Here's another data point, if I may say so. I feel uneasy calling this information "data", or even "information", for that matter. Information, after all, is defined as a measure of decrease in uncertainty. In that sense, I'll be happy if it's not misinformation. This is as clean an experiment as I could manage today. After I sent my earlier observation involving gimp, I have rebooted the machine and started X11 with one bare, undecorated xterm (gimp is too complicated and may have problems of its own -- although it works flawlessly on my linux machine). I used that xterm for a few minutes to read email at work, then launched an ssh with a vnc tunnel to another machine and left it alone for a while. Worked in VNC for a few minutes and quit. Then for a longer while, all I had on my screen was that (idle) xterm window, a firefox window, and Activity Monitor. I didn't touch the machine for several hours, but I was in the same room and would have heard the fan turn on if it were up to something. It wasn't. Then I opened gmail in that firefox window to check this thread and instantly -- I mean, in seconds -- the fan turns on, and sure enough, it is X11. No quartz-wm, no any-wm, no other clients. The prior report fully confirmed. If it is, indeed, a queue problem, then it probably has to do with the kind of stuff thrown in it, rather than quantity, or perhaps the queue itself is broken. Unlike in my first experiment, where I consciously tried to stress it, this time I almost didn't touch it. Not with my hands, anyway. Maybe there is something invisible taking place. An interesting thing, probably not related, is how that load is balanced among the two CPUs. I let it spin for a while without touching the inputs, staring at the Activity Monitor. Typically, it uses one CPU with the other being completely idle. Then, in a few minutes, it gradually moves from to the other CPU. Or, at least, Activity Monitor shows the transition as gradual (maybe it smoothes the graph). Then, without any disturbance from the outside, it suddenly migrates back. So it goes back and forth in a somewhat random pattern. Once during about 5 minues, I saw it loading both CPUs equally, at 50% each, before finally taking over one and releasing the other. I realize that scheduling is many architecture layers down from X11, but I'm irrationally curious to see how this would play out on a snigle-CPU system, if such exist. --Gene