[regression][DRI] SubBuffer rendering is much slower in compiz 0.9.8.0 than it was in 0.9.7

I'm guessing there's a logic change somewhere in the gles2 code, where something previously only done per output is now done on the whole screen.

framebuffer_object comes to mind, but it's not a factor here, as mentioned above.

Maybe it's because we use triangles instead of quads now. So the number of vertices etc is 50% higher in gles2.

Confirmed with a single monitor too. Even if I turn off all the new gles2 features in the opengl plugin, the gles2 branch gets 15-20% lower scores from glmark2 and glxgears (env vblank_mode=0).

This might be due to the fact that gles2 has to render everything as triangles instead of quads. So we're now pushing around 2x the polygons and 1.5x the number of vertices that trunk does.

OK, the problem is not our use of triangles instead of quads. I just tested quads and the slowdown remains.

Bisected. The major cause of this regression is:
http://bazaar.launchpad.net/~compiz-linaro-team/compiz/gles2/revision/3255

I'm pretty sure revision 3255 came from smspillaz. Though it's not obvious.

Ah okay, its the fact that we draw from an fbo to the backbuffer. Seems fair.

I'll have a look into using glBlitFramebuffer

Okay, I have a half implementation of using glBlitFramebuffer, and have some ideas as to how we might be able to preserve vsync while not using fbo's at all (still necessary for nvidia).

In progress.

I suspect the cause is either glClear (very expensive) being called too much, or this:
- gScreen->glPaintCompositedOutput (tmpRegion, scratchFbo, mask);
+ gScreen->glPaintCompositedOutput (screen->region (), scratchFbo, mask);

I'm a little confused by that.

Changing from tmpRegion to screen->region () effectively meant that the entire backing framebuffer object is painted to the backbuffer, rather than a small portion of it. This is necessary because we are always using glXSwapBuffers and the backbuffer becomes invalidated upon swap.

Why would this change have anything to do with the slowdown if the slowdown still occurrs when painting from a framebuffer object is turned off as you put in the description?

No comment, until I actually test my theories and maybe come up with more.

You could be right about glClear though. I notice that clearBuffers is not set to false again anywhere in screen.cpp

The point is that forcing the glXCopySubBufferMESA code path is still slow after r3255. I was surprised too.

Sorry, my mistake.

The slowdown in r3255 is only due to using the FBO on every frame. If I modify r3255 to use regional redraws then its still fast. So the regression in regional redraw performance happened after r3255.

This is a little bit mind-bending, but I've found the problem, at least on my precise desktop using Intel Sandy Bridge graphics...

1. I disable all the fancy new opengl rendering options so compiz should use regional redraws, so useFbo is always false.
2. On startup, I get a couple of frames with (mask & COMPOSITE_SCREEN_DAMAGE_ALL_MASK) so doubleBuffer.render is called with fullscreen==true. This calls: GLXDoubleBuffer::swap() --> copyFrontToBack() because useFbo was false --> glCopyPixels().
3. After startup, all frames use GLXDoubleBuffer::blit() --> GL::copySubBuffer(), which is now slow.

To make rendering fast again, all I have to do is break the sequence in #2. Either:
  (a) Force fullscreen=false; or
  (b) Comment out copyFrontToBack(); or
  (c) Comment out glCopyPixels()

So the problem, on my desktop where I've done all my performance comparisons, is that calling glCopyPixels just once or twice on startup will forever make GL::copySubBuffer take a slow rendering path thereafter. If I ensure glCopyPixels is never touched on startup then Mesa stays in fast mode for GL::copySubBuffer.

All,

I have modified this bug to just be about a very specific problem I find on my machine. If you're concerned about general performance of compiz 0.9.8.0 with the default settings then please look at bug 1024304 instead.

Fix committed into lp:compiz at revision 3370

This bug was fixed in the package compiz - 1:0.9.8.2+bzr3377-0ubuntu1

---------------
compiz (1:0.9.8.2+bzr3377-0ubuntu1) quantal-proposed; urgency=low

  [ Sam Spilsbury ]
  * debian/python-compizconfig.install
    - Install compizconfig-python.pc
  * debian/patches/100_expo_layout.patch
    - re-add the expo layout that used to be in precise (LP: #1047067)
    - add some testcases

  [ Timo Jyrinki ]
  * New upstream snapshot.
    - Fix multiple window placement bugs (LP: #974242) (LP: #976032)
    - Don't waste CPU looping through and looking at all the windows if you're
      rendering an output that has no damage on it. (LP: #1014986)
    - Updated convert files to fix some typos in the key names. (LP: #1041631)
    - Fix crash when imgsvg is loaded, due to missing symbol
      (decor_apply_gravity from libdecoration). (LP: #956986)
    - Treat unresolved symbols at link time as an error, rather than letting
      them through and cause strange crashes later. (LP: #1043143)
    - Refactors a little bit of the upgrade code and gets it under test to
      prepare to fix (LP: #1042537)
    - Updated AUTHORS from the full bzr log, and re-sort the list.
      (LP: #1042095)
    - Fixes FTBFS for kde4-window-decorator (LP: #1041310)
    - Fix obvious omissions from the introduction of unminimize_*,
      which were causing the unminimize animation settings to be ignored
      (LP: #1040455)
    - resize plugin: don't crash if resize wasn't initiated externally
      (LP: #1045191)
    - Clean up capitalization (LP: #1045652)
    - Avoid division by zero, if plugins try to deform a window down to size
      zero. (LP: #1045235)
    - Make "Unredirect Fullscreen Windows" more reliable. This fixes the
      problem with unredirection failing to engage at all (LP: #1041066) when
      gtk-window-decorator creates offscreen windows that are stacked on top.
      This also fixes the problem with unredirect hiding all windows,
      because it thinks the desktop window should be stacked on top
      (LP: #980663).
    - Ensure unredirected windows don't stay unredirected if they're no longer
      on top. (LP: #1041047)
    - Fix launching terminal functionality and make show-hud default key
      visible. Update the defaults to org.compiz.integrated to reflect the
      actual gnome values pre-gnome-3. (LP: #1040081) (LP: #1046199)
      (LP: #1046190)
    - Fix show-hud, bump COMPIZ_GNOME_INTEGRATED_SETTINGS_LIST_SIZE.
      (LP: #1046212)
    - Fixed: Windows with an alpha-channel, like gnome-terminal, were not
      being considered as possibly covering fullscreen windows. But they most
      certainly can. This ensures such RGBA windows are visible if they're
      stacked above a fullscreen window. (LP: #1046661)
    - Remove ListToStringList (LP: #1046184)
    - Fix typo causing CMake Error (LP: #1045665)
    - Transitions gtk-window-decorator over to use GSettings. Add a testing
      framework for the options code. (LP: #1042323)
    - Also need kdeworkspace since kdecorationbridge.h is there
      (LP: #1046770)
    - Implements some cleanup that was suggested on the merge for the original
      port to gsettings. Other issues fixed as wel...

quantal has seen the end of its life and is no longer receiving any updates. Marking the quantal task for this ticket as "Won't Fix".