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NAME

       PMAPI - introduction to the Performance Metrics Application Programming Interface

C SYNOPSIS

       #include <pcp/pmapi.h>

        ... assorted routines ...

       cc ... -lpcp

DESCRIPTION

       Within  the framework of the Performance Co-Pilot (PCP), client applications are developed
       using the Performance Metrics Application Programming Interface  (PMAPI)  that  defines  a
       procedural  interface  with  services  suited  to  the  development of applications with a
       particular interest in performance metrics.

       This description presents an overview  of  the  PMAPI  and  the  context  in  which  PMAPI
       applications  are  run.   The  PMAPI  is  more fully described in the Performance Co-Pilot
       Programmer's Guide, and the manual pages for the individual PMAPI routines.

PERFORMANCE METRICS - NAMES AND IDENTIFIERS

       For a description of the Performance Metrics Name Space (PMNS) and  associated  terms  and
       concepts, see PCPIntro(1).

       Not  all  PMIDs  need  be  represented  in the PMNS of every application.  For example, an
       application which monitors disk traffic will likely use a name space which references only
       the PMIDs for I/O statistics.

       Applications which use the PMAPI may have independent versions of a PMNS, constructed from
       an  initialization  file  when  the  application  starts;   see   pmLoadASCIINameSpace(3),
       pmLoadNameSpace(3), pmnscomp(1) and pmns(4).

       Internally  (below  the  PMAPI)  the  implementation of the Performance Metrics Collection
       System (PMCS) uses only the PMIDs,  and  a  PMNS  provides  an  external  mapping  from  a
       hierarchic  taxonomy  of  names to PMIDs that is convenient in the context of a particular
       system or particular use of the PMAPI.  For  the  applications  programmer,  the  routines
       pmLookupName(3)  and  pmNameID(3)  translate  between  names in a PMNS and PMIDs, and vice
       versa.  The PMNS may be traversed using pmGetChildren(3).

PMAPI CONTEXT

       An application using the PMAPI may manipulate several concurrent contexts, each associated
       with  a  source  of  performance  metrics, e.g. pmcd(1) on some host, or an archive log of
       performance metrics as created by pmlogger(1).

       Contexts are identified by a ``handle'', a small integer value that is returned  when  the
       context is created; see pmNewContext(3) and pmDupContext(3).  Some PMAPI functions require
       an explicit ``handle'' to identify the  correct  context,  but  more  commonly  the  PMAPI
       function  is  executed  in the ``current'' context.  The current context may be discovered
       using pmWhichContext(3) and changed using pmUseContext(3).

       If a PMAPI context has not been explicitly established (or the  previous  current  context
       has been closed using pmDestroyContext(3)) then the current PMAPI context is undefined.

       In  addition  to  the  source  of  the  performance metrics, the context also includes the
       instance profile and collection time  (both  described  below)  which  controls  how  much
       information is returned, and when the information was collected.

INSTANCE DOMAINS

       When  performance metric values are returned across the PMAPI to a requesting application,
       there may be more than one value for a particular metric.  Multiple values, or  instances,
       for a single metric are typically the result of instrumentation being implemented for each
       instance of a set of similar components or services in a system, e.g.  independent  counts
       for  each  CPU,  or  each  process,  or  each  disk,  or each system call type, etc.  This
       multiplicity of values is not enumerated in the name space but  rather,  when  performance
       metrics  are  delivered  across  the  PMAPI  by  pmFetch(3),  the  format  of  the  result
       accommodates values for one or more instances, with an instance-value  pair  encoding  the
       metric value for a particular instance.

       The  instances  are identified by an internal identifier assigned by the agent responsible
       for instantiating the  values  for  the  associated  performance  metric.   Each  instance
       identifier  has  a corresponding external instance identifier name (an ASCII string).  The
       routines pmGetInDom(3), pmLookupInDom(3) and pmNameInDom(3) may be used to  enumerate  all
       instance identifiers, and to translate between internal and external instance identifiers.

