How to disable/limit pixmap cache in X

[Jim Kronebusch]

I have spent the better part of the last 2 weeks trying to figure out what has been
plaguing my thin client network (and many others all over the world).  It turns out that
there is a feature in X used by many applications that allow them to cache image files
to X in the form of pixmaps.  This appears to be a good thing for standard Desktop
systems as it improves performance for many apps when dealing with images.  However this
is a massive problem for Linux thin clients that appears to have been getting
increasingly worse over the last few years.  Typical thin clients do not have local swap
space, and usually run low amounts of RAM (128MB or so).  Applications such as Firefox
and OpenOffice are very quick to start sucking up the RAM to use as an area to store
pixmaps.  Simple OpenOffice documents (especially when printing to low memory printers)
or websites with many images can quickly chew up all available system RAM on a thin
client starving necessary applications from the RAM they need and causing a complete
system freeze requiring a hard reboot.

It appears that the ability to cache pixmaps to RAM is not a necessity and rather a
feature to help speed up image access.  So I joined this list to see if anyone knows of
a way to say limit in X the amount of pixmap storage available to applications, or how
to even prevent them from caching pixmaps at all.

In the world of thin clients this is an extremely large problem in desperate need of
resolution.  Pixmap caching seems to be the most significant problem causing instability
in Linux thin client technology.  Consider this my desperate cry for help.

If such a feature is not available at this time, if someone could let me know if this is
even a feasible possibility that would be great.  Again a feature to only limit pixmap
cache space or to disable it completely is needed.  Limiting an applications memory
usage as a whole causes the applications to fail when exceeded.  This is not a request
for regulating system memory as a whole, just the caching of pixmaps.

Any thoughts or suggestions on this would be very appreciated by the entire Linux thin
client community (LTSP, Edubuntu, SkoleLinux, MilleXTerm, etc).

Thanks,

Jim Kronebusch
Cotter Tech Department
453-5188


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