DBI::Profile(3) User Contributed Perl Documentation DBI::Profile(3)NAMEDBI::Profile - Performance profiling and benchmarking for the DBI
SYNOPSIS
The easiest way to enable DBI profiling is to set the DBI_PROFILE envi‐
ronment 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 when the program exits. You can also enable profiling
by setting the Profile attribute of any DBI handle:
$dbh->{Profile} = 2;
Then the summary will be printed when the handle is destroyed.
Many other values apart from are possible - see "ENABLING A PROFILE"
below.
DESCRIPTION
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 con‐
cepts which are important to understand clearly.
Method Dispatch
Every method call on a DBI handle passes through a single 'dis‐
patch' 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
data about a single DBI method call is called a profile sample.
Data Filtering
If the method call was invoked by the DBI or by a driver then the
call is ignored for profiling because the time spent will be
accounted for by the original 'outermost' call for your code.
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.
Data Storage Tree
The profile data is accumulated as 'leaves on a tree'. The 'path'
through the branches of the tree to a particular leaf is determined
dynamically for each sample. This is a key feature of DBI profili‐
ing.
For each profiled method call the DBI walks along the Path and uses
each value in the Path to step into and grow the Data tree.
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}
But it's not 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 dynamic values such as
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 automati‐
cally replaced by corresponding dynamic values when they're used.
These magic cookies always start with a punctuation character.
For example a value of '"!MethodName"' in the Path causes the cor‐
responding entry in the Data to be the name of the method that was
called. For example, if the Path was:
[ 'foo', '!MethodName', 'selectall_arrayref' ]
and the selectall_arrayref() method was called, then the profile
sample data for that call will be merged into the tree at:
$h->{Profile}->{Data}->{foo}->{selectall_arrayref}->{bar}
Profile Data
Profile data is stored at the 'leaves' of the tree as references to
an array of numeric values. For example:
[
106, # 0: count of samples at this node
0.0312958955764771, # 1: total duration
0.000490069389343262, # 2: first duration
0.000176072120666504, # 3: shortest duration
0.00140702724456787, # 4: longest duration
1023115819.83019, # 5: time of first sample
1023115819.86576, # 6: time of last sample
]
After the first sample, later samples always update elements 0, 1,
and 6, and may update 3 or 4 depending on the duration of the sam‐
pled call.
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 each other:
$h->{Profile} = 0;
$h->{Profile} = "/DBI::Profile";
$h->{Profile} = DBI::Profile->new();
$h->{Profile} = {};
$h->{Profile} = { Path => [] };
Similarly, these examples have the same effect as each other:
$h->{Profile} = 6;
$h->{Profile} = "6/DBI::Profile";
$h->{Profile} = "!Statement:!MethodName/DBI::Profile";
$h->{Profile} = { Path => [ '!Statement', '!MethodName' ] };
If a non-blessed hash reference is given then the DBI::Profile module
is automatically "require"'d and the reference is blessed into that
class.
If a string is given then it is processed like this:
($path, $module, $args) = split /\//, $string, 3
@path = split /:/, $path
@args = split /:/, $args
eval "require $module" if $module
$module ⎪⎪= "DBI::Profile"
$module->new( Path => \@Path, @args )
So 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"".
Numbers can be used as a shorthand way to enable common Path values.
The simplest way to explain how the values are interpreted is to show
the code:
push @Path, "DBI" if $path_elem & 0x01;
push @Path, "!Statement" if $path_elem & 0x02;
push @Path, "!MethodName" if $path_elem & 0x04;
push @Path, "!MethodClass" if $path_elem & 0x08;
push @Path, "!Caller2" if $path_elem & 0x10;
So "2" is the same as "!Statement" and "6" (2+4) is the same as
"!Statement:!Method". Those are the two most commonly used values.
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(Profile=>2):dbname
dbi:DriverName(Profile=>{Username}:!Statement/MyProfiler/Foo:42):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 a reference to an array. Each element controls the
value to use at the corresponding level of the profile Data tree.
