A few minutes ago, I realized my disk was paging when I ran Vim. Took a quick look at gkrellm, and yes, in fact, I was almost out of swap space, and physical memory was maxed out. The culprit was Firefox, as usual; firefox-bin was responsible for roughly a gigabyte of X pixmap memory.
So I spent some time digging, and realized that I’d had a window open to the Nagios status map for a few hours, which includes a 992 x 1021 pixel PNG. The page refreshes every minute or so. So I closed Firefox, brought up xrestop, opened the status map again, and watched. Sure enough, X pixmap usage for Firefox jumped up by about 2500K per refresh. In the last 10 minutes or so, that number has ballooned to roughly 50MB.
What gets me is that this is the same image being loaded again and again. It’s not just the back-page cache–it looks like Firefox is keeping every image it loads in X memory, and it never goes away: closing the tab, closing the window, clearing the cache… it looks like nothing short of ending the process frees those pixmaps. :-(