Multiprocess rendering in wayland - webkitgtk+ --- socket issue? at at
Wed Jul 17 19:15:27 PDT 2013

> On Mon, Jul 08, 2013 at 09:38:26AM +0200, Jonas Ådahl wrote:
>> On Mon, Jul 8, 2013 at 8:05 AM, Iago Toral <itoral at> wrote:
>> > Hi,
>> >
>> > I am working on porting WebKitGTK+ to Wayland and we are having some
>> > difficulties figuring out the proper way to deal with the multiprocess
>> > architecture introduced with WebKit2.
>> >
>> > In WebKit2 we have two processes that are responsible for rendering
>> the
>> > contents of a webpage. The WebProcess takes care of parsing HTML,
>> > identifying the various layers that are part of that HTML (that will
>> be
>> > rendered separately) and the composition of all these layers to create
>> the
>> > final view of the page. This composition stage is done with OpenGL.
>> Once the
>> > composition is done, the other process (UIProcess) needs a way to
>> access the
>> > results of the composition and paint them on the screen.
>> >
>> > In X11, this is achieved by having the WebProcess render the
>> composition
>> > results to an offscreen XWindow and sharing the XWindow ID between the
>> two
>> > processes. XComposite is used to redirect the XWindow to a pixmap. The
>> > pixmap is painted in the UIProcess.
>> >
>> > As far as we know, there is no API in Wayland to allow two different
>> > processes to share a surface, so we are not sure if the architecture I
>> > describe above is currently supported in Wayland.
>> >
>> > So I guess my questions are:
>> > - Is there a way to share a surface between two processes?
>> > - If not, is there a way to implement this architecture in Wayland as
>> it is
>> > now?
>> > - Would it be possible/interesting to add surface sharing API to
>> Wayland so
>> > that it supports this type of architectures naturally?
>> I proposed an extension[0] for solving this a while back, but since
>> then as far as I know the general consensus has been to use nested
>> compositing[1] for sharing surfaces between processes. The nested
>> compositing is possible now, but if I remember correctly, it will
>> require an extra draw, as there is no Wayland EGL API for directly
>> providing buffers from a nested client to a surface of the host
>> client.
> It's actually almost identical to the sharing that WebKit2 does under
> X - same amount of copies.  The WebProcess will render into an
> EGLSurface (just like under X where it renders to a EGLSurface for a
> redirected window).  On eglSwapBuffers(), the compositor is notified
> that new contents is available and can bind the color buffer as a
> texture, similar to how a a compositor is notified of damage for a
> window under X.
> There's a few important things to notice in the nested example:
>  - We establish a direct connection between WebProcess and UIProcess,
>    so it's even a little more efficient under Wayland since you don't
>    have to go through the X server to notify the UIProcess when the
>    content is changed.
>  - We explicity avoid setting up a listen socket and pass the fd to
>    nested-client (WebProcess) by leaving it open across exec and
>    setting the WAYLAND_SOCKET environment variable to the fd number.
>    wl_display_connect() in the client will look for this and use that
>    fd when possible instead of trying to locate a listen socket.

Currently kalyan and I meet one issue that MiniBrowser UI responding speed
is very slow when WebGL is rendered by 60FPS in normal window and 46FPS in
full screen window (1280 x 1024). We haven't find the root cause. May 2
sockets (one is for Weston, another is for Mini server) will cause race? I
am not sure.

Yan Wang

>  - We're using the same mechanism for buffer sharing that weston and
>    othere full compositors use (Wayland EGL and the
>    EGL_WL_bind_wayland_display extension).  This way, we can reuse the
>    hw-enablement that we did to enable weston in the first place.
>  - nested.c is a toytoolkit client, so it uses window.c and cairo-egl;
>    nested-client.c is a standalone EGL/GLES2 wayland client.
>  - There's a lot of code in the example that is required to get it up
>    and run, but isn't directly related to the nested functionality;
>    all the toytoolkit code in nested.c and all the EGL/GLES2
>    boilerplate in nested-client.c.  In something like WebKit2,
>  - There is some kind of issue in there that looks like nested only
>    runs at half the frame rate.  I suspect we're waiting for the frame
>    events or sending them out too late or something - it's not that
>    the nested compositor model is inherently slow.
>> Regarding the mentioned extension, I had a hacked up
>> proof-of-concept working, but have not continued working on it
>> considering that it nested compositing and added EGL API is supposed
>> to be the way forward. If I have understood the situation wrong, I'd
>> be happy to continue with the previously proposed protocol extension.
> Your extension made a lot sense in that it allowed an application to
> forward buffers to the main compositor to let it do the final
> compositing (and or color conversion) and save a copy.  However, in
> too many cases you want to also clip or scale or otherwise further
> process the content.  So I think it makes sense to first go with the
> nested approach to let the client (UIProcess) use the buffer as a
> texture and then later explore a way to create a subsurface and attach
> the buffer as contents.
> Kristian
>> [0]
>> [1]
>> Jonas
>> >
>> > We would really love to see WebKitGTK+ fully ported to Wayland and
>> this is
>> > probably the major obstacle we have at the moment so we are really
>> looking
>> > forward to seeing the options we have available to make this happen.
>> >
>> > Also, notice I am adding two colleagues of mine to the CC who are not
>> > subscribed to this list. If you reply, please remember to keep them in
>> the
>> > loop.
>> >
>> > Iago
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