Alog

There is no need for a particularly tight coupling between the logfile-collection mechanism and the display mechanism. In fact, a standardized logfile format has been recently proposed to allow users to use quite different display programs to view the same logfile [Worley 1992]. Although the main focus of this paper is on display programs, for concreteness we describe alog, the mechanism currently being used to collect logfiles for upshot and PADL.

Alog is a macro package for C (and a subroutine package for Fortran) designed for the efficient collection of an event log. Its main features are:

simplicity

It has a fixed format consisting of 6 integers (event type, process number, user task number, user data item, time cycle (since timers roll over), time stamp, and a short (12-character) user string. Details can be found in [Herrarte and Lusk 1991].

speed

Logging of events consists of putting the above data in an in-memory buffer. In parallel jobs, separate buffers are used, even on a shared-memory machine, so no locking is necessary. Logs are written out to separate files on disk only at the end of the run.

portability

Alog is built on a portable microsecond clock package (distributed with alog), that takes advantage of machine-specific high-resolution timers on various machines to produce a as closely as possible a microsecond-resolution timestamp.

automatic alignment

Not only on workstation networks, but also on distributed-memory machines, the clocks read by individual processes are not synchronized. We use logged synchronization events to place data in the logfile that can be used by a post-processing program to align the separate log files into a single file of events in time order.

uninterpreted events

Although several systems have built alog events logging into the system itself (p4, PCN, Strand), the user is actually free to log any events he wants. We have found logging most useful when the events refer to high-level, application-dependent events.

states are secondary

While many display mechanisms include the notion of an event duration, in alog events are atomic, and it is a separate step for the user to assign pairs of event types to entry to and exit from a user-specified state.

The post-processing step of alog is easy to tune to add information to the logfile that is designed to aid the display program that will read the file. This is done in the case of upshot and PADL.