[FreeNX-kNX] More info on resizing slowness

[Mario Becroft]

Sorry about all the long posts. I have some more details on my problem
where things will slow down if the nxclient window is resized. (Recall
that I am running it in the virtual desktop mode where everything
appears inside a single nxclient window.)

I said the problem happened only when the nxclient window was resized
immediately after being created. Actually, this is wrong. The problem
can happen any time when the nxclient window is resized.

I can't put my finger on what exactly causes the problem and how it
manifests.

SYMPTOMS

It is most obvious in gnome-terminal, where scrolling becomes extremely
slow. It is also noticable in Adobe Reader 8, which becomes about 50%
slower than usual at scrolling. A lot of other programs seem to be
unaffected by the problem, or the difference is too slight to notice.

Obviously gnome-terminal is showing a lot of text, so the problem could
be here, but then again other programs that show text are
unaffected. xterm, for instance, is always fast no matter what. Maybe
it's just that because gnome-terminal is quite slow at the best of
times, the problem is more evident there than with programs that are
naturally faster.

HOW TO REPRODUCE

The problem is triggered by resizing the nxclient window. Resizing the
window may cause the problem to start happening, stop happening, or it
may have no effect on the problem. Once the problem has started, it
continues happening until you resize the window again, or logout and
resume the session, at which point it may persist or may go away.

The problem seems to happen more often when there are a lot of windows
on the screen. With only a single gnome-terminal window on-screen, for
instance, the problem rarely happens, but with some heavier programs
like Adobe Reader, OpenOffice, and Inkscape on the screen, it happens
more frequently.

NOTES

Note that once the problem has started happening (as a result of
resizing the window) starting and stopping the affected applications
makes no difference. For instance you can resize the window, triggering
the problem, then start a gnome-terminal, which will show the slow
behaviour.

The amount of traffic on the wire is identical for the same operation
with and without the problem.

When the problem is happening, the X server that nxclient is displaying
on uses more CPU than usual (for instance, 100% while scrolling a
gnome-terminal, compared to about 40% usually). The X server on the thin
clients is x.org version 7.1, on a standard CentOS 5.2 configuration.

Moving the nxclient window around and pressing control-alt-e or
control-alt-j has no effect. Switching to or from full screen with
control-alt-f has the same effect as resizing the window--it may make
the problem come or go.

The other weird thing is that the problem never happens when running
nxclient on my laptop. nxclient also runs somewhat faster on my laptop
than on our thin clients in general--maybe just because it has better
graphics hardware (ATI firegl compared to Intel i810).

I have tried playing with the link type setting and the cache sizes in
nxclient, but nothing seems to make much of a difference.

This is just so weird I can't begin to understand what could be
happening. Has anyone else seen anything like this? Any ideas how I
could diagnose it?

I think the next step might be for me to run oprofile and find out what
it is doing when it is slow.

-- 
Mario Becroft <mb at gem.win.co.nz>