The stock JVM is exceptionally bizarre about this because of the insane decision to pad all fields to memory boundaries (which means on a 32 bit machine a class

The stock JVM is exceptionally bizarre about this because of the insane decision to pad all fields to memory boundaries (which means on a 32 bit machine a class with 10 individual bit fields consumes 40 bytes of memory instead of something sane, like, say, 2). It also has an enormous amount of header overhead; a smallint on a competent Lisp or Smalltalk image takes up, say, one 4 byte word and can use 30 bits of that for data. Java's Integer class which is similar in intent is (in stock JVM) somewhere between 12 and 32 depending on VM settings. I've never seen another VM quite like it. In a previous job we got burned pretty hard by the amount of sheer overhead in this and I had to spend about six months converting everything over to use raw int arrays and copy-on-write objects so we could fit a reasonable but not really huge dataset into memory. Despite this, Hotspot is very good and the stock JVM GC is very useful and it is reasonably straightforward to get visibility into the state of the heap and what is going on with the GC. I've never worked in a professional capacity that has anything comparable to VisualVM, although I gather Smalltalk and commercial Lisps have comparable offerings, and you only have to program in Ruby for a while to learn to appreciate the GC quality.