Re: [RFC] Multiple QEMU AIO implementations

Marcelo Tosatti
2008-04-16T23:52:20Z

On Wed, Apr 16, 2008 at 04:36:42PM -0500, Anthony Liguori wrote:
> This isn't fully cooked yet, but pretty close. The basic idea is to
> make the aio usage in block-raw go to a set of function pointers and
> allow multiple simultaneous AIO implementations.

The AIO API looks great. Looking forward to a linux-aio backend.

> I converted the posix-aio support to this, and also introduced a "unix"
> aio which just uses O_NONBLOCK and select(). The later only supports 1
> simultaneous request per-fd but currently posix-aio is limited to that
> too. At least with my QEMU testing, the unix aio implementation
> outperforms posix-aio by a factor of 2.

Unfortunately O_NONBLOCK is not honoured for block backed storage,
so its a no-go.

> And it uses no signals...
>
> I'm inclined to suggest that we use signalfd with posix-aio, and for
> older guests, just fall back to unix aio. We can also introduce a
> linux-aio and use that when possible.

I think Avi's suggestion to emulate signalfd with a separate thread
which does sigtimedwait+write-to-a-pipe is sensate.

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