Re: [patch 2/2] QEMU: decrease console "refresh rate" with -nographic

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Apr 15, 2008 at 08:40:09AM -0500, Anthony Liguori wrote:
> Avi Kivity wrote:
> >Anthony Liguori wrote:
> >>
> >>BTW, when we set O_ASYNC on the tap fd, we're eliminating 
> >>O_NONBLOCK.  This means that we have to poll loop select() when 
> >>readv()'ing packets instead of just reading until hitting AGAIN.  
> >>This means at least an extra syscall per packet.

Yeah, I noticed that problem too.

> >
> >I didn't know that O_ASYNC and O_NONBLOCK were mutually exclusive.  
> >Can you point me at the relevant documentation?
> 
> I don't know that they are, but we're doing an:
> 
> fcntl(fd, F_SETFL, O_ASYNC);
> 
> F_SETFL is not additive so the previous O_NONBLOCK gets dropped.

Fortunately read() will only be issued for the tap fd when select()
returns with its fd set.

And when that happens there is always a packet available for reading...


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