mmv(4) Memory Mapped Values for Performance Co-Pilot




The files in $PCP_TMP_DIR/mmv are generated by mmv_stats_init() function from libpcp_mmv library. There could be multiple files in this directory, each file representing a single source of the performance metrics. The metrics are harvested by the mmv PMDA which exports them to the rest of the Performance Co-Pilot infrastructure.


Each file starts with the following header:

0    4    tag == "MMV\0"

4    4    Version

8    8    Generation 1

16    8    Generation 2

24    4    Number of TOC entries

28    4    Flags

32    4    Process identifier (PID)

36    4    Cluster identifier

The generation numbers are timestamps at the time of file creation, and must match for the file to be considered by the MMV PMDA.

The flags can specify ways in which the client would like the MMV PMDA to behave - e.g. the MMV_FLAG_PROCESS flag specifies that only if the process identified by PID is currently running should those values be exported.

Finally, if set, the cluster identifier is a hint to the MMV PMDA as to what cluster should be used with this application when forming the individual metric identifiers. A performance metric identifier (see PMDA(3)) consists of the PMDA domain number, the cluster number, and the individual item numbers described in the Metrics section.

The header is followed by at least 2 TOC sections: one section for metrics and another for values. The TOC section has the following format:


0    4    Section Type (see mmv_stats.h)

4    4    Number of entries in the section

8    8    Section's offset from the start of the file

The section types are:

1: Indoms (instance domain definitions)
2: Instances
3: Metrics (metric definitions)
4: Values
5: String

The only mandatory sections are Metrics and Values. Indoms and Instances sections only appear if there are metrics with multiple instances. String sections only appear if there are metrics with string values, or when Metrics or Indoms are defined with help text.

The entries in the Indoms section have the following format:


0    4    Unique serial number for this domain

4    4    Number of entries in the domain

8    8    Offset to first instance

16    8    Short help text offset

24    8    Long help text offset

The entries in the Instances section have the following format:


0    8    Offset into the indom section

8    4    Unused padding (zero filled)

12    4    Internal instance identifier

16    64    External instance identifier

The entries in the Metrics section have the following format:


0    64    Metric Name

64    4    Metric Item (see PMDA(3))

68    4    Metric Type (see mmv_stats.h)

72    4    Semantics (see PMAPI(3))

76    4    Dimensions (see PMAPI(3))

80    4    Instance Domain ID

84    4    Unused padding (zero filled)

88    8    Short help text offset

96    8    Long help text offset

The entries in the Values section have the following format:


0    8    pmAtomValue (see PMAPI(3))

8    8    Extra space for STRING and ELAPSED

16    8    Offset into the Metrics section

24    8    Offset into the Instances section

Each entry in the strings section is a 256 byte character array, containing a single NULL-terminated character string. So each string has a maximum length of 256 bytes, which includes the terminating NULL.