Desktops Running On XMir
Daniel van Vugt
daniel.van.vugt at canonical.com
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.
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.
> Jono Bacon
> Ubuntu Community Manager
> www.ubuntu.com <http://www.ubuntu.com> / www.jonobacon.org
> www.identi.ca/jonobacon <http://www.identi.ca/jonobacon>
> www.twitter.com/jonobacon <http://www.twitter.com/jonobacon>
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