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Re: [Qemu-devel] RFC: running the user interface in a thread ...


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] RFC: running the user interface in a thread ...
Date: Thu, 21 Jan 2016 09:58:11 +0000
User-agent: Mutt/1.5.24 (2015-08-30)

On Tue, Jan 19, 2016 at 01:51:09PM +0100, Kevin Wolf wrote:
> Am 18.01.2016 um 10:54 hat Gerd Hoffmann geschrieben:
> >   Hi folks,
> > 
> > I'm starting to investigate if and how we can move the user interface
> > code into its own thread instead of running it in the iothread and
> > therefore avoid blocking the guest in case some UI actions take a little
> > longer.
> > 
> > opengl and toolkits tend to be bad at multithreading.  So my idea is to
> > have a single thread dedicated to all the UI + rendering stuff, possibly
> > let even the virglrenderer run in ui thread context.
> > 
> > I think we have to make that opt-in per user interface, so we can go
> > forward step by step.
> > 
> > The ui thread will need quite some stuff provided by the mainloop.  Wait
> > for all kinds of events (from vnc socket, x11 connection, ...).
> > Probably timers too.  Wait for events from other threads (guest screen
> > updates).
> > 
> > Suggestions how to tackle that?
> > Can I reuse the qemu mainloop code outside of the iothread?
> 
> That should be possible. The block layer runs additional main loops in
> dataplane threads. I think AioContext is the keyword here, so that you
> process only events in your own UI context.
> 
> I'm copying Stefan who knows this stuff a bit better than me.
> 
> > Maybe it'll be better to go straight for a glib main loop?

That sounds good in theory (but see below) since AioContext integrates
with the glib main loop because it is a GSource.  QEMU code should use
AioContext where we have high resolution timers and APIs for file
descriptor, EventNotifier, and BH.

But I think using multiple glib main loops has limitations/problems.
CCing Daniel Berrange because I vaguely remember this topic being
discussed.

> > Is it possible to wait for file handle events and posix condition
> > variables at the same time?

I don't think POSIX condition variables can be monitored from an event
loop.

Use BH or EventNotifier instead.  BH has optimizations to avoid syscalls
where possible while EventNotifier is just a plain eventfd/pipe
(requires a write() syscall for each notification).

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