mmv — Memory Mapped Values for Performance Co-Pilot
Synopsis
$PCP_TMP_DIR/mmv/<file>
Description
The files in $PCP_TMP_DIR/mmv are generated either by the mmv_stats_init(3) and mmv_stats2_init(3) functions from the libpcp_mmv library, or by a native language module such as Parfait (Java), Speed (Golang) or Hornet (Rust).
There could be multiple files in this directory, each file representing a single source of the performance metrics. The metrics are harvested by pmdammv(1) which exports them to the rest of the Performance Co-Pilot infrastructure.
File Format
Each file starts with the following header:
| Offset | Length | Name |
| 0 | 4 | tag == "MMV\0" |
| 4 | 4 | Version (1 or 2) |
| 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 version number specifies which mapping layout format is in use. There are two, both are very similar, as described below. The sole purpose of the MMV version 2 format is to allow the use of longer metric and instance names. If names longer than MMV_NAMEMAX are not in use, it is best to use MMV version 1 format as this allows older versions of PCP to also consume the data. Support for v2 format was added in the pcp-3.11.4 release.
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:
| Offset | Length | Value |
| 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
6: Labels
The only mandatory sections are Metrics and Values. Indoms and Instances sections of either version only appear if there are metrics with multiple instances. String sections only appear if there are metrics with string values, when Metrics or Indoms are defined with help text, or in v2 MMV format where metric names and instance names are stored in string sections (providing significantly more space than v1 MMV format for these names). Label sections only appear if there are metrics annotated with labels (name/value pairs). Labels are supported in v3 MMV format.
The entries in the Indoms sections have the following format:
| Offset | Length | Value |
| 0 | 4 | Unique serial number for this domain |
| 4 | 4 | Number of entries in the domain |
| 8 | 8 | Offset to first Instance or Instance2 |
| 16 | 8 | Short help text offset |
| 24 | 8 | Long help text offset |
The entries in the Instances (v1) section have the following format:
| Offset | Length | Value |
| 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 Instances (v2) section have the following format:
| Offset | Length | Value |
| 0 | 8 | Offset into the indom section |
| 8 | 4 | Unused padding (zero filled) |
| 12 | 4 | Internal instance identifier |
| 16 | 8 | External instance identifier string offset |
The entries in the Metrics (v1) section have the following format:
| Offset | Length | Value |
| 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 Metrics (v2) section have the following format:
| Offset | Length | Value |
| 0 | 8 | Metric Name string offset |
| 8 | 4 | Metric Item (see PMDA(3)) |
| 12 | 4 | Metric Type (see mmv_stats.h) |
| 16 | 4 | Semantics (see PMAPI(3)) |
| 20 | 4 | Dimensions (see PMAPI(3)) |
| 24 | 4 | Instance Domain ID |
| 28 | 4 | Unused padding (zero filled) |
| 32 | 8 | Short help text offset |
| 40 | 8 | Long help text offset |
The entries in the Values section have the following format:
| Offset | Length | Value |
| 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.
The entries in the Labels (v3) section have the following format:
| Offset | Length | Value |
| 0 | 4 | Flags (PM_LABEL_[CLUSTER|ITEM|INDOM|INSTANCES]|OPTIONAL) |
| 4 | 4 | Identifier for given type (indom, cluster or item) |
| 8 | 4 | Internal Instance or PM_IN_NULL |
| 12 | 244 | Payload (Name and Value JSONB String) |
Each entry in the payload is a 244 byte (maximum) character array, containing a single NULL-terminated name:value pair in JSON format. Insignificant whitespace must not be present. Label names consist only of alphanumeric characters or underscores, and must begin with an alphabetic. Upper and lower case characters are considered distinct.
See Also
PCPIntro(1), pmdammv(1), PMAPI(3), mmv_stats_init(3), pcp.conf(5) and pcp.env(5).
Referenced By
mmv_inc_value(3), mmv_lookup_value_desc(3), mmv_stats2_init(3), mmv_stats_registry(3), pmdammv(1).