Re: [Qemu-devel] Re: [PATCH 2/7] Enable I/O thread and VNC threads by default

[Anthony Liguori]

On 02/08/2011 05:15 AM, Aurelien Jarno wrote:
> Anthony Liguori a écrit :
>    
> > On 02/08/2011 04:06 AM, Aurelien Jarno wrote:
> >      
> > > Yes, it's slow. But is it a problem? You assume that people use QEMU
> > > only for emulating SMP platforms. This is a wrong assumption. Beside the
> > > x86 target, only sparc really supports SMP emulation.
> > >
> > >        
> > It's *not* just about performance.
> >
> > TCG requires a signal to break out of a tight chained TB loop.  If you
> > have a guest in a tight loop waiting for something external (like
> > polling on a in-memory flag), the device emulation will not get to run
> > until a signal is fired.
> >
> > Unless you set SIGIO on every file descriptor that selects polls on (and
> > you can't because there are a number that just don't support SIGIO),
> > then you have a race condition.
> >
> >      
> In practice you will get a signal when the next timer event expire. I
> agree it's suboptimal, but it works, and has been like that for here.
>    

During early boot up before the periodic timer is enabled can cause 
quite a noticable issue here.

I think it's cris specifically that does polling I/O in the early 
startup before any periodic timer is enabled.

> Having that fixed through an I/O thread is actually quite nice, however
> it should not be done ignoring all the *current* drawbacks of the
> iothread mode. We know them (at least for some of them), so let's try to
> solve them.
>    

Yes, agree 100%.

> And now, I don't buy the argument "it's been there for years", it was
> *disabled* by default.
>    

Yeah, I think we need to enable it by default and commit to fixing all 
of the outstanding issues.

I think we've fixed all that we're aware of but we probably won't find 
the rest unless we enable it universally.

Regards,

Anthony Liguori