## 4.2 Changes in the implementation that affect usability and performance

### 4.2.1 Garbage collection

Entries in thread stacks and closures that are no longer reachable are now garbage collected.

### 4.2.2 Oz dictionaries

The implementation of Oz dictionaries has been replaced. In particular, the new implementation can efficiently handle lookups for absent entries (when it is necessary to confirm that an entity is not in the dictionary), and can handle subsequent numeric keys space-efficiently.

### 4.2.3 Performance of the distribution subsystem

The distribution subsystem and the distribution's marshaler have been analyzed and tuned in order to achieve high throughput between Mozart processes.

### 4.2.4 Pickling format

The pickling format has been incompatibly changed for the sake of space and run-time efficiency.

### 4.2.5 Bugs..

In general, we tried to keep up with the bug reports. There were too many of them to list them all here..