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

[Frediano Ziglio]

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

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).

Frediano