DBI::Profile(3pm) User Contributed Perl Documentation DBI::Profile(3pm)
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 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 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 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
profiling.
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
automatically 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
corresponding entry in the Data to be the name of the method that
was called. For example, if the Path was:
[ 'foo', '!MethodName', 'bar' ]
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
sampled 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 splitting 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.
If the value of Path is anything other than an array reference, it is
treated as if it was:
[ '!Statement' ]
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 particular
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 handles this is the
statement that was last prepared or executed on that database handle.
That can lead to a little 'fuzziness' 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 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. Currently the first call doesn't
record the true location. That may change.
!Caller
Use a string showing the filename and line number of the code calling
the method.
!Caller2
Use a string showing the filename and line number of the code calling
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.
!Time
Use the current value of time(). Rarely used. See the more useful
"!Time~N" below.
!Time~N
Where "N" is an integer. Use the current value of time() but with
reduced precision. The value used is determined in this way:
int( time() / N ) * N
This is a useful way to segregate a profile into time slots. For
example:
[ '!Time~60', '!Statement' ]
Code Reference
The subroutine is passed the handle it was called on and the DBI method
name. The current Statement is in $_. The statement string should not
be modified, so most subs start with "local $_ = $_;".
The list of values it returns is used at that point in the Profile Path.
Any undefined values are treated as the string ""undef"".
The sub can 'veto' (reject) a profile sample by including a reference to
undef ("\undef") in the returned list. That can be useful when you want
to only profile statements that match a certain pattern, or only profile
certain methods.
Subroutine Specifier
A Path element that begins with '"&"' is treated as the name of a
subroutine in the DBI::ProfileSubs namespace and replaced with the
corresponding code reference.
Currently this only works when the Path is specified by the
"DBI_PROFILE" environment variable.
Also, currently, the only subroutine in the DBI::ProfileSubs namespace
is '&norm_std_n3'. That's a very handy subroutine when profiling code
that doesn't use placeholders. See DBI::ProfileSubs for more
information.
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.
Reference to a Scalar
Specifies that the current value of the referenced scalar be used at
that point in the Path. This provides an efficient way to get
'contextual' values into your profile.
Other Values
Any other values are stringified and used literally.
(References, and values that begin with punctuation characters are
reserved.)
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 formatted version of the profile data tree.
If the results are being formatted 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
calculated 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 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.
PROFILE OBJECT METHODS
format
See "REPORTING".
as_node_path_list
@ary = $dbh->{Profile}->as_node_path_list();
@ary = $dbh->{Profile}->as_node_path_list($node, $path);
Returns the collected data ($dbh->{Profile}{Data}) restructured into a
list of array refs, one for each leaf node in the Data tree. This 'flat'
structure is often much simpler for applications to work with.
The first element of each array ref is a reference to the leaf node.
The remaining elements are the 'path' through the data tree to that
node.
For example, given a data tree like this:
{key1a}{key2a}[node1]
{key1a}{key2b}[node2]
{key1b}{key2a}{key3a}[node3]
The as_node_path_list() method will return this list:
[ [node1], 'key1a', 'key2a' ]
[ [node2], 'key1a', 'key2b' ]
[ [node3], 'key1b', 'key2a', 'key3a' ]
The nodes are ordered by key, depth-first.
The $node argument can be used to focus on a sub-tree. If not specified
it defaults to $dbh->{Profile}{Data}.
The $path argument can be used to specify a list of path elements that
will be added to each element of the returned list. If not specified it
defaults to a ref to an empty array.
as_text
@txt = $dbh->{Profile}->as_text();
$txt = $dbh->{Profile}->as_text({
node => undef,
path => [],
separator => " > ",
format => '%1$s: %11$fs / %10$d = %2$fs avg (first %12$fs, min %13$fs, max %14$fs)'."\n";
sortsub => sub { ... },
);
Returns the collected data ($dbh->{Profile}{Data}) reformatted into a
list of formatted strings. In scalar context the list is returned as a
single concatenated string.
A hashref can be used to pass in arguments, the default values are shown
in the example above.
The "node" and <path> arguments are passed to as_node_path_list().
The "separator" argument is used to join the elements of the path for
each leaf node.
The "sortsub" argument is used to pass in a ref to a sub that will order
the list. The subroutine will be passed a reference to the array
returned by as_node_path_list() and should sort the contents of the
array in place. The return value from the sub is ignored. For example,
to sort the nodes by the second level key you could use:
sortsub => sub { my $ary=shift; @$ary = sort { $a->[2] cmp $b->[2] } @$ary }
The "format" argument is a "sprintf" format string that specifies the
format to use for each leaf node. It uses the explicit format parameter
index mechanism to specify which of the arguments should appear where in
the string. The arguments to sprintf are:
1: path to node, joined with the separator
2: average duration (total duration/count)
(3 thru 9 are currently unused)
10: count
11: total duration
12: first duration
13: smallest duration
14: largest duration
15: time of first call
16: time of first call
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_PROFILE 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
collected data. The dbi_profile_merge_nodes() function can be used to
merge leaf node values.
dbi_profile_merge_nodes
use DBI qw(dbi_profile_merge_nodes);
$time_in_dbi = dbi_profile_merge_nodes(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_nodes(
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;
The second argument need not be just leaf nodes. If given a reference to
a hash then the hash is recursively searched for leaf nodes and all
those found are merged.
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_nodes(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.
Prior to DBI 1.56 the dbi_profile_merge_nodes() function was called
dbi_profile_merge(). That name still exists as an alias.
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 !Statement. If $statement is undef then $h->{Statement}
will be used. Similarly $method is the string to use if the Path
specifies !MethodName. There is no default value for $method.
The $h->{Profile}{Path} attribute is processed by dbi_profile() in the
usual way.
The $h parameter is usually a DBI handle but it can also be a reference
to a hash, in which case the dbi_profile() acts on each defined value in
the hash. This is an efficient way to update multiple profiles with a
single sample, and is used by the DashProfiler module.
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->connect
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).
For asynchronous queries, time spent while the query is running on the
backend is not counted.
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 end 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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