NAME | C SYNOPSIS | DESCRIPTION | OTHER CONSIDERATIONS | INSTANCE NAME MATCHING | FILES | SEE ALSO | COLOPHON

PMDACACHE(3)              Library Functions Manual              PMDACACHE(3)

NAME         top

       pmdaCacheStore,  pmdaCacheStoreKey, pmdaCacheLookup, pmdaCacheLookup‐
       Name, pmdaCacheLookupKey, pmdaCacheOp,  pmdaCachePurge,  pmdaCacheRe‐
       size - manage a cache of instance domain information for a PMDA

C SYNOPSIS         top

       #include <pcp/pmapi.h>
       #include <pcp/pmda.h>

       int pmdaCacheStore(pmInDom indom, int flags, const char *name,
               void *private);
       int pmdaCacheStoreKey(pmInDom indom, int flags, const char *name,
               int keylen, const void *key, void *private);
       int pmdaCacheLookup(pmInDom indom, int inst, char **name,
               void **private);
       int pmdaCacheLookupName(pmInDom indom, const char *name, int *inst,
               void **private);
       int pmdaCacheLookupKey(pmInDom indom, const char *name, int keylen,
               const void *key, char **oname, int *inst, void **private);
       int pmdaCacheOp(pmInDom indom, int op);
       int pmdaCachePurge(pmInDom indom, time_t recent);
       int pmdaCacheResize(pmInDom indom, int maximum);

       cc ... -lpcp_pmda -lpcp

DESCRIPTION         top

       The pmdaCache family of routines provide services to support the
       maintenance of complex instance domains for Performance Co-Pilot
       PMDAs.  There is potentially one cache of information for each
       instance domain, and for each instance the cache maintains:
       - external instance name (supplied by the PMDA)
       - internal instance identifier (assigned by pmdaCacheStore or
         calculated from a ``hint'' by pmdaCacheStoreKey)
       - state, where active instances are visible and part of the current
         instance domain, and inactive instances are hidden, but not
         forgotten; pmdaCacheStore or pmdaCacheStoreKey may be used to
         change the state of an instance
       - an optional opaque pointer to data that is associated with the
         instance, but maintained by the PMDA
       - an optional opaque key that is used as a ``hint'' to
         pmdaCacheStoreKey when guessing the initial internal instance
         identifier
       - the last time the cache was saved and the instance had been marked
         as active at some point since the previous cache load or save
         operation

       The semantics of a PCP instance domain require a number of rules to
       be followed, namely:
       1. Each internal instance identifier must be unique and in the range
          0 to 2^31 - 1.  This rule is enforced by the pmdaCache family of
          routines.
       2. The external instance name must be unique.  When the instance name
          contains a space, it is further constrained such that the name to
          the left of the first space (the short name) must also be unique.
          Refer to the INSTANCE NAME MATCHING section below.  The PMDA must
          honor this rule, the pmdaCache family of routines will detect
          attempts to violate this rule.
       3. Where an external instance name corresponds to some object or
          entity, there is an expectation that the association between the
          name and the object is fixed, e.g. ``/dev/hda'' is always the name
          of the same disk on a particular system.  This rule is perhaps the
          responsibility of the PMDA, but is often a characteristic of the
          environment in which the PMDA runs.
       4. It is preferable, although not mandatory, for the association
          between and external instance name and an internal instance
          identifier to be persistent.  This rule is supported by the
          pmdaCache family of routines.
       5. When opaque keys are used, the values of the keys must be unique
          across all instances within an instance domain.  This rule is
          enforced by the pmdaCache family of routines.

       The visible interface to the cache is oriented towards the PMDA
       developer who is most concerned about the names of instances, while
       the details of how the rest of the PCP infrastructure expects the
       internal instance identifiers to be managed is not relevant.

       Instances are updated in the cache for instance domain indom by
       calling pmdaCacheStore or pmdaCacheStoreKey with the external name of
       the instance passed via name.  The opaque pointer private may be used
       to associate additional data with the entry in the cache; if no such
       data is required, private should be NULL.  Any manipulation of the
       additional data (including allocation or freeing) is the
       responsibility of the PMDA caller, as the cache simply maintains the
       pointer to the data (passed via private).

       The upper bound for identifiers allocated for any given indom cache
       can be optionally reduced from the default (2^31 - 1) to some lesser
       maximum, using pmdaCacheResize.  This maximum will then be persisted
       and restored in the usual manner, and can thus be associated
       permanently with a cache once set.  This has applications when using
       these interfaces as general purpose identifier caches, and is less
       applicable when using them for instance domain caching.

