mdata: buffer metadata change tracker plugin

[vpp.git] / src / plugins / mdata / mdata_doc.md

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+# Buffer metadata change tracker {#mdata_doc}
+
+## Introduction
+
+The mdata plugin uses the vlib main loop "before" performance counter
+hook to snapshoot buffer metadata before calling the node dispatch
+function. Similarly, the plugin uses the main loop "after" hook to
+compare a vectors' worth of buffer metadata after the fact.
+
+The comparison function is a simple octet-by-octet A != B check. We
+accumulate changed octets per-node across the entire run, using a
+single spinlock-protected accumulator.
+
+The "show buffer metadata" command produces a report of all fields
+whose values are changed by nodes visited during a given run.
+
+Since many fields in the vnet_buffer_opaque_t are union members,
+it may appear that a certain node changes numerous fields. The entire
+point of the exercise is to warn developers that if a packet visits
+node N, data placed into opaque union field F *will* be affected.
+
+One should never assume much about buffer metadata immutability across
+arbitrary subgraphs. This tool generates accurate reports, to the
+extent that one exercises the required subgraph trajectories.