JEP 294: Linux/s390x Port

Summary

Port JDK 9 to Linux/s390x.

Motivation

s390x (also known under the names "System z" or "z/Architecture") is a mainframe architecture developed and supported by IBM. Several Linux distributions including Ubuntu, RHEL/Fedora and SuSE run on s390x.

Current cloud deployments with packages like TomEE, Cassandra, Spark, Hadoop and Neo4j, to name just a few, rely heavily on Java. Because most of these packages are open source they run best on OpenJDK, which is currently not available for Linux/s390x.

The reason for this JEP is to resolve this deficiency.

Description

SAP has a complete (i.e., template interpreter, C1, and C2 JIT) and certified (Java SE 1.4-8) s390x port which has been used in production for years. In the s390x Port Project we have already contributed this port, which runs on IBM System z models z10 or newer (64-bit only).

The focus of this JEP is not the porting effort itself, which has been mostly completed, but rather the integration of the port into the JDK 9 master repository.

Currently we have a patch queue with less than 10 changesets and with very few shared changes which shouldn't affect current platforms at all. Besides two minimal build changes in the top-level and jdk repositories, all the other changes are in the hotspot repository only.

We have already started opening JBS issues for the shared changes and some of them are already under review.

Alternatives

Currently, the Zero port can be used to run the JDK on Linux/s390x but it is quite slow (because it only uses the old, deprecated C++ interpreter) and is not very well tested. It is no real alternative for running workloads like application servers or database applications written in Java.

IBM's Developer Kit for Linux is also available for Linux/s390x but it is currently not open source and Java applications usually require some configuration/tuning in order to run with it. Also, it can not be used for testing new features of upcoming Java versions because it is only released after the JDK itself is GA.

Testing

This port has been used in production for years within our commercial SAP JVM offering. We are already building and testing the OpenJDK version of the port daily (by running jtreg and JCK tests, SPEC benchmarks, and several SAP applications), and are committed to do so in the future. Of course we will also maintain the port in the future and update it conforming to all upcoming requirements.

Risks and Assumptions

Unlike our previous PowerPC/AIX Port, this port requires only minimal changes to the existing code base, because:

• s390x is a CISC architecture with a strong memory model
• no OS-relevant changes are required because we only target Linux

We don't foresee any impact on the existing platforms at all.