Dropped Events

Overview

Some backends, such as the streaming backend implementation, may fail to transmit a trace event due to bandwidth limits or other breakdowns.

To handle such cases, Tonbandgerät backends can signal to the trace handling core that an individual trace event could not be submitted and traced.

Because Tonbandgerät buffering and re-transmitting suche events could both significantly slow down the firmware and make the time required to complete an individual trace hook even more unpredictable, no effort is made to ensure all events are actually transmitted.

However, Tonbandgerät does track such dropped events and ensures a dropped event marker is inserted into the trace, informing the user that some events were dropped and the trace might not perfectly reflect reality.

Note

If your trace contains dropped events, all recordings past that point may be corrupted.

The Dropped Event Counter

Tonbandgerät maintains a single global counter that increments each time an event is dropped. This counter is shared across all cores.

Whenever a drop occurs, Tonbandgerät will attempt to encode and submit a dropped event counter event (a special trace event that carries the current value of the counter) on every subsequent trace event until one such counter event is successfully transmitted. This ensures the converter always learns about losses as soon as the channel recovers, assuming the dropped-event detecting reported by the backend is accurate.

If the counter event itself cannot be transmitted (the channel is still congested), the counter is incremented again and the actual event that triggered the attempt is abandoned. The next trace event will again try to submit a dropped event counter message.

Periodic Submission

As an additional safety measure, Tonbandgerät can also submit a dropped event counter event periodically, even when no drops have occurred. This is controlled by tband_configTRACE_DROP_CNT_EVERY:

// tband_config.h
#define tband_configTRACE_DROP_CNT_EVERY 50  // default

When set to a value N > 0, a dropped event counter event is submitted approximately every N trace events. This gives the converter a regular heartbeat it can use to confirm that no events were silently lost and provides another assurance that the trace is accurate.

Set to 0 to disable periodic submission entirely. In that case, a counter event is only emitted when the drop count actually changes. This saves a small amount of bandwidth at the cost of the safety net.

Tip

The overhead of periodic submission is very small at the default interval of 50. It is generally worth keeping enabled unless bandwidth is extremely constrained.

In the Trace Viewer

When the converter detects a non-zero or increasing dropped event counter in the trace stream, it emits a log message to inform you that events were lost. This appears in the converter output alongside the converted trace.