       For cases where the PMDA developer wishes to influence the allocation
       of internal instance identifiers, e.g. for instance domains with more
       than one natural dimension, or where there is a desire to allocate
       the same instance identifier each time the PMDA is started, even on
       different hosts, pmdaCacheStoreKey may be used.  In this case, an
       initial ``hint'' for the instance identifier is provided as an opaque
       key via the first keylen bytes in key (which could be any sort of
       data, including binary values) else if keylen is less than 1 or key
       is NULL then name is used as the ``hint''.  The ``hint'' is hashed to
       produce an initial instance identifier in the range 0 to 2^31 - 1 (or
       lesser maximum, if set).  If this instance identifier is already
       allocated, then the value is rehashed.  This procedure is repeated
       until an unallocated instance identifier is found, or
       pmdaCacheStoreKey gives up and returns PM_ERR_GENERIC.  For each
       instance domain, the ``hint'' must be unique across all instances,
       else pmdaCacheStoreKey returns PM_ERR_INST.

       The flags argument controls how the instance should be processed in
       the cache as follows:

       PMDA_CACHE_ADD
              Insert the entry into the cache if it is not already there and
              mark it active.  If the entry is already in the cache mark it
              active.

       PMDA_CACHE_HIDE
              Mark the entry in the cache as inactive, but remember the
              details of the association between the external instance name
              and the internal instance identifier.  Entries that are
              inactive will be hidden from cache traversal via
              PMDA_CACHE_WALK_NEXT operations, but remain visible to
              pmdaCacheLookup, pmdaCacheLookupName and pmdaCacheLookupKey
              requests.

       PMDA_CACHE_CULL
              Remove the entry from the cache.

       On success pmdaCacheStore or pmdaCacheStoreKey will return the
       internal instance identifier of the associated cache entry.  Valid
       instance identifiers are guaranteed to be unique and non-negative.
       Failure will be indicated by a negative value (suitable for decoding
       with pmErrStr(3)) and most likely PM_ERR_INST to indicate the
       requested instance is not in the cache, or -EINVAL to indicate a
       potential violation of the short name uniqueness property (see the
       INSTANCE NAME MATCHING section below).

       pmdaCacheLookup is used to search the entries in the cache based on
       the internal instance identifier inst.

       On success the return value will be PMDA_CACHE_ACTIVE or
       PMDA_CACHE_INACTIVE (depending on the active or inactive state of the
       cache entry), name (if not NULL) and private (if not NULL) will be
       set to the external instance name and the associate additional data
       area as provided when the instance was last activated via
       pmdaCacheStore or pmdaCacheStoreKey.

       pmdaCacheLookup failure is indicated by a negative return value
       suitable for decoding with pmErrStr(3).

       The pmdaCacheLookup interface is required by the PMDA's fetch
       callback that is registered via pmdaSetFetchCallback(3).  Here the
       internal instance identifier is passed to the fetch callback to
       identifier for which instance a value is required.  Typical usage is
       shown in the code fragment below.

         static int
         foo_callback(pmdaMetric *mdesc, unsigned int inst, pmAtomValue *atom)
         {
             mydata   *mdp;
             char     *name;
             int      sts;

             sts = pmdaCacheLookup(mdesc->m_desc.indom, inst, &name, (void **)&mdp);
             /*
              * expect sts == PMDA_CACHE_ACTIVE except for cataclysmic events
              * use mdp as required, name may be useful for diagnostics
              */
             ...

       pmdaCacheLookupName is used to search the entries in the cache based
       on the external instance name name.

       On success the return value will be PMDA_CACHE_ACTIVE or
       PMDA_CACHE_INACTIVE (depending on the active or inactive state of the
       cache entry), inst (if not NULL) and private (if not NULL) will be
       set to the internal instance identifier and the associate additional
       data area as provided when the instance was last activated via
       pmdaCacheStore or pmdaCacheStoreKey.

       pmdaCacheLookupName failure is indicated by a negative return value
       suitable for decoding with pmErrStr(3).

       The pmdaCacheLookupName interface is useful for PMDAs wishing to
       update an instance domain based on the external instance names.

       pmdaCacheLookupKey is used to search the entries in the cache based
       on an opaque key (or ``hint'') previously used in a call to
       pmdaCacheStoreKey.  The ``hint'' is provided via the first keylen
       bytes in key.  For symmetry with pmdaCacheStoreKey, if keylen is less
       than 1 or key is NULL then name is used as the ``hint'' (although the
       results will be the same as calling pmdaCacheLookupName in this
       case).

       On success the return value will be PMDA_CACHE_ACTIVE or
       PMDA_CACHE_INACTIVE (depending on the active or inactive state of the
       cache entry), oname (if not NULL), inst (if not NULL) and private (if
       not NULL) will be set to the external instance name, the internal
       instance identifier and the associate additional data area as
       provided when the instance was last activated via pmdaCacheStore or
       pmdaCacheStoreKey.

       pmdaCacheLookupKey failure is indicated by a negative return value
       suitable for decoding with pmErrStr(3).

