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Tim Bunce

NAME

DBI::Profile - Performance profiling and benchmarking for the DBI

SYNOPSIS

The easiest way to enable DBI profiling is to set the DBI_PROFILE environment variable to 2 and then run your code as usual:

  DBI_PROFILE=2 prog.pl

This will profile your program and then output a textual summary grouped by query. You can also enable profiling by setting the Profile attribute of any DBI handle:

  $dbh->{Profile} = 2;

Other values are possible - see "ENABLING A PROFILE" below.

DESCRIPTION

DBI::Profile is new and experimental and subject to change.

The DBI::Profile module provides a simple interface to collect and report performance and benchmarking data from the DBI.

For a more elaborate interface, suitable for larger programs, see DBI::ProfileDumper and dbiprof. For Apache/mod_perl applications see DBI::ProfileDumper::Apache.

OVERVIEW

Performance data collection for the DBI is built around several concepts which are important to understand clearly.

Method Dispatch

Every method call on a DBI handle passes through a single 'dispatch' function which manages all the common aspects of DBI method calls, such as handling the RaiseError attribute.

Data Collection

If profiling is enabled for a handle then the dispatch code takes a high-resolution timestamp soon after it is entered. Then, after calling the appropriate method and just before returning, it takes another high-resolution timestamp and calls a function to record the information. That function is passed the two timestamps plus the DBI handle and the name of the method that was called. That information about a single DBI method call is called the profile sample data.

Data Filtering

If the method call was invoked by the DBI or by a driver then the call is currently ignored for profiling because the time spent will be accounted for by the original 'outermost' call.

For example, the calls that the selectrow_arrayref() method makes to prepare() and execute() etc. are not counted individually because the time spent in those methods is going to be allocated to the selectrow_arrayref() method when it returns. If this was not done then it would be very easy to double count time spent inside the DBI.

In future releases it may be possible to alter this behaviour.

Data Storage Tree

The profile data is stored as 'leaves on a tree'. The 'path' through the branches of the tree to the particular leaf that will store the profile sample data for a profiled call is determined dynamically. This is a powerful feature.

For example, if the Path is

  [ 'foo', 'bar', 'baz' ]

then the new profile sample data will be merged into the tree at

  $h->{Profile}->{Data}->{foo}->{bar}->{baz}

It wouldn't be very useful to merge all the call data into one leaf node (except to get an overall 'time spent inside the DBI' total). It's more common to want the Path to include the current statement text and/or the name of the method called to show what the time spent inside the DBI was for.

The Path can contain some 'magic cookie' values that are automatically replaced by corresponding dynamic values when they're used. For example DBIprofile_Statement (exported by DBI::profile) is automatically replaced by value of the Statement attribute of the handle. For example, is the Path was:

  [ 'foo', DBIprofile_Statement, 'bar' ]

and the value of $h->{Statement} was:

  SELECT * FROM tablename

then the profile data will be merged into the tree at:

  $h->{Profile}->{Data}->{foo}->{SELECT * FROM tablename}->{bar}

The default Path is just [ DBIprofile_Statement ] and so by default the profile data is aggregated per distinct Statement string.

For statement handles this is always simply the string that was given to prepare() when the handle was created. For database handles this is the statement that was last prepared or executed on that database handle. That can lead to a little 'fuzzyness' because, for example, calls to the quote() method to build a new statement will typically be associated with the previous statement. In practice this isn't a significant issue and the dynamic Path mechanism can be used to setup your own rules.

Profile Data

Profile data is stored at the 'leaves' of the tree as references to an array of numeric values. For example:

    [
      106,                    # count
      0.0312958955764771,     # total duration
      0.000490069389343262,   # first duration
      0.000176072120666504,   # shortest duration
      0.00140702724456787,    # longest duration
      1023115819.83019,       # time of first event
      1023115819.86576,       # time of last event
    ]

ENABLING A PROFILE

Profiling is enabled for a handle by assigning to the Profile attribute. For example:

  $h->{Profile} = DBI::Profile->new();

The Profile attribute holds a blessed reference to a hash object that contains the profile data and attributes relating to it. The class the Profile object is blessed into is expected to provide at least a DESTROY method which will dump the profile data to the DBI trace file handle (STDERR by default).

All these examples have the same effect as the first:

  $h->{Profile} = {};
  $h->{Profile} = "DBI::Profile";
  $h->{Profile} = "2/DBI::Profile";
  $h->{Profile} = 2;

If a non-blessed hash reference is given then the DBI::Profile module is automatically require'd and the reference is blessed into that class.

