Multi-threaded composition is actually mostly serialized by SurfaceStack::guard.

[Daniel van Vugt]

Multi-threaded composition is actually mostly serialized by SurfaceStack::guard.

Mir has one thread per DisplayBufferCompositor with performance in mind. However those threads get serialized and provide almost no benefit because SurfaceStack::for_each_if() needs to lock its guard mutex in:

void mc::DefaultDisplayBufferCompositor::compose(
    mir::geometry::Rectangle const& view_area,
    std::function<void(std::shared_ptr<void> const&)> save_resource)
{
    renderer->clear(local_frameno);

    mc::RenderingOperator applicator(*renderer, save_resource);
    FilterForVisibleSceneInRegion selector(view_area);
    scene->for_each_if(selector, applicator); <-------------- This function is locked

    overlay_renderer->render(view_area, save_resource);
}

Unfortunately the function that is locked is the one which potentially does most of the work, unless your GL driver likes to defer absolutely everything.

A simple solution would be to use a read-write lock in SurfaceStack. However I do wonder if finally executing GLRenderer concurrently in multiple threads is something that class is truely ready for... ?

[Daniel van Vugt (vanvugt)]

I think this was resolved by: lp:~kdub/mir/no-rendering-operator

[Launchpad Janitor (janitor)]

This bug was fixed in the package mir - 0.1.9+14.10.20140430.1-0ubuntu1

---------------
mir (0.1.9+14.10.20140430.1-0ubuntu1) utopic; urgency=medium

  [ Daniel van Vugt ]
  * New upstream release 0.1.9 (https://launchpad.net/mir/+milestone/0.1.9)
    - mirclient ABI unchanged, still at 7. Clients do not need rebuilding.
    - mirserver ABI bumped to 19. Shells need rebuilding.
    - More libmirserver class changes and reorganization, including;
      . Moving things from shell:: to scene::
      . Rewriting/refactoring surface factories.
    - Added an id() to Renderable.
    - Scene/Renderer interfaces:
      . Scene is no longer responsible for its own iteration (no for_each
        any more). Instead you should iterate over the list returned by
        Scene::generate_renderable_list().
    - Bugs fixed:
      . Stale socket issue. (LP: #1285215)
      . Qt render gets blocked on EGLSwapBuffers. (LP: #1292306)
      . Lock order violated found in helgrind (potential deadlock).
        (LP: #1296544)
      . [regression] SwitchingBundle in framedropping mode can hang.
        (LP: #1306464)
      . [DPMS] Display backlight turns back on almost immediately after
        being turned off. (LP: #1231857)
      . Wrong frame is seen on wake up/resume/unlock. (LP: #1233564)
      . Nested platform is not testable (LP: #1299101)
      . [regression] mir_demo_server_shell crashes on display resume.
        (LP: #1308941)
      . Multi-threaded composition is actually mostly serialized by
        SurfaceStack::guard. (LP: #1234018)
      . Mirscreencast slows down compositing and makes it very jerky.
        (LP: #1280938)
      . Mirscreencast can cause clients to render faster than the screen
        refresh rate. (LP: #1294361)
      . Screen turns on when a new session/surface appears. (LP: #1297876)
      . mir-doc package is >56MB in size, expands to >100MB of files.
        (LP: #1304998)
      . [regression] Clang: 'mir::test::doubles::MockSurface::visible'
        hides overloaded virtual function [-Woverloaded-virtual].
        (LP: #1301135)
      . [regression] GLRenderer* unit tests have recently become noisy.
        (LP: #1308905)
      . FocusController::set_focus_to() no longer seems to raise a session
        to the top. (LP: #1302689)

  [ Ubuntu daily release ]
  * New rebuild forced
 -- Ubuntu daily release <email address hidden> Wed, 30 Apr 2014 13:26:58 +0000