       To avoid a persistent cache growing without bound, pmdaCachePurge can
       be used to cull all entries that have not been active in the last
       recent seconds.  For performance reasons, the time accounting is
       imprecise and the entries are timestamped at the time of the next
       cache save operation after the entry has been added or marked active
       (refer to PMDA_CACHE_SAVE and PMDA_CACHE_SYNC below).  On success
       pmdaCachePurge returns the number of culled entries, else in the case
       of an error the return value is negative (and suitable for decoding
       with pmErrStr(3)).

       pmdaCacheOp may be used to perform additional operations on the cache
       as follows:

       PMDA_CACHE_LOAD
              The cache can optionally be maintained as a persistent
              external file, so that the mapping of instance names to
              instance identifiers is persistent across executions of a
              PMDA.  This operation loads the cache from the external file,
              and then all new cache entries are marked inactive, and the
              additional data pointer is set to NULL.  Entries loaded from
              the external file are checked against the current cache
              contents and if the instance name and instance identifiers
              match then the state in the cache (active or inactive) is not
              changed. Should a mismatch be found (same instance name and
              different instance identifier, or same instance identifier and
              different instance name, or some but not all of the instance
              identifier, the instance name and the ``hint'' match) then the
              entry from the external file is ignored and a warning is
              issued on stderr.  Typically a PMDA would only perform this
              operation once per execution.

       PMDA_CACHE_SAVE
              If any instance has been added to, or deleted from, the
              instance domain since the last PMDA_CACHE_LOAD,
              PMDA_CACHE_SAVE or PMDA_CACHE_SYNC operation, the entire cache
              is written to the external file as a bulk operation.  This
              operation is provided for PMDAs that are not interested in
              using pmdaCachePurge and simply want the external file to
              reflect the set of known instances without accurate details of
              when they were last marked active.

              Returns the number of instances saved to the external file,
              else 0 if the external file was already up to date.

       PMDA_CACHE_STRINGS
              Annotates this cache as being a special-purpose cache used for
              string de-duplication in PMDAs exporting large numbers of
              string valued metrics.  This can be used to reduce the memory
              footprint of the PMDA (duplicate strings hash to the same
              bucket, and are stored in memory once only).  Key comparisons
              are not terminated at the first space but rather the entire
              string is used for matching.  These are specialised caches not
              useful for general purpose instance domain handling.

       PMDA_CACHE_SYNC
              Within an instance domain, if any instance has been added to,
              or deleted from, or marked active since the last
              PMDA_CACHE_LOAD, PMDA_CACHE_SAVE or PMDA_CACHE_SYNC operation,
              the entire cache is written to the external file as a bulk
              operation.  This operation is similar to PMDA_CACHE_SAVE, but
              will save the instance domain more frequently so the
              timestamps more accurately match the semantics expected by
              pmdaCachePurge.

              Returns the number of instances saved to the external file,
              else 0 if the external file was already synchronized.

       PMDA_CACHE_CHECK
              Returns 1 if a cache exists for the specified instance domain,
              else 0.

       PMDA_CACHE_REUSE
              When a new instance is added to the cache, the default
              strategy is to assign instance identifiers in a monotonic
              increasing manner.  Once the maximum possible instance
              identifier value has been assigned, the strategy changes to
              one where starting from 0, the next available unused instance
              identifier will be used.  Calling pmdaCacheOp with
              PMDA_CACHE_REUSE forces an irreversible change to a second
              (reuse) strategy where the next unallocated instance
              identifier will be used.  This may be useful in cases where
              there is a desire to restrict the allocated instance
              identifiers to smaller values.  The prevailing strategy will
              be saved and restored across PMDA_CACHE_SAVE and
              PMDA_CACHE_LOAD operations.  If pmdaCacheStoreKey is ever
              used, the associated instance domain will be changed to
              PMDA_CACHE_REUSE mode.

       PMDA_CACHE_REORG
              Reorganize the cache to allow faster retrieval of active
              entries, and the cost of slower retrieval for inactive
              entries, and reclaim any culled entries.  The cache may be
              internally re-organized as entries are added, so this
              operation is not required for most PMDAs.

       PMDA_CACHE_WALK_REWIND
              Prepares for a traversal of the cache in ascending instance
              identifier sequence.

       PMDA_CACHE_WALK_NEXT
              Fetch the next active instance identifier from the cache.
              Requires a prior call using PMDA_CACHE_WALK_REWIND and will
              return -1 when all instances have been processed.

              Only one cache walk can be active at any given time, nesting
              calls to PMDA_CACHE_WALK and PMDA_CACHE_REWIND will interfere
              with each other.

       PMDA_CACHE_ACTIVE
              Changes every inactive entry in the cache to be marked active.

