Ensure a .dmp file extension when loading IBM system dumps into MAT
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For platforms that run the IBM JVM, the strategic direction for memory analysis is to move away from PHDs and towards system dumps because they have so much more information. System dumps are the operating system core dumps usually produced with crashes (AIX/Linux=core, z/OS=SVCDUMP, Windows=minidump). In recent versions of the IBM JVM, a system dump can be loaded directly into the Memory Analyzer Tool without running jexract on it. The versions are: Java 5 >= SR12, Java 6 >= SR9, Java 626, and Java 7. The matching WAS versions are WAS >= 220.127.116.11 and >= 18.104.22.168 and >= 8.
However, this is all a bit non-obvious because it isn't made clear that MAT will only list files in its File Open dialog with an extension of .dmp. So if you have a system dump from a recent IBM JVM, ensure it has a .dmp extension before trying to open it in MAT.
By the way, here are the recommended generic JVM arguments to completely move away from PHDs to system dumps for OOMs. Also see this TechNote on prerequisites such as proper ulimits and physical memory.