If a string is given then it is split on '/' characters and the first value is used to select the Path to be used (see below). The second value, if present, is used as the name of a module which will be loaded and it's new method called. If not present it defaults to DBI::Profile. Any other values are passed as arguments to the new method. For example: "2/DBIx::OtherProfile/Foo/42".

Various common sequences for Path can be selected by simply assigning an integer value to Profile. The simplest way to explain how the values are interpreted is to show the code:

    push @Path, "DBI"                       if $path & 0x01;
    push @Path, DBIprofile_Statement        if $path & 0x02;
    push @Path, DBIprofile_MethodName       if $path & 0x04;
    push @Path, DBIprofile_MethodClass      if $path & 0x08;

So using the value "1" causes all profile data to be merged into a single leaf of the tree. That's useful when you just want a total.

Using "2" causes profile sample data to be merged grouped by the corresponding Statement text. This is the most frequently used.

Using "4" causes profile sample data to be merged grouped by the method name ('FETCH', 'prepare' etc.). Using "8" is similar but gives the fully qualified 'glob name' of the method called. For example: '*DBD::Driver::db::prepare', '*DBD::_::st::fetchrow_hashref'.

The values can be added together to create deeper paths. The most useful being 6 (statement then method name) or 10 (statement then method name with class). Using a negative number will reverse the path. Thus -6 will group by method name then statement.

The spliting and parsing of string values assigned to the Profile attribute may seem a little odd, but there's a good reason for it. Remember that attributes can be embedded in the Data Source Name string which can be passed in to a script as a parameter. For example:

    dbi:DriverName(RaiseError=>1,Profile=>2):dbname

And also, if the DBI_PROFILE environment variable is set then The DBI arranges for every driver handle to share the same profile object. When perl exits a single profile summary will be generated that reflects (as nearly as practical) the total use of the DBI by the application.

THE PROFILE OBJECT

The DBI core expects the Profile attribute value to be a hash reference and if the following values don't exist it will create them as needed:

Data

A reference to a hash containing the collected profile data.

Path

The Path value is used to control where the profile for a method call will be merged into the collected profile data. Whenever profile data is to be stored the current value for Path is used.

The value can be one of:

Array Reference

Each element of the array defines an element of the path to use to store the profile data into the Data hash.

Undefined value (the default)

Treated the same as [ $DBI::Profile::DBIprofile_Statement ].

Subroutine Reference NOT YET IMPLEMENTED

The subroutine is passed the DBI method name and the handle it was called on. It should return a list of values to uses as the path. If it returns an empty list then the method call is not profiled.

The following 'magic cookie' values can be included in the Path and will be

DBIprofile_Statement

Replaced with the current value of the Statement attribute for the handle the method was called with. If that value is undefined then an empty string is used.

DBIprofile_MethodName

Replaced with the name of the DBI method that the profile sample relates to.

DBIprofile_MethodClass

Replaced with the fully qualified name of the DBI method, including the package, that the profile sample relates to. This shows you where the method was implemented. For example:

  'DBD::_::db::selectrow_arrayref' =>
      0.022902s
  'DBD::mysql::db::selectrow_arrayref' =>
      2.244521s / 99 = 0.022445s avg (first 0.022813s, min 0.022051s, max 0.028932s)

The "DBD::_::db::selectrow_arrayref" shows that the driver has inherited the selectrow_arrayref method provided by the DBI.

But you'll note that there is only one call to DBD::_::db::selectrow_arrayref but another 99 to DBD::mysql::db::selectrow_arrayref. That's because after the first call Perl has cached the method to speed up method calls. You may also see some names begin with an asterix ('*'). Both of these effects are subject to change in later releases.

Other magic cookie values may be added in the future.

REPORTING

Report Format

The current profile data can be formatted and output using

    print $h->{Profile}->format;

To discard the profile data and start collecting fresh data you can do:

    $h->{Profile}->{Data} = undef;

The default results format looks like this:

  DBI::Profile: 0.001015s (5 calls) programname @ YYYY-MM-DD HH:MM:SS
  '' =>
      0.000024s / 2 = 0.000012s avg (first 0.000015s, min 0.000009s, max 0.000015s)
  'SELECT mode,size,name FROM table' =>
      0.000991s / 3 = 0.000330s avg (first 0.000678s, min 0.000009s, max 0.000678s)

Which shows the total time spent inside the DBI, with a count of the total number of method calls and the name of the script being run, then a formated version of the profile data tree.

If the results are being formated when the perl process is exiting (which is usually the case when the DBI_PROFILE environment variable is used) then the percentage of time the process spent inside the DBI is also shown.

In the example above the paths in the tree are only one level deep and use the Statement text as the value (that's the default behaviour).

