize structures
long i;
long iter = total_size / seg_size; //No. of iterations to reach desired file size (total_size)
io_context_t myctx;
if(0 != io_queue_init(iter, &myctx))
{
perror("Could not initialize io queue");
exit(EXIT_FAILURE);
}
struct iocb * ioq[iter];
//loop through iter times to reach desired file size
for (i = 0; i < iter; i++) {
struct iocb *io = (struct iocb*) malloc(sizeof (struct iocb));
io_prep_pwrite(io, fd, buf[bufIndex], seg_size, offset);
io_set_callback(io, work_done);
ioq[i] = io;
offset += seg_size;
bufIndex ++;
if (bufIndex > seg_rows - 1) //If entire matrix written, start again from index 0
bufIndex = 0;
}
printf("done preparing. Now submitting..\n");
if(iter != io_submit(myctx, iter, ioq))
{
perror("Failure on submit");
exit(EXIT_FAILURE);
}
printf("now awaiting completion..\n");
wait_count = iter;
int res;
while (wait_count) {
res = io_wait_run(myctx, iter);
if (res < 0)
io_error("io_wait_run", res);
}
close(fd);
}
void cleanup() {
free(buf[0]);
free(buf);
}
int file_open(char *filename) {
int fd;
if (-1 == (fd = open(filename, O_DIRECT | O_CREAT | O_WRONLY | O_TRUNC, 0666))) {
printf("\nError opening file. \n");
exit(-1);
}
return fd;
}

Answer 1

First of all, good job using libaio instead of POSIX aio.
Are there any restrictions on the usage of O_DIRECT ?
I'm not 100% sure this is the real problem, but O_DIRECT has some requirements (quoting mostly from TLPI):
The data buffer being transferred must be aligned on a memory boundary that is a multiple of the block size (use posix_memalign)
The file or device offset at which data transfer commences must be a multiple of the block size
The length of the data to be transferred must be a multiple of the block size
At a glance, I can see you are not taking aby precautions to align memory in allocate_2D_matrix.
If I do not open the file using O_DIRECT, things work fine, but it
beats the purpose of having async writes.
This happens not to be the case. Asynchronous I/O works well without O_DIRECT (for instance think of the number of system calls slashed).