Help with AIO

[Hagai Avrahami]

Hi

When benchmarking AIO read, I reached more than 1024 concurrent Async IO -
1. I saw a weird error log
2. Nginx did not fallback to regular blocking read

I looked on the source code --  src/os/unix/ngx_linux_aio_read.c  line:105
and I think  there is a problem in errno handling

// Current Code
//----------------------------------------------------------------------------
    *n = io_submit(ngx_aio_ctx, 1, piocb);

    if (n == 1) {
        ev->active = 1;
        ev->ready = 0;
        ev->complete = 0;

        return NGX_AGAIN;
    }

    n = -n;    <---------------------------------------------------------??

    if (n == NGX_EAGAIN) {
        return ngx_read_file(file, buf, size, offset);
    }

    ngx_log_error(NGX_LOG_CRIT, file->log, n,
                  "io_submit(\"%V\") failed", &file->name);

    if (n == NGX_ENOSYS) {
        ngx_file_aio = 0;
        return ngx_read_file(file, buf, size, offset);
    }

    return NGX_ERROR;*



// I think it should be
//----------------------------------------------------------------------------
    *n = io_submit(ngx_aio_ctx, 1, piocb);

    if (n == 1) {
        ev->active = 1;
        ev->ready = 0;
        ev->complete = 0;

        return NGX_AGAIN;
    }

    if (n == 0)
    {
        ngx_log_error(NGX_LOG_CRIT, file->log, 0,
                  "io_submit(\"%V\") failed - 0 - I/O request blocks",
&file->name);

        return NGX_ERROR;
    }
    else
    {
        n = errno;
    }

* *  if (n== NGX_EAGAIN) {
        return ngx_read_file(file, buf, size, offset);
    }

    ngx_log_error(NGX_LOG_CRIT, file->log, n,
                  "io_submit(\"%V\") failed", &file->name);

    if (n== NGX_ENOSYS) {
        ngx_file_aio = 0;
        return ngx_read_file(file, buf, size, offset);
    }

    return NGX_ERROR;*
//----------------------------------------------------------------------------


your help will be appreciated
Thanks
Hagai

On Tue, Sep 6, 2011 at 9:33 AM, Maxim Dounin <mdounin at mdounin.ru> wrote:

> Hello!
>
> On Tue, Sep 06, 2011 at 01:21:00AM +0300, Hagai Avrahami wrote:
>
> > Hi
> >
> > I configured Nginx with *aio *mode *on *and 1 *worker process*
> > In benchmarking I saw limit of 1024 concurrent sessions even configured
> to
> > 4096 *worker connections*
>
> Sure it's not limitation of the benchmarking tool you use?  Most
> tools out there use select() and hence limited to 1024
> concurrent connections.
>
> > When I looked in the source code I could see that  io_setup is called
> with
> > fix value of 1024 max events
> > can anyone please explain me the hard coded limit
> > Is it OK to change it or it is derived from other limit I am not aware of
> > any help will be appreciated
>
> io_setup() maxevents is a limit on number of aio requests allowed
> to be submitted simulteneously; if nginx hits this limit, it will
> fallback to normal blocking read.
>
> If you are going to change it, make sure to keep total for all
> processes lower than /proc/sys/fs/aio-max-nr.
>
> Maxim Dounin
>
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>



-- 

*Hagai Avrahami*
Qwilt | Work: +972-72-2221644| Mobile: +972-54-4895656 |
hagaia at qwilt.com<yoav at qwilt.com>
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