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Timestamping

The support for Timestamps is optional and so the contents of this chapter are informative only.

In many systems it is desirable to periodically insert a timestamp packet into the trace stream, effectively marking that point in the stream with a time value.

This can be used to judge "time" between various point in the trace stream and, more notably, to be able to correlate trace streams from different harts (i.e. this point in hart A’s stream occurred at roughly the same time as that point in hart B’s trace stream). The former helps one to judge performance of sections of code execution (to the granularity of timestamp insertion). The latter helps debugging multi-hart MP problems.

An implementation may have the following:

  • A timestamp is (up to) a 64-bit time value.

  • Configurable options for generating timestamp values such as a hart’s 'time' values or 'cycle' values.

  • Options could may also include things like taking 'time' values with the low 4 or 8 bits dropped off which would create a coarser granularity time values

  • Timestamp generation may be enabled or disabled. If enabled, a timestamp packet would be generated periodically which may be based on configurable interval or rate, e.g. once every 2n items where 'n' and 'items' are configurable among some limited set of choices. The choices could be:

    • Time

    • Time scaled down. An implementation specific scaled or divided down derivative of time. This may be useful in providing a smaller coarser graularity values

    • Time Interpolated up. An implementation specific interpolated up derivative of time. This may be useful in providing higher resolution time values

    • Cycle

    • Implementation specific

  • A timestamp packet may also be generated in conjunction with a sync packet

  • Timestamp packets are highly compressible and variable in size depending on the number of low bits of the current value that have changed wrt the last emitted timestamp value. If timestamp packets are emitted rarely (but not as rare as sync packets), then they will tend to be, say, 2-4 bytes in size (still much less than the full up to 64-bit size). If timestamp packets are emitted somewhat frequently, then they will tend to be 1-2 bytes in size. If timestamp packets are emitted very frequently, then they will tend to be <1 byte in size. Timestamp values associated with sync packets would always be the full implemented size.