[Spice-devel] [PATCH spice-server 2/2] docs: Add some notes on event scheduling and threading

[Christophe Fergeau]

On Thu, Mar 28, 2019 at 10:27:46AM -0400, Frediano Ziglio wrote:
> > On Thu, Mar 28, 2019 at 04:25:31AM -0400, Frediano Ziglio wrote:
> > > > 
> > > > On Mon, Mar 11, 2019 at 02:03:33PM +0000, Frediano Ziglio wrote:
> > > > > Signed-off-by: Frediano Ziglio <fziglio at redhat.com>
> > > > > ---
> > > > >  docs/spice_threading_model.txt | 8 ++++++++
> > > > >  1 file changed, 8 insertions(+)
> > > > > 
> > > > > diff --git a/docs/spice_threading_model.txt
> > > > > b/docs/spice_threading_model.txt
> > > > > index 9351141c8..25a3a030c 100644
> > > > > --- a/docs/spice_threading_model.txt
> > > > > +++ b/docs/spice_threading_model.txt
> > > > > @@ -39,6 +39,14 @@ connect, disconnect and migrate. Connect and migrate
> > > > > are
> > > > > asynchronous (the job
> > > > >  is done while the current thread is doing something else) while
> > > > >  disconnect
> > > > >  is
> > > > >  synchronous (the main thread will wait for termination).
> > > > >  
> > > > > +One aspect to take into consideration is the event scheduling. SPICE
> > > > > uses
> > > > > some
> > > > > +`SpiceCoreInterface` to handle events. As the events will be handled
> > > > > from
> > > > > a
> > > > > +thread based on the core interface you have to use the correct core.
> > > > > Each
> > > > > +channel has an associated core interface which can be retrieved using
> > > > > +`red_channel_get_core_interface`. There's also a main core interface
> > > > > you
> > > > > can get
> > > > > +using `reds_get_core_interface`. `reds_core_timer_*` and
> > > > > `reds_core_watch_*`
> > > > > +functions use the main core interface.
> > > > 
> > > > Do we need a few words as to when to use the main core interface?
> > > > Apart from this, looks good to me.
> > > > 
> > > > Christophe
> > > > 
> > > 
> > > It sounds a nice idea.
> > > 
> > > But honestly I cannot came with an easy rule beside "If code runs on
> > > main thread like Qemu character devices or everything not running in
> > > a channel you can use the main core interface."
> > 
> > Yes, something like your rule would work "Code running in the QEMU
> > thread should use the main core interface. Code running in the cursor or
> > display channel (through RedWorker) should use xxx interface.. Code
> > running in other channels should use yyy. Be aware that a channel's
> 
> IMHO all code running in channels should use channel core, no matter
> if they run on main or not (in the past I adjusted this).
> Note that cursor channel code can run in both main and not main
> for instance, not all cursor channels runs under RedWorker.
> I think it's easier to avoid too much exceptions.
> 
> > ClientCbs run in a different thread context than the rest of the
> > channel" (though the last sentence may no longer be accurate with the
> > work you are doing in that area).
> 
> ClientCbs will be removed, one less thing to know, and new callbacks/vfunc
> will work on the channel thread so not different from other channel code.
> 
> > 
> > Christophe
> > 
> 
> Taking into account that ClientCbs part will be soon (I hope) obsolete
> and that part of "channel core for channel core" part is partially there
> I would update to
> 
> +One aspect to take into consideration is the event scheduling. SPICE uses some
> +`SpiceCoreInterface` to handle events. As the events will be handled from a
> +thread based on the core interface you have to use the correct core. Each
> +channel has an associated core interface which can be retrieved using
> +`red_channel_get_core_interface`. There's also a main core interface you can get
> +using `reds_get_core_interface`. `reds_core_timer_*` and `reds_core_watch_*`
> +functions use the main core interface.


> +Even though multiple channel types run in the main thread and so could use
> +directly the main code interface, for coherence, rule simplicity and to allows
> +the code to be moved in a separate thread easily if needed, it's advisable to
> +use the channel core interface (that will be the main core interface in this
> +case).

I'm not sure if this paragraph makes things clearer or more confusing
(because too many details). Maybe remove it?

> +Currently character devices and not channel code runs in the main thread.

Is this here to mean character device code should be using reds_core_*?

Overall looks good to me.

> OT: I though QEMU moved to GLib to handle events (so the main core interface
> was using GLib) but for performance reasons epoll or other implementations
> are used (that's the reason for SOCKET limitation for Windows).

Ah ok, I assumed epoll was there for historical reasons, not because
of performance. 

Christophe
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