Re: [Qemu-devel] [PATCH][RFC] Linux AIO support when using O_DIRECT

[Anthony Liguori]

Christoph Hellwig wrote:
> On Mon, Mar 23, 2009 at 12:14:58PM -0500, Anthony Liguori wrote:
>   
> > I'd like to see the O_DIRECT bounce buffering removed in favor of the  
> > DMA API bouncing.  Once that happens, raw_read and raw_pread can  
> > disappear.  block-raw-posix becomes much simpler.
> >     
>
> See my vectored I/O patches for doing the bounce buffering at the
> optimal place for the aio path. Note that from my reading of the
> qcow/qcow2 code they might send down unaligned requests, which is
> something the dma api would not help with.
>   

I was going to look today at applying those.

> For the buffered I/O path we will always have to do some sort of buffering
> due to all the partition header reading / etc.  And given how that part
> isn't performance critical my preference would be to keep doing it in
> bdrv_pread/write and guarantee the lowlevel drivers proper alignment.
>   

I really dislike having so many APIs.  I'd rather have an aio API that 
took byte accesses or have pread/pwrite always be emulated with a full 
sector read/write

> > We would drop the signaling stuff and have the thread pool use an fd to  
> > signal.  The big problem with that right now is that it'll cause a  
> > performance regression for certain platforms until we have the IO thread  
> > in place.
> >     
>
> Talking about signaling, does anyone remember why the Linux signalfd/
> eventfd support is only in kvm but not in upstream qemu?
>   

Because upstream QEMU doesn't yet have an IO thread.

TCG chains together TBs and if you have a tight loop in a VCPU, then the 
only way to break out of the loop is to receive a signal.  The signal 
handler will call cpu_interrupt() which will unchain TBs allowing TCG 
execution to break once you return from the signal handler.

An IO thread solves this in a different way by letting select() always 
run in parallel to TCG VCPU execution.  When select() returns you can 
send a signal to the TCG VCPU thread to break it out of chained TBs.

Not all IO in qemu generates a signal so this a potential problem but in 
practice, if we don't generate a signal for disk IO completion, a number 
of real world guests breaks (mostly non-x86 boards).

Regards,

Anthony Liguori