[Qemu-devel] Re: [PATCH] Implement an fd pool to get real AIO with posix-aio

[Avi Kivity <avi@redhat.com>]

Anthony Liguori wrote:
> This patch implements a simple fd pool to allow many AIO requests with
> posix-aio.  The result is significantly improved performance (identical to that
> reported for linux-aio) for both cache=on and cache=off.
>
> The fundamental problem with posix-aio is that it limits itself to one thread
> per-file descriptor.  I don't know why this is, but this patch provides a simple
> mechanism to work around this (duplicating the file descriptor).
>
> This isn't a great solution, but it seems like a reasonable intermediate step
> between posix-aio and a custom thread-pool to replace it.
>
>  
> +static int raw_fd_pool_get(BDRVRawState *s)
> +{
> +    int i;
> +
> +    for (i = 0; i < RAW_FD_POOL_SIZE; i++) {
> +        /* already in use */
> +        if (s->fd_pool[i] != -1)
> +            continue;
> +
> +        /* try to dup file descriptor */
> +        s->fd_pool[i] = dup(s->fd);
> +        if (s->fd_pool[i] != -1)
> +            return s->fd_pool[i];
> +    }
> +
> +    /* we couldn't dup the file descriptor so just use the main one */
> +    return s->fd;
> +}
> +
>   

dup()ing the fd on each request is unnecessary work; would be better to 
cache the duped fd.

Of course, if this is just a stepping stone, it doesn't matter very much.

-- 
error compiling committee.c: too many arguments to function