Section: Software
LS3: Large Scale Simulator
Participants : Pierre Sens [ correspondent ] , Jean-Michel Busca.
LS3 is a discrete event simulator originally developed for Pastis, a peer-to-peer file system based on Pastry. LS3 allows to build a network of tenths of thousands nodes on a single computer, and simulate its execution by taking into account message transmission delays. LS3 transparently simulates communication layers between nodes, and executes the same application code (including Pastry, Past and higher layers) as in a real execution of the system.
LS3's modular design consists of three independent layers, allowing the simulator to be reused in areas other than Pastis and Pastry:
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At the kernel level, the system being simulated is described in a generic way in terms of entities triggered by events. Each entity has a current state and a current virtual time, and can be programmed either in synchronous mode (blocking wait of the next event) or in asynchronous mode (activation of an event handler). A multi-threaded event engine delivers events in chronological order to each entity by applying a conservative scheduling policy, based on the analysis of event dependencies.
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At the network level, the system being simulated is modeled in terms of nodes sending and receiving messages, and connected through a network. The transmission delay of a message is derived from the distance between the sending and the receiving nodes in the network, according to the selected topology. Three topologies can be used: local network (all nodes belong to the same local network), two-level hierarchy (nodes are grouped into LANs connected through WANs) and sphere (nodes are located on a sphere). It is possible to set the jitter rate of transmission delays, as well as the rate of message loss in the network.
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The stubs Pastry level interfaces Pastry with LS3: it defines a specialization of Pastry nodes that allows them to interface with standard LS3 nodes. Several parameters and policies that drive the behaviour and the structure of a Pastry network can be set at this level, including: the distribution of node ids, the selection of boostrap nodes, the periodicity of routing tables checks and the rate of node churn. It is also possible to simulate the ping messages that nodes send to supervise each other, and set failure detection thresholds.
Some figures: LS3 can simulate a network of 20 000 Pastry nodes with no application within 512 Mb of RAM, and it takes approximately 12 minutes on a single processor Pentium M 1,7 GHz to build such network. When simulating the Pastis application, event processing speed is about 500 evt/s. The speedup factor depends on the simulated load: as an example, speedup ranges from 20 for a single user to 0,05 for 400 simultaneous users.
