Optimistic Fossil Collection for Time Warp Simulation

Optimistic Fossil Collection is a fully distributed mechanism to reclaim memory from the state and event histories of a Time Warp simulation. Each fossil collector executes with a Logical Process (LP) and operates independently of other fossil collectors. More precisely, each fossil collector examines event arrival t imes and creates a statistical model of the expected ,variance from LVT. From this model, it is possible to determine the probability that the LP will, in the future, rollback distance X from LVT. Thus, the fossil collector can examine the time-stamps of items in the state and event histories to find the probability that they will be needed in the future. Comparing this probability against a user-specified risk factor, the fossil collector decides if the item can be marked as a fossil and scavenged. Optimistic fossil collection does, however, allow for the possibility for simulation jailure. Consequently, it may be desirable to periodically have complete checkpoints taken and archived during the simulation for possible restart with a smaller risk factor specijied. This method of memory management assumes that there is an underlying stationary distribution for the rollback lengths during a time interval t. This is a reasonable assumption since empirical studies have shown that rollback lengths in Time Warp are relatively constant in length. This assumption can, however, also be relaxed and models that operate without an underlying assumption about the distribution of rollback lengths. This paper reviews the design and implementation of two rollback models for optimistic fossil collection. The first assumes a geometrically distributed rollback length; the second assumes an arbitrary distribution of rollback lengths with fixed mean and variance. An implementation of the mechanism is also reported that describes our experiences with one im*Support for this work was providedin part by the Advanced Research Projects Agency under contract J-FBI-93-116, monitored by the Department of Justice. plementation of optimistic fossil collection.