speeding up distributed multihead (xdmx)

Adam Jackson ajax at nwnk.net
Mon May 15 20:22:06 UTC 2017

On Sat, 2017-05-13 at 22:27 -0400, Joshua Marshall wrote:

> This summer I've been charged with updating my university's display
> wall.  It looks like the software features of X11 eclipse those of
> DisplayCluster, but in speaking with the DisplayCluster author, it
> _actually works_.  In particular, the performance over 16 screens was
> wholly unacceptable and traced it to a part of the server which
> serializes a nondescript something.
> Now, does anybody know what he is referring to, if the performance
> issues persist, and how hard it might be to fix them?

I looked up the DisplayCluster white paper, and while it does say that
dmx doesn't scale beyond 16 tiles that's perhaps an unwarranted
conclusion. I suspect what it refers to is that the stock X server
source has an internal limitation of 16 screens. So yeah, performance
beyond 16 tiles is going to be unacceptable, because it's zero ;)

That's literally just a #define controlling the size of a few arrays
though, you can change MAXSCREENS to whatever you want when you build.
Alternatively, subdivide: one Xdmx frontend talking to four Xdmx
middle-ends each talking to 16 tiles, for example.

There could certainly be performance problems beyond that; more details
would be helpful. Xdmx definitely has other limitations that might make
it unsuitable for your use (old OpenGL support, 32 kilopixel coordinate
limit, no compressed image transfer) so I don't want to oversell it,
but if you do encounter issues please let us know so we can determine
whether they're fixable.

- ajax

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