The merged profile data at the 'leaves' of the tree are presented as total time spent, count, average time spent (which is simply total time divided by the count), then the time spent on the first call, the time spent on the fastest call, and finally the time spent on the slowest call.

The 'avg', 'first', 'min' and 'max' times are not particularly useful when the profile data path only contains the statement text. Here's an extract of a more detailed example using both statement text and method name in the path:

  'SELECT mode,size,name FROM table' =>
      'FETCH' =>
          0.000076s
      'fetchrow_hashref' =>
          0.036203s / 108 = 0.000335s avg (first 0.000490s, min 0.000152s, max 0.002786s)

Here you can see the 'avg', 'first', 'min' and 'max' for the 108 calls to fetchrow_hashref() become rather more interesting. Also the data for FETCH just shows a time value because it was only called once.

Currently the profile data is output sorted by branch names. That may change in a later version so the leaf nodes are sorted by total time per leaf node.

Report Destination

The default method of reporting is for the DESTROY method of the Profile object to format the results and write them using:

    DBI->trace_msg($results, 0);  # see $ON_DESTROY_DUMP below

to write them to the DBI trace() filehandle (which defaults to STDERR). To direct the DBI trace filehandle to write to a file without enabling tracing the trace() method can be called with a trace level of 0. For example:

    DBI->trace(0, $filename);

The same effect can be achieved without changing the code by setting the DBI_TRACE environment variable to 0=filename.

The $DBI::Profile::ON_DESTROY_DUMP variable holds a code ref that's called to perform the output of the formatted results. The default value is:

  $ON_DESTROY_DUMP = sub { DBI->trace_msg($results, 0) };

Apart from making it easy to send the dump elsewhere, it can also be useful as a simple way to disable dumping results.

CHILD HANDLES

Child handles inherit a reference to the Profile attribute value of their parent. So if profiling is enabled for a database handle then by default the statement handles created from it all contribute to the same merged profile data tree.

CUSTOM DATA COLLECTION

Using The Path Attribute

  XXX example to be added later using a selectall_arrayref call
  XXX nested inside a fetch loop where the first column of the
  XXX outer loop is bound to the profile Path using
  XXX bind_column(1, \${ $dbh->{Profile}->{Path}->[0] })
  XXX so you end up with separate profiles for each loop
  XXX (patches welcome to add this to the docs :)

Adding Your Own Samples

The dbi_profile() function can be used to add extra sample data into the profile data tree. For example:

    use DBI;
    use DBI::Profile (dbi_profile dbi_time);

    my $t1 = dbi_time(); # floating point high-resolution time

    ... execute code you want to profile here ...

    my $t2 = dbi_time();
    dbi_profile($h, $statement, $method, $t1, $t2);

The $h parameter is the handle the extra profile sample should be associated with. The $statement parameter is the string to use where the Path specifies DBIprofile_Statement. If $statement is undef then $h->{Statement} will be used. Similarly $method is the string to use if the Path specifies DBIprofile_MethodName. There is no default value for $method.

The $h->{Profile}{Path} attribute is processed by dbi_profile() in the usual way.

It is recommended that you keep these extra data samples separate from the DBI profile data samples by using values for $statement and $method that are distinct from any that are likely to appear in the profile data normally.

SUBCLASSING

Alternate profile modules must subclass DBI::Profile to help ensure they work with future versions of the DBI.

CAVEATS

Applications which generate many different statement strings (typically because they don't use placeholders) and profile with DBIprofile_Statement in the Path (the default) will consume memory in the Profile Data structure for each statement.

If a method throws an exception itself (not via RaiseError) then it won't be counted in the profile.

If a HandleError subroutine throws an exception (rather than returning 0 and letting RaiseError do it) then the method call won't be counted in the profile.

Time spent in DESTROY is added to the profile of the parent handle.

Time spent in DBI->*() methods is not counted. The time spent in the driver connect method, $drh->connect(), when it's called by DBI->connect is counted if the DBI_PROFILE environment variable is set.

Time spent fetching tied variables, $DBI::errstr, is counted.

DBI::PurePerl does not support profiling (though it could in theory).

A few platforms don't support the gettimeofday() high resolution time function used by the DBI (and available via the dbi_time() function). In which case you'll get integer resolution time which is mostly useless.

On Windows platforms the dbi_time() function is limited to millisecond resolution. Which isn't sufficiently fine for our needs, but still much better than integer resolution. This limited resolution means that fast method calls will often register as taking 0 time. And timings in general will have much more 'jitter' depending on where within the 'current millisecond' the start and and timing was taken.

This documentation could be more clear. Probably needs to be reordered to start with several examples and build from there. Trying to explain the concepts first seems painful and to lead to just as many forward references. (Patches welcome!)