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:

1. Init some structs
2. Prepare aio (io_prep_pwrite)
3. Submit io requests (io_submit)
4. Check for event completion (io_getevents)
5. 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).