PMDACACHE

NAME

C SYNOPSIS

cc ... -lpcp_pmda -lpcp  

DESCRIPTION

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external instance name (supplied by the PMDA)

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internal instance identifier (assigned by pmdaCacheStore)

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state, where active instances are visible and part of the current instance domain, and inactive instances are hidden, but not forgotten; pmdaCacheStore may be used to change the state of an instance

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an opaque pointer to data that is associated with the instance, but maintained by the PMDA

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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.

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 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 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 and pmdaCacheLookupName requests.

PMDA_CACHE_CULL

Remove the entry from the cache.

On success pmdaCacheStore 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.

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.

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.

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) 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_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 the second (reuse) strategy. 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.

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.

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

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.  

INSTANCE NAME MATCHING

The following table summarizes the ``short name'' matching semantics for an instance domain.

FILES

SEE ALSO

Index

NAME

C SYNOPSIS

DESCRIPTION

OTHER CONSIDERATIONS

INSTANCE NAME MATCHING

FILES

SEE ALSO