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

Marcelo Tosatti
2008-04-18T15:09:56Z

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.