The bigger session for the utility instance was too big to attach: http://tech.kevinallasso.org/mem_release_session/
What version of MacOS are you running, and what is your hardware?
OS X 10.7.5 Model Name: Mac mini Model Identifier: Macmini5,2 Processor Name: Intel Core i5 Processor Speed: 2.5 GHz Number of Processors: 1 Total Number of Cores: 2 L2 Cache (per Core): 256 KB L3 Cache: 3 MB Memory: 4 GB Boot ROM Version: MM51.0077.B10 SMC Version (system): 1.75f0 Serial Number (system): C07GQ3KHDJD1 Hardware UUID: 089E18A8-92DE-5EA4-90BC-5C62BFF40D5C
Memory allocated by the JS engine is not affected by jemalloc_purge_freed_pages, so this is not a JS bug.
glandium, any wisdom you have to add would be well-received :)