Issue 42925 that was reported for Tomcat 6 also applies to Tomcat 5.5. When using the Apache Portable Runtime, the sendfile thread can take 100% CPU load when a poll descriptor time-out occurs. This happens because Poll.maintain() is never called for the sendfile thread. The proposed patch (http://issues.apache.org/bugzilla/attachment.cgi?id=20930) can also be applied to Tomcat 5.5.
Link to the original issue: http://issues.apache.org/bugzilla/show_bug.cgi?id=42925
A port of the TC6 fix has been formerly proposed.
Fixed in 5.5.x and will be included in 5.5.26 onwards.