Desktops Running On XMir

Daniel van Vugt daniel.van.vugt at
Tue Jun 25 02:06:30 UTC 2013

I was thinking about this just today, and the fact that it works means 
XMir is probably not working how it should (!)

1. Double-buffered compositors (Usually only Compiz. Sometimes Gnome) 
have >=two buffers of their own. Only the front one gets sent to Mir's 
backbuffer...? This would be sub-optimal because:
   (a) The X compositor is maintaining two buffers at the same time that 
Mir is. It's a waste of resources in the least
   (b) It creates at least one frame of lag.

2. Single-buffered desktops (all others?), which are updated by region 
blits only. No full-screen buffer swapping in the X compositor, because 
for most of them there is no "compositor". Last I checked, single 
buffering cannot work in Mir because we haven't implemented it. You have 
to simulate it by copying the X buffer to Mir's backbuffer (see 
mir/examples/fingerpaint.c). This is again sub-optimal because of the 
copying and lag.

So given these desktops do work under XMir (I checked), and given Mir 
does not understand single buffering, that means our XMir approach 
probably has some slow buffer copying that needs to be eliminated. It 
might feel native, but benchmarks will disagree.

Also, last I checked, RAOF's plan was to use composition bypass to solve 
this problem (when it's ready). Thus skipping the extra step we still 
have and achieving equal performance to native X.

P.S. I just measured 9-16% improvement in windowed Compiz/XMir 
performance by tweaking the compiz config to eliminate Compiz' double 
buffering (untick everything under CCSM > OpenGL). This is safe under 
XMir because Mir does the real buffer swapping.

- Daniel

On 25/06/13 04:43, Jono Bacon wrote:
> Hi All,
> In case anyone didn't see it, Thomas did some testing of other desktops
> running on XMir on Mir:
>   * XFCE -
>   * LXDE -
>   * GNOME 3 -
> He couldn't test KDE because of an archive issue (non-Mir related). He
> will test this soon. Unity 7 already runs on XMir.
> I asked him about performance and he said it feels native.
> Thanks,
>     Jono
> --
> Jono Bacon
> Ubuntu Community Manager
> <> /
> <>
> <>
> <>

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