In general, hybrid monitoring is the best technique for monitoring computer systems. Unlike pure software monitoring, it needs an extra hardware monitor. But the hardware monitor can be implemented in an object-independent way except for the adaptation to the object system. In using a standard hybrid interface, many types of parallel object systems can be monitored. As the main parts can be reused, it is worthwhile to provide expensive features such as a global clock. One example of such an universally applicable hardware monitor system is our ZM4 system .
The ZM4 is structured as a master/slave system with a control and evaluation computer (CEC) as the master, and an arbitrary number of monitor agents (MA) as slaves (see fig. 2). The distance between these MAs can be up to 1,000 meters. Conceptually, the CEC is the host of the whole monitor system. It controls the measurement activities of the MAs, stores the measured data and provides the user with SIMPLE, a powerful and universal toolset for evaluation of the measured data (see section 5).
The MAs are IBM PCs which are equipped with up to 4 dedicated probe units (DPUs). The MAs control the DPUs and buffer the measured event traces on their local disks. The DPUs are printed circuit boards which link the MA to the nodes of the object system. The DPUs are responsible for event recognition, time stamping, event recording and for high-speed buffering of event traces.
A local clock with a resolution of 100 ns and a time stamping mechanism are integrated into the DPU. The clock of each DPU obtains all information for preparing precise and globally valid time stamps from the measure tick generator (MTG) via the tick channel. Time stamps in a physically distributed configuration may be adjusted after the measurement, according to the known wire length. While the tick channel together with the synchronization mechanism is our own development, we used commercially available parts for the data channel, i.e. ETHERNET with TCP/IP. The data channel forms the communication subsystem of ZM4, and it is used to distribute control information and measured data.
The ZM4's architectural flexibility has been achieved by two properties: easy interfacing and a scalable architecture. The DPU can easily be adapted to different object systems. Up to now, interfaces have been built for SUN Sparc, DIRMU, Transputer, IBM PC, SUPRENUM and some embedded systems. ZM4 is fully scalable in terms of MAs and DPUs. The smallest configuration consists of one MA with one DPU, and can monitor up to four object nodes. Larger object systems are matched by more DPUs and MAs respectively.
The idea of a scalable architecture influenced the designs of many other monitor systems. Some important implementations are M , Netmon-II , Spy , TMP , TOPSYS  and TRAMS . The ZM4 is distinguished from other approaches by the following features:
For a detailed description of the ZM4 hardware and comparisons to other monitor systems, see .