       PMDA_CACHE_INACTIVE
              Changes every active entry in the cache to be marked inactive.

       PMDA_CACHE_CULL
              Remove every entry from the cache.

       PMDA_CACHE_SIZE
              Return the number of entries in the cache (includes active,
              inactive and any culled entries that have not yet been
              reclaimed).

       PMDA_CACHE_SIZE_ACTIVE
              Return the number of active entries in the cache.

       PMDA_CACHE_SIZE_INACTIVE
              Return the number of inactive entries in the cache.

       PMDA_CACHE_DUMP
              Dump the current state of the cache on stderr.

       PMDA_CACHE_DUMP_ALL
              Like PMDA_CACHE_DUMP, but also dump the internal hashing
              structures used to support lookup by instance name, lookup by
              instance identifier and the collision statistics for ``hint''
              hashing from pmdaCacheStoreKey.

       pmdaCacheOp returns a non-negative value on success, and failure is
       indicated by a negative return value (suitable for decoding with
       pmErrStr(3)).

OTHER CONSIDERATIONS         top

       When the pmdaCache routines are used for particular instance domain,
       pmdaInstance(3) and the instance domain enumeration behind
       pmdaFetch(3) will attempt to extract instance domain information from
       the cache, thereby avoiding reference to the pmdaIndom data
       structures that have historically been used to define instance
       domains and service instance requests.  A PMDA can adopt a hybrid
       approach and choose to implement some instance domains via the
       traditional pmdaIndom method, and others via the pmdaCache approach,
       however attempts to manage the same instance domain by both methods
       will result in the pmdaCache method silently prevailing.

       If all instances in a PMDA are to be serviced from a pmdaCache then a
       pmdaIndom is not required, and the pmdaInit(3) call becomes

             pmdaInit(dp, NULL, 0, metrictab, nmetrics);

       However, the PMDA will need to explicitly initialize the indom field
       of the pmDesc in the metrictab entries, as this cannot be done by
       pmdaInit(3) if indomtab is missing entries for the instance domains
       maintained in the cache.

       Independent of how the instance domain is being maintained, to
       refresh an instance domain prior to a fetch or an instance domain
       operation, the standard methods of a ``wrapper'' to the
       pmdaInstance(3) and pmdaFetch(3) methods should be used.

       Refer to the simple PMDA source code for an example use of the
       pmdaCache routines.

       When using pmdaCacheStoreKey, if there is a desire to ensure the
       given ``hint'' generates the same initial instance identifier across
       all platforms, then the caller should ensure the endian and word-size
       issues are considered, e.g. if the natural data structure used for
       the key is an array of 32-bit integers, then htonl(3) should be used
       on each element of the array before calling pmdaCacheStoreKey or
       pmdaCacheLookupKey.

INSTANCE NAME MATCHING         top

       The following table summarizes the ``short name'' matching semantics
       for an instance domain (caches other than PMDA_CACHE_STRINGS style).

      ┌────────┬─────────────────┬───────────────────────────────────────────┐
      │name in │ pmdaCacheLookup │ result                                    │
      │cache   │ name            │                                           │
      ├────────┼─────────────────┼───────────────────────────────────────────┤
      │foodle  │ foo             │ no match (PM_ERR_INST)                    │
      │foo     │ foodle          │ no match (PM_ERR_INST)                    │
      │foo     │ foo             │ match                                     │
      │foo bar │ foo             │ match on short name (instance identifier) │
      │foo bar │ foo bar         │ match on full name (instance identifier)  │
      │foo     │ foo bar         │ bad match (-EDOM)                         │
      │foo bar │ foo blah        │ bad match (-EDOM)                         │
      └────────┴─────────────────┴───────────────────────────────────────────┘

FILES         top

       Cache persistence uses files with names constructed from the indom
       within the $PCP_VAR_DIR/config/pmda directory.

SEE ALSO         top

       BYTEORDER(3), PMAPI(3), PMDA(3), pmdaInit(3), pmdaInstance(3),
       pmdaFetch(3), pmdaSetFetchCallback(3), pmErrStr(3) and pmGetInDom(3).

COLOPHON         top

       This page is part of the PCP (Performance Co-Pilot) project.
       Information about the project can be found at ⟨http://www.pcp.io/⟩.
       If you have a bug report for this manual page, send it to
       pcp@oss.sgi.com.  This page was obtained from the project's upstream
       Git repository ⟨git://git.pcp.io/pcp⟩ on 2017-04-25.  If you discover
       any rendering problems in this HTML version of the page, or you
       believe there is a better or more up-to-date source for the page, or
       you have corrections or improvements to the information in this
       COLOPHON (which is not part of the original manual page), send a mail
       to man-pages@man7.org

Performance Co-Pilot                 PCP                        PMDACACHE(3)