Ubuntu Manpage: pmdaInit, pmdaRehash, pmdaSetFlags

Provided by: libpcp-pmda3-dev_3.10.8build1_amd64

NAME

       pmdaInit, pmdaRehash, pmdaSetFlags - initialize a PMDA

C SYNOPSIS

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

       void pmdaInit(pmdaInterface *dispatch, pmdaIndom *indoms, int nindoms,
               pmdaMetric *metrics, int nmetrics);
       void pmdaRehash(pmdaExt *pmda, pmdaMetric *metrics, int nmetrics);
       void pmdaSetFlags(pmdaInterface *dispatch, int flags);

       cc ... -lpcp_pmda -lpcp

DESCRIPTION

       pmdaInit initializes a PMDA so that it  is  ready  to  receive  PDUs  from  pmcd(1).   The
       function  expects  as  arguments  the  instance  domain  table  (indoms)  and  the  metric
       description table (metrics) that are initialized by the PMDA. The  arguments  nindoms  and
       nmetrics should be set to the number of instances and metrics in the tables, respectively.

       Much  of  the pmdaInterface structure can be automatically initialized with pmdaDaemon(3),
       pmdaGetOpt(3) and pmdaDSO(3).  pmdaInit completes the PMDA initialization phase with three
       operations.   The first operation adds the domain and instance numbers to the instance and
       metric tables.  Singular metrics (metrics without an  instance  domain)  should  have  the
       instance  domain  PM_INDOM_NULL  set  in  the  indom  field  of  the pmDesc structure (see
       pmLookupDesc(3)).  Metrics with an instance domain should set this field to be the  serial
       number of the instance domain in the indoms table.

       The  instance  domain  table may be made empty by setting indoms to NULL and nindoms to 0.
       This allows the caller to provide custom  Fetch  and  Instance  callback  functions.   The
       metric  table may be made empty by setting metrics to NULL and nmetrics to 0.  This allows
       the caller to provide custom Fetch and Descriptor callback functions.

EXAMPLE

       For example, a PMDA has three metrics: A, B and C, and two instance domains X and Y,  with
       two instances in each instance domain.  The instance domain and metrics description tables
       could be defined as:

            static pmdaInstid _X[] = {
                { 0, "X1" }, { 1, "X2" }
            };

            static pmdaInstid _Y[] = {
                { 0, "Y1" }, { 1, "Y2" }
            };

            static pmdaIndom indomtab[] = {
            #define X_INDOM 0
                { X_INDOM, 2, _X },
            #define Y_INDOM 3
                { Y_INDOM, 2, _Y }
            };

            static pmdaMetric metrictab[] = {
            /* A */
                { (void *)0,
                  { PMDA_PMID(0,0), PM_TYPE_U32, PM_INDOM_NULL, PM_SEM_INSTANT,
                    { 0,0,0,0,0,0} }, },
            /* B */
                { (void *)0,
                  { PMDA_PMID(0,1), PM_TYPE_U32, X_INDOM, PM_SEM_INSTANT,
                    { 0,0,0,0,0,0} }, },
            /* C */
                { (void *)0,
                  { PMDA_PMID(0,2), PM_TYPE_DOUBLE, Y_INDOM, PM_SEM_INSTANT,
                    { 0,1,0,0,PM_TIME_SEC,0} }, }
            };

       The metric description table defines metric A with  no  instance  domain,  metric  B  with
       instance  domain  X  and  metric C with instance domain Y.  Metric C has units of seconds,
       while the other metrics have  no  units  (simple  counters).   pmdaInit  will  take  these
       structures  and  assign  the  PMDA(3) domain number to the it_indom field of each instance
       domain.  This identifier also replaces the indom field of  all  metrics  which  have  that
       instance domain, so that they are correctly associated.

       The  second  stage opens the help text file, if one was specified with the -h command line
       option (see pmdaGetOpt(3)) or as a helptext argument to pmdaDSO(3) or pmdaDaemon(3).

       The final stage involves preparing the metric table lookup strategy.

METRIC LOOKUP

When fetch and descriptor requests are made of the PMDA, each requested PMID must be
mapped to a metric table entry. There are currently three strategies for performing this
mapping - direct, linear and hashed. Each has its own set of tradeoffs and an appropriate
strategy should be selected for each PMDA.

If all of the metric PMID item numbers correspond to the position in the metrics table,
then direct mapping is used. This is the most efficient of the lookup functions as it
involves a direct array index (no additional memory is required nor any additional
processing overhead). If the PMID numbering requirement is met by the PMDA, it is ideal.
This strategy can be explicitly requested by calling pmdaSetFlags(pmda,
PMDA_FLAG_EXT_DIRECT) before calling pmdaInit. In this case, if the direct mapping is not
possible (e.g. due to an oversight on the part of the PMDA developer), a warning is logged
and the linear strategy is used instead.

The second strategy (linear search) is the default, when a direct mapping cannot be
established. This provides greater flexibility in the PMID numbering scheme, as the PMDA
item numbers do not have to be unique (hence, the PMID cluster numbers can be used more
freely, which is often extremely convenient for the PMDA developer). However, lookup
involves a linear walk from the start of the metric table until a matching PMID is found,
for each requested PMID in a request.

The third strategy (hash lookup) can be requested by calling pmdaSetFlags(pmda,
PMDA_FLAG_EXT_HASHED) before calling pmdaInit. This strategy is most useful for PMDAs
with large numbers of metrics (many hundreds, or thousands). Such PMDAs will almost
always use the cluster numbering scheme, so the direct lookup scheme becomes
inappropriate. They may also be prepared to sacrifice a small amount of additional memory
for a hash table, mapping PMID to metric table offsets, to speed up lookups in their vast
metric tables.

This final strategy can also be used by PMDAs serving up dynamically numbered metrics.
For this case, the pmdaRehash function should be used to replace the metric table when new
metrics become available, or existing metrics are removed. The PMID hash mapping will be
recomputed at the same time that the new metric table is installed.

DIAGNOSTICS

pmdaInit will set dispatch->status to a value less than zero if there is an error that
would prevent the PMDA(3) from successfully running. pmcd(1) will terminate the
connection to the PMDA(3) if this occurs.

pmdaInit may issue any of these messages:

PMDA interface version interface not supported
The interface version is not supported by pmdaInit.

Using pmdaFetch() but fetch call back not set
The fetch callback, pmdaFetch(3), requires an additional callback to be
provided using pmdaSetFetchCallBack(3).

Illegal instance domain inst for metric pmid
The instance domain inst that was specified for metric pmid is not within
the range of the instance domain table.

No help text path specified
The help text callback, pmdaText(3), requires a help text file for the
metrics to have been opened, however no path to the help text was specified
as a command line option, or as an argument to pmdaDSO(3) or pmdaDaemon(3).
This message is only a warning.

Direct mapping for metrics disabled @ num
The unit numbers of the metrics did not correspond to the index in the
metric description table. The direct mapping failed for metric number num
in the metrics table. This is less efficient but is not fatal and the
message is only a warning.

Hashed mapping for metrics disabled @ num
A memory allocation failure occurred while building the hash table to index
the metric description table. This is a non-fatal warning message - a
fallback to linear searching will be automatically performed should this
situation arise.

CAVEAT

       The  PMDA  must be using PMDA_INTERFACE_2 or later, as specified in the call to pmdaDSO(3)
       or pmdaDaemon(3).