       All of the instance identifiers for a particular performance metric are collectively known
       as an instance domain.  Multiple performance metrics may share the same instance domain.

       If only one instance is ever available for a particular performance metric,  the  instance
       identifier  in  the result from pmFetch(3) assumes the special value PM_IN_NULL and may be
       ignored by the application, and only one instance-value pair appears  in  the  result  for
       that  metric.   Under these circumstances, the associated instance domain (as returned via
       pmLookupDesc(3)) is set to PM_INDOM_NULL to indicate  that  values  for  this  metric  are
       singular.

       The  difficult  issue  of  transient performance metrics (e.g. per-filesystem information,
       hot-plug replaceable hardware modules, etc.) means that repeated  requests  for  the  same
       PMID  may  return  different  numbers  of  values,  and/or  some changes in the particular
       instance identifiers returned.  This means applications  need  to  be  aware  that  metric
       instantiation  is  guaranteed  to  be valid at the time of collection only.  Similar rules
       apply to the transient semantics of the associated metric values.  In general however,  it
       is  expected  that  the  bulk of the performance metrics will have instantiation semantics
       that are fixed over the execution life-time of any PMAPI client.

THE TYPE OF METRIC VALUES

       The PMAPI supports a wide range of format and type encodings for the values of performance
       metrics,  namely  signed  and unsigned integers, floating point numbers, 32-bit and 64-bit
       encodings of all of  the  above,  ASCII  strings  (C-style,  NULL  byte  terminated),  and
       arbitrary aggregates of binary data.

       The  type  field in the pmDesc structure returned by pmLookupDesc(3) identifies the format
       and type of the values for a particular  performance  metric  within  a  particular  PMAPI
       context.

       Note  that the encoding of values for a particular performance metric may be different for
       different PMAPI  contexts,  due  to  differences  in  the  underlying  implementation  for
       different  contexts.  However it is expected that the vast majority of performance metrics
       will have consistent value encoding across all versions of all implementations, and  hence
       across all PMAPI contexts.

       The  PMAPI  supports  routines  to  automate the handling of the various value formats and
       types, particularly for the common case where conversion to a canonical format is desired,
       see pmExtractValue(3) and pmPrintValue(3).

THE DIMENSIONALITY AND SCALE OF METRIC VALUES

       Independent  of how the value is encoded, the value for a performance metric is assumed to
       be drawn from a set of values that can be described in terms of their  dimensionality  and
       scale  by  a  compact  encoding  as follows.  The dimensionality is defined by a power, or
       index, in each of 3 orthogonal dimensions, namely Space, Time and Count (or Events,  which
       are  dimensionless).  For example I/O throughput might be represented as Space/Time, while
       the running total of system calls is Count, memory allocation is Space and average service
       time  is Time/Count.  In each dimension there are a number of common scale values that may
       be used to better encode ranges that might otherwise exhaust the  precision  of  a  32-bit
       value.   This  information  is  encoded  in the pmUnits structure which is embedded in the
       pmDesc structure returned from pmLookupDesc(3).

       The routine pmConvScale(3) is provided to convert values in conjunction with  the  pmUnits
       structures  that  defines  the  dimensionality  and  scale  of the values for a particular
       performance metric as returned from pmFetch(3), and the desired dimensionality  and  scale
       of the value the PMAPI client wishes to manipulate.

INSTANCE PROFILE

       The  set  of  instances  for  performance  metrics  returned from a pmFetch(3) call may be
       filtered or restricted using an instance profile.  There is one instance profile for  each
       PMAPI  context  the  application  creates, and each instance profile may include instances
       from one or more instance domains.

       The routines pmAddProfile(3) and pmDelProfile(3) may be used  to  dynamically  adjust  the
       instance profile.

COLLECTION TIME

       For  each  set  of  values  for  performance  metrics  returned via pmFetch(3) there is an
       associated ``timestamp'' that serves to identify when the performance metric  values  were
       collected; for metrics being delivered from a real-time source (i.e. pmcd(1) on some host)
       this would typically be not long before they were  exported  across  the  PMAPI,  and  for
       metrics  being delivered from an archive log, this would be the time when the metrics were
       written into the archive log.

