How to disable/limit pixmap cache in X

[Brian Paul]

Jim Kronebusch wrote:
> On Wed, 19 Sep 2007 11:34:19 +0300, Daniel Stone wrote
>> On Tue, Sep 18, 2007 at 10:02:17PM -0500, Jim Kronebusch wrote:
>>> 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.
>> Um.  If someone says 'put this image into a window', you need to know
>> _which image to put_, which requires the server to store the pixmap, at
>> least temporarily.  If this is undesirable, you could patch the
>> applications to just free the pixmaps as soon as they're done using
>> them, or (in Firefox's case), just to not leak gigabytes of the bloody
>> things.
>>
>> Limiting pixmap storage will mean your apps will just get horribly
>> confused and exit immediately, which is probably not what you want.
>>
>> So, either way, the answer comes down to 'look at the app'.  (Or, 'don't
>> use Firefox'.)
> 
> This problem isn't just related to Firefox (although it appears to be the worst
> offender).  OpenOffice has trouble as well coming in a close second depending on what
> you are doing in it (typing a letter obviously is not a problem).  And various other
> apps do this to some degree as well.
> 
> The problem in the thin client world is that when an app runs wild with pixmaps it is
> possible to chew up all available RAM.  This in turn hard freezes thin clients.
> 
> So even though it is not good practice and can cause apps to crash, I am looking for a
> way to limit how much RAM can be used for the purpose of storing pixmap data.  If such a
> setting was available an offending application may simply die a horrible death, but at
> least the client itself can still function without requiring a hard reboot and loss of
> data in other open applications.
> 
> This would be a band-aid to keep thin clients running until a method is found to work
> with the offending applications.  Problem is there are so many apps there needs to be a
> way to set a limit, then if apps start crashing we can track down their devels and try
> to get a fix into the app.  I realize this isn't a real problem for full desktops, but
> please understand how big of a deal this is for the usability of Linux Thin clients.

Maybe you could write a shim library (inserted between the app and Xlib 
with LD_PRELOAD) that intercepted pixmap create/destroy calls and kept 
an approximate count of memory usage.  When some memory threshold is 
hit, exit.

Of course, it would rely on clients actually using the shim.  But maybe 
you can control that in your environment.

Something like this could probably be implemented on the server side as 
an extension, but don't quote me on that.

-Brian