Small Datum

I filed a bug for the Sun JDK a long time ago, sometime around 1998. At the time growing the heap didn't work and the workaround was to set -Xms and -Xmx to the same value. That worked as long as you knew a reasonable value -- too small and your process dies, too large and you waste memory. Now I get to revisit Java GC. I am running Linkbench and the bin/linkbench script that starts the bench client uses -Xmx=1000 to limit the heap to 1000MB of RAM. Someone else wrote that script and I didn't know it was done until a test failed when the heap was too small. I use Linkbench on large and small servers and need to be careful about memory usage on the small servers so I ran a few tests to understand the impact on performance and memory usage for Linkbench run with -Xmx=1000, -Xmx=2000 and -Xmx not set. I used a test with a ~10G database (maxid1=10M) and two levels of concurrency -- 16 clients, 64 clients. For the tests I report metrics from the Linkbench client process: max