The elements of Path array can be one of the following types:
Special Constant
!Statement
Use the current Statement text. Typically that's the value of the
Statement attribute for the handle the method was called with. Some
methods, like commit() and rollback(), are unrelated to a particu‐
lar statement. For those methods !Statement records an empty
string.
For statement handles this is always simply the string that was
given to prepare() when the handle was created. For database han‐
dles 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.
!MethodName
Use the name of the DBI method that the profile sample relates to.
!MethodClass
Use the fully qualified name of the DBI method, including the pack‐
age, 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::selec‐
trow_arrayref but another 99 to DBD::mysql::db::selectrow_arrayref.
Currently the first call Pern't record the true location. That may
change.
!Caller
Use a string showing the filename and line number of the code call‐
ing the method.
!Caller2
Use a string showing the filename and line number of the code call‐
ing the method, as for !Caller, but also include filename and line
number of the code that called that. Calls from DBI:: and DBD::
packages are skipped.
!File
Same as !Caller above except that only the filename is included,
not the line number.
!File2
Same as !Caller2 above except that only the filenames are included,
not the line number.
Code 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 used at this point
in the Path. If it returns an empty list then the method call is
not profiled.
Attribute Specifier
A string enclosed in braces, such as '"{Username}"', specifies that
the current value of the corresponding database handle attribute
should be used at that point in the Path.
Other Values
Any other values are stringified and used literally.
(References, and values that begin with punctuation characters are
reserved.)
Only the first 100 elements in Path are used.
If the value of Path is anything other than an array reference, it is
treated as if it was:
[ DBI::Profile::!Statement ]
REPORTING
Report Format
The current accumulated 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 42.7% (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. If the process is not exiting then the percentage is calcu‐
lated using the time between the first and last call to the DBI.
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 Pro‐
file 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 MANIPULATION
Recall that "$h-"{Profile}->{Data}> is a reference to the collected
data. Either to a 'leaf' array (when the Path is empty, i.e., DBI_PRO‐
FILE env var is 1), or a reference to hash containing values that are
either further hash references or leaf array references.
Sometimes it's useful to be able to summarise some or all of the col‐
lected data. The dbi_profile_merge() function can be used to merge
leaf node values.
dbi_profile_merge
use DBI qw(dbi_profile_merge);
$time_in_dbi = dbi_profile_merge(my $totals=[], @$leaves);
Merges profile data node. Given a reference to a destination array, and
zero or more references to profile data, merges the profile data into
the destination array. For example:
$time_in_dbi = dbi_profile_merge(
my $totals=[],
[ 10, 0.51, 0.11, 0.01, 0.22, 1023110000, 1023110010 ],
[ 15, 0.42, 0.12, 0.02, 0.23, 1023110005, 1023110009 ],
);
$totals will then contain
[ 25, 0.93, 0.11, 0.01, 0.23, 1023110000, 1023110010 ]
and $time_in_dbi will be 0.93;
For example, to get the time spent 'inside' the DBI during an http
request, your logging code run at the end of the request (i.e. mod_perl
LogHandler) could use:
my $time_in_dbi = 0;
if (my $Profile = $dbh->{Profile}) { # if DBI profiling is enabled
$time_in_dbi = dbi_profile_merge(my $total=[], $Profile->{Data});
$Profile->{Data} = {}; # reset the profile data
}
If profiling has been enabled then $time_in_dbi will hold the time
spent inside the DBI for that handle (and any other handles that share
the same profile data) since the last request.
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 asso‐
ciated with. The $statement parameter is the string to use where the
Path specifies !Statement. If $statement is undef then $h->{Statement}
will be used. Similarly $method is the string to use if the Path speci‐
fies !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 !Statement in the
Path (the default) will consume memory in the Profile Data structure
for each statement. Use a code ref in the Path to return an edited
(simplified) form of the 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->con‐
nect is counted if the DBI_PROFILE environment variable is set.
Time spent fetching tied variables, $DBI::errstr, is counted.
Time spent in FETCH for $h->{Profile} is not counted, so getting the
profile data doesn't alter it.
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 use‐
less.
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 gen‐
eral will have much more 'jitter' depending on where within the 'cur‐
rent 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!)
perl v5.8.8 2006-02-07 DBI::Profile(3)