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NAME | C SYNOPSIS | DESCRIPTION | OTHER CONSIDERATIONS | INSTANCE NAME MATCHING | FILES | SEE ALSO | COLOPHON |
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PMDACACHE(3) Library Functions Manual PMDACACHE(3)
pmdaCacheStore, pmdaCacheStoreKey, pmdaCacheLookup,
pmdaCacheLookupName, pmdaCacheLookupKey, pmdaCacheOp,
pmdaCachePurge, pmdaCachePurgeCallback, pmdaCacheResize - manage a
cache of instance domain information for a PMDA
#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
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 in‐
stance domain, and for each instance the cache maintains:
- external instance name (supplied by the PMDA)
- internal instance identifier (assigned by pmdaCacheStore or cal‐
culated 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 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 charac‐
teristic 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 pm‐
daCache 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 rele‐
vant.
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 manipu‐
lation of the additional data (including allocation or freeing) is
the responsibility of the PMDA caller, as the cache simply main‐
tains 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 domain
caching.
For cases where the PMDA developer wishes to influence the alloca‐
tion 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 re‐
hashed. 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 PM‐
DA_CACHE_WALK_NEXT operations, but remain visible to pmda‐
CacheLookup, pmdaCacheLookupName and pmdaCacheLookupKey re‐
quests.
PMDA_CACHE_CULL
Remove the entry from the cache.
On success pmdaCacheStore or pmdaCacheStoreKey will return the in‐
ternal 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 decod‐
ing 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 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 val‐
ue 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 pmdaCacheLookupName
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 pri‐
vate (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 pmda‐
CacheStore 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, 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 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 ex‐
ternal file, so that the mapping of instance names to in‐
stance identifiers is persistent across executions of a PM‐
DA. 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 iden‐
tifiers match then the state in the cache (active or inac‐
tive) is not changed. Should a mismatch be found (same in‐
stance name and different instance identifier, or same in‐
stance 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 in‐
stance domain since the last PMDA_CACHE_LOAD, PM‐
DA_CACHE_SAVE, PMDA_CACHE_WRITE or PMDA_CACHE_SYNC opera‐
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 instances
without accurate details of when they were last marked ac‐
tive.
Returns the number of instances saved to the external file,
else 0 if the external file was already up to date.
PMDA_CACHE_WRITE
A variant of PMDA_CACHE_SAVE where the entire cache is un‐
conditionally written to the external file as a bulk opera‐
tion, 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 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 com‐
parisons are not terminated at the first space but rather
the entire string is used for matching. These are spe‐
cialised caches not useful for general purpose instance do‐
main handling.
PMDA_CACHE_SYNC
Within an instance domain, if any instance has been added
to, or deleted from, or marked active since the last PM‐
DA_CACHE_LOAD, PMDA_CACHE_SAVE, PMDA_CACHE_WRITE or PM‐
DA_CACHE_SYNC operation, the entire cache is written to the
external file as a bulk operation. This operation is simi‐
lar 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 do‐
main, 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 in‐
stance identifier will be used. Calling pmdaCacheOp with
PMDA_CACHE_REUSE forces an irreversible change to a second
(reuse) strategy where the next unallocated instance iden‐
tifier will be used. This may be useful in cases where
there is a desire to restrict the allocated instance iden‐
tifiers 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 PM‐
DA_CACHE_REUSE mode.
PMDA_CACHE_REORG
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 oper‐
ation 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, nest‐
ing 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 ac‐
tive.
PMDA_CACHE_INACTIVE
Changes every active entry in the cache to be marked inac‐
tive.
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 re‐
claimed).
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)).
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 do‐
mains 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 prevail‐
ing.
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 in‐
stance domains maintained in the cache.
Independent of how the instance domain is being maintained, to re‐
fresh 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 pm‐
daCache 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 struc‐
ture 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
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) │
└─────────┴─────────────────┴───────────────────────────────────────────┘
Cache persistence uses files with names constructed from the indom
within the $PCP_VAR_DIR/config/pmda directory.
BYTEORDER(3), PMAPI(3), PMDA(3), pmdaInit(3), pmdaInstance(3),
pmdaFetch(3), pmdaLabel(3), pmErrStr(3) and pmGetInDom(3).
This page is part of the PCP (Performance Co-Pilot) project. In‐
formation 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@groups.io. This page was obtained from the project's upstream
Git repository ⟨https://github.com/performancecopilot/pcp.git⟩ on
2025-02-02. (At that time, the date of the most recent commit
that was found in the repository was 2025-01-30.) 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)
Pages that refer to this page: indomcachectl(1), pmdainstance(3), pmjsonget(3)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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