Why does Xvfb not support the backing store?

[Irek Szczesniak]

On Wed, 16 Mar 2005, Roland Mainz wrote:

> Likely because Xvfb doesn't call the neccesary hooks to setup
> backing store. If I remember it correctly SuSE Linux has a patch for
> that in their RPMs... sndirsch may be able to answer the question...

Thanks for your e-mail, Roland.  I sent an e-mail to Stefan Dirsch
(sndirsch), and promptly got the patch from him.  The patch was to
remove the comment in this line:

     /* miInitializeBackingStore(pScreen); */

But this comment hints that perhaps there is something wrong with the
backing store in Xvfb, and so the backing store was disabled.  And
maybe there is, because the backing store that I got is not fully
functional.

I tested the newly compiled Xvfb on both Linux and Mac OS X.  In both
cases I ran Xvfb and opened 40 windows.  On Linux I was able to grab
the contents of 6 windows (very nice images, no overlappings).
However, on the rest of windows I got such messages:

> ~/pics >xwd -id 0x140009
> X Error of failed request:  BadDrawable (invalid Pixmap or Window parameter)
>   Major opcode of failed request:  14 (X_GetGeometry)
>   Resource id in failed request:  0x140009
>   Serial number of failed request:  6
>   Current serial number in output stream:  6

X manuals say that an X server does not have to keep backing store for
some windows when it runs out of memory.  So I ran Xvfb with options
"-ld 0 -ls 0", which make Xvfb ask for as much memory as necessary.
Unfortunately, it didn't help.

On Mac OS X worked only for windows with simple graphics, like xclock
or xterm.  When I displayed some photos, the backing store failed to
work.

Is there some limit on the amount of memory Xvfb can use?

Why do I get the error messages for some windows?


Thank you & best,
Irek