O Guia Definitivo do Yii 1.1

array(
......
'preload'=>array('log'),
'components'=>array(
......
'log'=>array(
'class'=>'CLogRouter',
'routes'=>array(
array(
'class'=>'CFileLogRoute',
'levels'=>'error',
'filter'=>'CLogFilter',
),
...other log routes...
),
),
),
)
Starting from version 1.0.7, Yii supports logging call stack information in the messages that
are logged by calling Yii::trace. This feature is disabled by default because it lowers
performance. To use this feature, simply define a constant named YII_TRACE_LEVEL at
the beginning of the entry script (before including yii.php) to be an integer greater than 0.
Yii will then append to every trace message with the file name and line number of the call
stacks belonging to application code. The number YII_TRACE_LEVEL determines how
many layers of each call stack should be recorded. This information is particularly useful
during development stage as it can help us identify the places that trigger the trace
messages.
Performance Profiling
Performance profiling is a special type of message logging. Performance profiling can be
used to measure the time needed for the specified code blocks and find out what the
performance bottleneck is.
To use performance profiling, we need to identify which code blocks need to be profiled.
We mark the beginning and the end of each code block by inserting the following methods:
Yii::beginProfile('blockID');
...code block being profiled...
Yii::endProfile('blockID');
where blockID is an ID that uniquely identifies the code block.