pmdacache(3) — Linux manual page


PMDACACHE(3)            Library Functions Manual            PMDACACHE(3)

NAME         top

       pmdaCacheStore, pmdaCacheStoreKey, pmdaCacheLookup,
       pmdaCacheLookupName, pmdaCacheLookupKey, pmdaCacheOp,
       pmdaCachePurge, pmdaCachePurgeCallback, pmdaCacheResize - 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 pmdaCachePurgeCallback(pmInDom indom, time_t recent, void
               (*callback)(void *));
       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 cur‐
         rent 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 pmdaCache‐
         StoreKey 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 rou‐
          tines 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 an 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 ex‐
       pects the internal instance identifiers to be managed is not rel‐

       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 appli‐
       cations when using these interfaces as general purpose identifier
       caches, and is less applicable when using them for instance do‐
       main caching.

       For cases where the PMDA developer wishes to influence the allo‐
       cation of internal instance identifiers, e.g. for instance do‐
       mains with more than one natural dimension, or where there is a
       desire to allocate the same instance identifier each time the PM‐
       DA is started, even on different hosts, pmdaCacheStoreKey may be
       used.  In this case, an initial ``hint'' for the instance identi‐
       fier 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 in‐
       stance 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 unal‐
       located 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 pmdaCache‐
       StoreKey returns PM_ERR_INST.

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

              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.

              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 PM‐
              DA_CACHE_WALK_NEXT operations, but remain visible to pmda‐
              CacheLookup, pmdaCacheLookupName and pmdaCacheLookupKey

              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 (suit‐
       able for decoding with pmErrStr(3)) and most likely PM_ERR_INST
       to indicate the requested instance is not in the cache, or -EIN‐
       VAL to indicate a potential violation of the short name unique‐
       ness 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 PM‐
       DA_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 addi‐
       tional 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 PM‐
       DA_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 acti‐
       vated 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'' (al‐
       though the results will be the same as calling pmdaCacheLookup‐
       Name in this case).

       On success the return value will be PMDA_CACHE_ACTIVE or PM‐
       DA_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 da‐
       ta area as provided when the instance was last activated via pm‐
       daCacheStore or pmdaCacheStoreKey.

       pmdaCacheLookupKey failure is indicated by a negative return val‐
       ue 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 account‐
       ing 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, PMDA_CACHE_WRITE and PM‐
       DA_CACHE_SYNC below).  On success pmdaCachePurge returns the num‐
       ber of culled entries, else in the case of an error the return
       value is negative (and suitable for decoding with pmErrStr(3)).

       The pmdaCachePurgeCallback function is similar to pmdaCachePurge
       except that a callback function will also be called with the pri‐
       vate data pointer associated with the cache entry to be culled.
       The callback is not made if private is NULL.  This would typical‐
       ly be used to free the private data when the associated entry is
       purged in PMDAs that do not separately maintain any references to
       the private data.

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

              The cache can optionally be maintained as a persistent ex‐
              ternal file, so that the mapping of instance names to in‐
              stance 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 cur‐
              rent 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.  Typi‐
              cally a PMDA would only perform this operation once per

              If any instance has been added to, or deleted from, the
              instance domain since the last PMDA_CACHE_LOAD, PM‐
              tion, 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 in‐
              stances 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.

              A variant of PMDA_CACHE_SAVE where the entire cache is un‐
              conditionally written to the external file as a bulk oper‐
              ation, independent of any previous cache operations or the
              state of the cache.

              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.

              Within an instance domain, if any instance has been added
              to, or deleted from, or marked active since the last PM‐
              DA_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 do‐
              main 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 synchro‐

              Returns 1 if a cache exists for the specified instance do‐
              main, else 0.

              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 in‐
              stance 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.

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

              Prepares for a traversal of the cache in ascending in‐
              stance identifier sequence.

              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, nest‐
              ing calls to PMDA_CACHE_WALK_NEXT and PM‐
              DA_CACHE_WALK_REWIND will interfere with each other.

              Changes every inactive entry in the cache to be marked ac‐

              Changes every active entry in the cache to be marked inac‐

              Remove every entry from the cache.

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

              Return the number of active entries in the cache.

              Return the number of inactive entries in the cache.

              Dump the current state of the cache on stderr.

              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)).


       When the pmdaCache routines are used for particular instance do‐
       main, 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 ap‐
       proach, however attempts to manage the same instance domain by
       both methods will result in the pmdaCache method silently pre‐

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

             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 in‐
       stance 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 call‐
       ing pmdaCacheStoreKey or pmdaCacheLookupKey.


       The following table summarizes the ``short name'' matching seman‐
       tics for an instance domain (caches other than PMDA_CACHE_STRINGS
   │ 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 (-EINVAL)                       │
   │ foo bar │ foo blah        │ bad match (-EINVAL)                       │

FILES         top

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

SEE ALSO         top

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

COLOPHON         top

       This page is part of the PCP (Performance Co-Pilot) project.  In‐
       formation about the project can be found at ⟨⟩.
       If you have a bug report for this manual page, send it to  This page was obtained from the project's
       upstream Git repository
       ⟨⟩ on 2024-06-14.
       (At that time, the date of the most recent commit that was found
       in the repository was 2024-06-14.)  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

Performance Co-Pilot               PCP                      PMDACACHE(3)

Pages that refer to this page: indomcachectl(1)pmdainstance(3)pmjsonget(3)