Thursday, September 23, 2010


Our friends at Oracle have recently published some impressive SPECjbb2005 scores. Hotspot got a little over 3.3 million bops using 8 JVMs on a Sun Fire X4800.

I'm curious about some of the command line options they used. Most of them look like they're JIT and GC tuning options (would anyone really use -XX:InlineSmallCode except for a benchmark?), but one looks a bit different: -XX:+UseCompressedStrings. I also noticed that they've prepended some libraries called alt-string.jar and alt-rt.jar onto the bootstrap classpath.

I've googled this option but not much turns up. My best guess is that they're representing String data using byte arrays instead of char arrays. This would save half the space (8-bits vs. 16-bits), but would only work well for ASCII. i.e. good if you're Western European, not so good if you're Eastern European or Asian. Plus it wouldn't be very significant for small strings, since the object headers of the String and char[] objects take up a big portion of space occupied by each String.

According to the SPEC submission the JVM which supports these options should be available this month, so maybe we'll see some doc for this option to satisfy my curiosity.

No comments:

Post a Comment