Q Sometimes, every few weeks, my iMac reports that there is no system memory available to run applications. When this last happened, I checked Activity Monitor and saw that it was SystemUIServer that was causing this problem, which returns to normal after a restart. Why should this happen?
A SystemUIServer is a part of Mac OS X that handles some of its human interface, specifically parts of the menu bar, including handling third-party menu enhancements.
Thus it is most probable that an app or process which uses the menu bar is suffering from a memory leak, that is it is failing to free up memory that it no longer requires. Such leaks progressively consume more and more memory until they exhaust that available, causing this error, and requiring a restart.
Keep a careful eye to see which app is likely to be responsible. In the past, problems like this have resulted from outdated buggy versions of Dropbox, for example.
It is perhaps more probable in this case that the culprit is actually a background service, perhaps an old extension or database server, which needs updating or removal. One way of establishing this is to restart with the Shift key held down, disabling non-Apple startup items, as that should make the problem disappear if caused by such a third-party service.
Because OS X automatically restarts SystemUIServer if it quits, one workaround when its memory use has become excessive would be to select SystemUIServer in Activity Monitor and force it to quit. Wait a little while and a fresh copy will run up. You can also get more detailed information by opening Terminal, and typing in the Shell command sysdiagnose SystemUIServer to generate full system information.
Updated from the original, which was first published in MacUser volume 26 issue 16, 2010.