Ubuntu Manpage: PMAPI - introduction to the Performance Metrics Application Programming Interface

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 Per‐
       formance Metrics Application Programming Interface (PMAPI) that defines a procedural interface with  ser‐
       vices 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 initializa‐
       tion file when the application starts; see pmLoadASCIINameSpace(3), pmLoadNameSpace(3), and PMNS(5).

       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)  andpmTraversePMNS.   The  pmFetchGroup(3)
       functions combine metric name lookup, fetch, and conversion operations.

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 a set of archives 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  cre‐
       ated;  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''  con‐
       text.  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 in‐
       formation 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 typ‐
ically 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 instantiat‐
ing the values for the associated performance metric. Each instance identifier has a corresponding ex‐
ternal instance identifier name (an ASCII string). The routines pmGetInDom(3), pmLookupInDom(3) and pm‐
NameInDom(3) may be used to enumerate all instance identifiers, and to translate between internal and ex‐
ternal 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 replace‐
able 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, name‐
       ly 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  ex‐
       pected  that the vast majority of performance metrics will have consistent value encoding across all ver‐
       sions 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 pmPrint‐
       Value(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  repre‐
       sented  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  in‐
       formation is encoded in the pmUnits structure which is embedded in the pmDesc structure returned from pm‐
       LookupDesc(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.
       Alternatively, the pmFetchGroup(3) functions can perform data  format  and  unit  conversion  operations,
       specified by textual descriptions of desired unit / scales.

INSTANCE PROFILE

       The set of instances for performance metrics returned from a pmFetch(3) call may be filtered or restrict‐
       ed  using an instance profile.  There is one instance profile for each PMAPI context the application cre‐
       ates, 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  ``time‐
       stamp''  that serves to identify when the performance metric values were collected; for metrics being de‐
       livered 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 a set of archives, this would be the
       time when the metrics were written into the archive.

       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 fre‐
       quency 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 war‐
       ranted nor practical (from an implementation viewpoint).

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

       A  PMAPI  application may call pmSetMode(3) to vary the requested collection time, e.g. to rescan perfor‐
       mance metrics values from the recent past, or to ``fast-forward'' through a set of archives.

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 appro‐
       priate 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), pmLookupLabels(3), pmLookupText(3) and pm‐
       Fetch(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(3), but in others (most notably for the result from pmFetch(3)), spe‐
       cial routines (e.g. pmFreeResult(3) and pmFreeLabelSets(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(3).  The larger, negative error codes  are  PMAPI  error  condi‐
       tions.

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

MULTI-THREADED APPLICATIONS

       The original design for PCP was based around single-threaded applications, or more strictly  applications
       in  which only one thread was ever expected to call the PCP libraries.  This restriction has been relaxed
       for libpcp to allow the most common PMAPI routines to be safely called from any thread in a multi-thread‐
       ed application.

       However the following groups of functions and services in libpcp are still  restricted  to  being  called
       from  a  single-thread,  and this is enforced by returning PM_ERR_THREAD when an attempt to call the rou‐
       tines in each group from more than one thread is detected.

       1.  Any use of a PM_CONTEXT_LOCAL context, as the DSO PMDAs that are called directly from libpcp may  not
           be thread-safe.

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(5).  Values for these variables may
       be obtained programmatically using the pmGetConfig(3) function.