       There is an issue here of  exactly  when  individual  metrics  may  have  been  collected,
       especially  given  their  origin  in potentially different Performance Metric Domains, and
       variability in the metric updating frequency at the lowest level of the Performance Metric
       Domain.   The  PMCS  opts  for  the  pragmatic approach, in which the PMAPI implementation
       undertakes to return all of the metrics with values accurate as of the timestamp,  to  the
       best  of our ability.  The belief is that the inaccuracy this introduces is small, and the
       additional burden of accurate individual timestamping for each returned  metric  value  is
       neither warranted nor practical (from an implementation viewpoint).

       Of  course, in the case of collection of metrics from multiple hosts the PMAPI client must
       assume the sanity  of  the  timestamps  is  constrained  by  the  extent  to  which  clock
       synchronization protocols are implemented across the network.

       A  PMAPI  application may call pmSetMode(3) to vary the requested collection time, e.g. to
       rescan performance metrics values from the recent past, or to ``fast-forward'' through  an
       archive log.

GENERAL ISSUES OF PMAPI PROGRAMMING STYLE

       Across the PMAPI, all arguments and results involving a ``list of something'' are declared
       to be arrays with an associated argument or function  value  to  identify  the  number  of
       elements  in  the  list.   This  has  been  done to avoid both the varargs(3) approach and
       sentinel-terminated lists.

       Where the size of a  result  is  known  at  the  time  of  a  call,  it  is  the  caller's
       responsibility  to  allocate (and possibly free) the storage, and the called function will
       assume the result argument is of an appropriate size.  Where a result is of variable  size
       and  that  size  cannot  be  known  in  advance (e.g. for pmGetChildren(3), pmGetInDom(3),
       pmNameInDom(3), pmNameID(3), pmLookupText(3) and pmFetch(3)) the PMAPI implementation uses
       a  range  of dynamic allocation schemes in the called routine, with the caller responsible
       for subsequently releasing the storage when no longer required.  In some cases this simply
       involves  calls  to free(3C), but in others (most notably for the result from pmFetch(3)),
       special routines (e.g. pmFreeResult(3)) should be used to release the storage.

       As a general rule, if the called routine returns an error status then no  allocation  will
       have been done, and any pointer to a variable sized result is undefined.

DIAGNOSTICS

       Where  error  conditions  may  arise,  the  functions that comprise the PMAPI conform to a
       single, simple error notification scheme, as follows;

       +  the function returns an integer

       +  values >= 0 indicate no error, and perhaps some positive status,  e.g.  the  number  of
          things really processed

       +  values  <  0  indicate  an  error,  with  a  global table of error conditions and error
          messages

       The PMAPI routine  pmErrStr(3)  translates  error  conditions  into  error  messages.   By
       convention, the small negative values are assumed to be negated versions of the Unix error
       codes as defined in <errno.h> and the strings  returned  are  as  per  strerror(3C).   The
       larger, negative error codes are PMAPI error conditions.

       One  error,  common  to  all  PMAPI  routines  that  interact with pmcd(1) on some host is
       PM_ERR_IPC, which indicates the communication link to pmcd(1) has been lost.

PCP ENVIRONMENT

       Most environment  variables  are  described  in  PCPIntro(1).   In  addition,  environment
       variables  with the prefix PCP_ are used to parameterize the file and directory names used
       by PCP.  On each installation, the file /etc/pcp.conf contains the local values for  these
       variables.   The  $PCP_CONF  variable  may be used to specify an alternative configuration
       file,  as  described  in  pcp.conf(4).   Values  for  these  variables  may  be   obtained
       programatically using the pmGetConfig(3) function.

SEE ALSO

       PCPIntro(1), PCPIntro(3), PMAPI(3), pmda(3), pmGetConfig(3), pcp.conf(4) and pcp.env(4).