The CMS event builder demonstrator and results with Myrinet

The data acquisition system for the CMS experiment at the Large Hadron Coll ider (LHC) will require a large and high performance event building network . Several switch technologies are currently being evaluated in order to com pare different architectures for the event builder. One candidate is Myrine t. This paper describes the demonstrator which has been setup to study a sm all-scale (16 x 16) event builder based on PCs running Linux connected to M yrinet and Ethernet switches. A detailed study of the Myrinet switch performance has been performed for v arious traffic conditions, including the behaviour of composite switches. R esults from event building studies are presented, including measurements oi l throughput, overhead and scaling. Traffic shaping techniques have been im plemented and the effect on the event building performance has been investi gated. The paper reports on performances and maximum event rate obtainable using custom software, not described, for the Myrinet control program and t he low-level communication layer, implemented in a driver for Linux, A high performance sender is emulated by creating a dummy buffer that remains res ident in the network interface and moving from the host only the first 64 b ytes used by the event building protocol. An approximate scaling in N is pr esented assuming a balanced system where each source sends on average data to all destinations with the same rate. (C) 2001 Elsevier Science B.V. All rights reserved.