Support for Compressed Strings being Dropped in HotSpot JVM?

by

Originally, this option was added to improve SPECjBB performance. The gains are due to reduced memory bandwidth requirements between the processor and DRAM. Loading and storing bytes in the byte[] consumes 1/2 the bandwidth versus chars in the char[].

However, this comes at a price. The code has to determine if the internal array is a byte[] or char[]. This takes CPU time and if the workload is not memory bandwidth constrained, it can cause a performance regression. There is also a code maintenance price due to the added complexity.

Because there weren’t enough production-like workloads that showed significant gains (except perhaps SPECjBB), the option was removed.

There is another angle to this. The option reduces heap usage. For applicable Strings, it reduces the memory usage of those Strings by 1/2. This angle wasn’t considered at the time of option removal. For workloads that are memory capacity constrained (i.e. have to run with limited heap space and GC takes a lot of time), this option can prove useful.

If enough memory capacity constrained production-like workloads can be found to justify the option’s inclusion, then maybe the option will be brought back.

Edit 3/20/2013: An average server heap dump uses 25% of the space on Strings. Most Strings are compressible. If the option is reintroduced, it could save half of this space (e.g. ~12%)!

Edit 3/10/2016: A feature similar to compressed strings is coming back in JDK 9 JEP 254.

More Related Contents:

  1. Can Java 8 code be compiled to run on Java 7 JVM?
  2. When will JVM use intrinsics
  3. Why defining class as final improves JVM performance?
  4. Measuring time spent on GC
  5. Why is the 64bit JVM faster than the 32bit one?
  6. PermGen elimination in JDK 8
  7. Difference between thread’s context class loader and normal classloader
  8. Fastest way to iterate over all the chars in a String
  9. How do I speed up the gwt compiler?
  10. Debug a java application without starting the JVM with debug arguments
  11. When is the static block of a class executed?
  12. java.nio.file.Path for a classpath resource
  13. Java user.home is being set to %userprofile% and not being resolved
  14. Java HashMap performance optimization / alternative
  15. Find out what JVM Eclipse is running on
  16. Exception 0xC0000005 from JNI_CreateJavaVM (jvm.dll)
  17. Java Exceptions counter on JVM HotSpot
  18. JTable Calls Custom Cell Renderer Method… Continuously
  19. Fastest way to sum integers in text file
  20. Java try/catch performance, is it recommended to keep what is inside the try clause to a minimum?
  21. What is the best macro-benchmarking tool / framework to measure a single-threaded complex algorithm in Java? [closed]
  22. Hash Set and Array List performances
  23. Java: how much time does an empty loop use?
  24. Why doesn’t the JVM cache JIT compiled code?
  25. What are bytecodes and how does the JVM handle them
  26. Why a sawtooth shaped graph?
  27. Java for-loop optimization
  28. Is Hibernate an overkill for an Android application? [duplicate]
  29. how to choose the jvm heap size?
  30. Trick behind JVM’s compressed Oops

Leave a Comment