Provided by: libpcp3-dev_6.3.3-1_amd64 
NAME
pmdaCacheStore, pmdaCacheStoreKey, pmdaCacheLookup, pmdaCacheLookupName, pmdaCacheLookupKey, pmdaCacheOp,
pmdaCachePurge, pmdaCachePurgeCallback, pmdaCacheResize - manage a cache of instance domain information
for a PMDA
C SYNOPSIS
#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
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 pmdaCache‐
StoreKey)
- state, where active instances are visible and part of the current instance domain, and inactive in‐
stances 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 inter‐
nal 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 en‐
forced 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 fami‐
ly 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 an external instance name and an
internal instance identifier to be persistent. This rule is supported by the pmdaCache family of rou‐
tines.
5. When opaque keys are used, the values of the keys must be unique across all instances within an in‐
stance 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 in‐
stance identifiers to be managed is not relevant.
Instances are updated in the cache for instance domain indom by calling pmdaCacheStore or pmdaCache‐
StoreKey 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 pro‐
duce an initial instance identifier in the range 0 to 2^31 - 1 (or lesser maximum, if set). If this in‐
stance 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 pmdaCache‐
Lookup, 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 asso‐
ciated 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_IN‐
ST 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 in‐
st.
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 ex‐
ternal instance name and the associate additional data area as provided when the instance was last acti‐
vated 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 pmdaSet‐
FetchCallBack(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 in‐
ternal 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'') previ‐
ously 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 impre‐
cise 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 PMDA_CACHE_SYNC below). On
success pmdaCachePurge returns the number of culled entries, else in the case of an error the return val‐
ue is negative (and suitable for decoding with pmErrStr(3)).
The pmdaCachePurgeCallback function is similar to pmdaCachePurge except that a callback function will al‐
so be called with the private data pointer associated with the cache entry to be culled. The callback is
not made if private is NULL. This would typically 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:
PMDA_CACHE_LOAD
The cache can optionally be maintained as a persistent external file, so that the mapping of in‐
stance 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 in‐
stance 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 PM‐
DA_CACHE_LOAD, PMDA_CACHE_SAVE, PMDA_CACHE_WRITE 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 al‐
ready up to date.
PMDA_CACHE_WRITE
A variant of PMDA_CACHE_SAVE where the entire cache is unconditionally written to the external
file as a bulk operation, independent of any previous cache operations or the state of the cache.
PMDA_CACHE_STRINGS
Annotates this cache as being a special-purpose cache used for string de-duplication in PMDAs ex‐
porting 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 match‐
ing. 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, PMDA_CACHE_WRITE 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 accu‐
rately match the semantics expected by pmdaCachePurge.
Returns the number of instances saved to the external file, else 0 if the external file was al‐
ready 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 PM‐
DA_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, at 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 PM‐
DA_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_NEXT and PM‐
DA_CACHE_WALK_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 in‐
stance 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
When the pmdaCache routines are used for particular instance domain, pmdaInstance(3) and the instance do‐
main 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 imple‐
ment 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 en‐
tries, 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 ini‐
tial instance identifier across all platforms, then the caller should ensure the endian and word-size is‐
sues 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 pmdaCache‐
LookupKey.
INSTANCE NAME MATCHING
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 (-EINVAL) │
│ foo bar │ foo blah │ bad match (-EINVAL) │
└─────────┴─────────────────┴───────────────────────────────────────────┘
FILES
Cache persistence uses files with names constructed from the indom within the $PCP_VAR_DIR/config/pmda
directory.
SEE ALSO
BYTEORDER(3), PMAPI(3), PMDA(3), pmdaInit(3), pmdaInstance(3), pmdaFetch(3), pmdaLabel(3), pmErrStr(3)
and pmGetInDom(3).
Performance Co-Pilot PCP PMDACACHE(3)