Our Jenkins master handles just under 300 jobs with about half using SCM (subversion) polling. After a major branching event, we updated to version 1.481 of Jenkins and 1.42 of the subversion plugin. We saw extremely high memory usage shortly after startup with noticeably degraded performance. Analyzing a heap dump with eclipse mat, we got the following summary:

One instance of "java.util.TaskQueue" loaded by "<system class loader>" occupies 792,697,824 (73.68%) bytes. The instance is referenced by org.tmatesoft.svn.core.wc.DefaultSVNRepositoryPool @ 0x3267810 , loaded by "java.net.URLClassLoader @ 0x1ce7c938". The memory is accumulated in one instance of "java.util.TimerTask[]" loaded by "<system class loader>".

After rolling back our subversion plugin to 1.39, with no other changes to our configuration, memory usage settled back into a much more nominal (for us) range.