Optimisation of Virtual Machine Garbage Collection Policies

Garbage collection represents an important feature of many modern systems that allows programmers to avoid explicit memory management and related common errors. However, its usage introduces problems in software performance engineering, because on the one hand the system exhibits better performances when there is low memory occupancy and on the other hand the garbage collection activity blocks other processes, so it may introduce a performance drawback. We propose a queueing model to analyse a system with a garbage collector, where customers arrive according to a Poisson process and the service time distribution depends on the amount of free memory. Customer arrivals correspond to process activations in the system. The model parameters allow one to specify the garbage collector service rate, and the distribution of the delays between successive activations. We show that the process underlying such a queueing model is a Quasi-Birth-Death stochastic process and we derive the steady-state analysis via Matrix Geometric Methods. Finally, we propose a heuristic based on this model to derive an appropriate and effective garbage collector activation rate in order to minimise the average system response time. The parametrisation is done using system statistics.