[Qemu-devel] Re: [PATCH 3/3] Implement linux-aio backend

[Marcelo Tosatti]

On Thu, Apr 17, 2008 at 02:26:52PM -0500, Anthony Liguori wrote:
> This patch introduces a Linux-aio backend that is disabled by default.  To
> use this backend effectively, the user should disable caching and select
> it with the appropriate -aio option.  For instance:
> 
> qemu-system-x86_64 -drive foo.img,cache=off -aio linux
> 
> There's no universal way to asynchronous wait with linux-aio.  At some point,
> signals were added to signal completion.  More recently, and eventfd interface
> was added.  This patch relies on the later.
> 
> We try hard to detect whether the right support is available in configure to
> avoid compile failures.

> +    do {
> +     err = io_submit(aio_ctxt_id, 1, iocbs);
> +    } while (err == -1 && errno == EINTR);
> +
> +    if (err != 1) {
> +     fprintf(stderr, "failed to submit aio request: %m\n");
> +     exit(1);
> +    }
> +
> +    outstanding_requests++;
> +
> +    return &aiocb->common;
> +}
> +
> +static void la_wait(void)
> +{
> +    main_loop_wait(10);
> +}

Sleeping in the context of vcpu's is extremely bad (eg virtio-block
blocks in write() throttling which kills performance). It should wait
on IO completions instead (qemu-kvm.c creates a pthread "waitqueue" to
resolve that issue).

Other than that looks fine to me, will give it a try.