  Lattice Gauge Theory is an integral part of particle physics that requires
high performance computing in the multi-Tflops regime. These requirements are
motivated by the rich research program and the physics milestones to be reached
by the lattice community. Over the last years the enormous gains in processor
performance, memory bandwidth, and external I/O bandwidth for parallel
applications have made commodity clusters exploiting PCs or workstations also
suitable for large Lattice Gauge Theory applications. For more than one year
two clusters have been operated at the two DESY sites in Hamburg and Zeuthen,
consisting of 32 resp. 16 dual-CPU PCs, equipped with Intel Pentium 4 Xeon
processors. Interconnection of the nodes is done by way of Myrinet. Linux was
chosen as the operating system. In the course of the projects benchmark
programs for architectural studies were developed. The performance of the
Wilson-Dirac Operator (also in an even-odd preconditioned version) as the inner
loop of the Lattice QCD (LQCD) algorithms plays the most important role in
classifying the hardware basis to be used. Using the SIMD Streaming Extensions
(SSE/SSE2) on Intel's Pentium 4 Xeon CPUs give promising results for both the
single CPU and the parallel version. The parallel performance, in addition to
the CPU power and the memory throughput, is nevertheless strongly influenced by
the behavior of hardware components like the PC chip-set and the communication
interfaces. The paper covers the physics motivation for using PC clusters as
well as a system description, operating experiences, and benchmark results for
various hardware.
