This issues documneted and fixed in 6.0 does also affect 5.5.27: https://issues.apache.org/bugzilla/show_bug.cgi?id=46085 This problem occures very often with JVM IBM / Multi-Core Maschine becuase its bascially related to the Java Memory Modell and this combination uses Memory Optimization much more.
I have proposed the same fix for 5.5.x
This has been fixed in 5.5.x and will be included in 5.5.28 onwards.