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DBI::Profile - Performance profiling and benchmarking for the DBI |
DBI::Profile - Performance profiling and benchmarking for the DBI
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.
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.
Performance data collection for the DBI is built around several concepts which are important to understand clearly.
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.
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.
[
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
]
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 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:
A reference to a hash containing the collected profile data.
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:
Data hash.
[ $DBI::Profile::DBIprofile_Statement ].
The following 'magic cookie' values can be included in the Path and will be
'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.
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: 0.001015 seconds (5 method calls) programname
'' =>
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.
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 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.
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 :)
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.
Alternate profile modules must subclass DBI::Profile to help ensure they work with future versions of the DBI.
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!)
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DBI::Profile - Performance profiling and benchmarking for the DBI |