I found out the issue.
The BufferingForwardingAppender
is inheriting from BufferingAppenderSkeleton
(as are other appenders making use of logging events buffering such as AdoNetAppender
, RemotingAppender
, SmtpAppender
..).
The BufferingAppenderSkeleton
is actually buffering logging events before actually delivering them to the target appender once a certain condition is met (buffer full for example).
According to documentation of the LoggingEvent
class (representing a logging event, and containing all values (message, threadid ...) of the event) :
Some logging events properties are considered "volatile", that is the values are correct at the time the event is delivered to appenders, but will not be consistent at any time afterwards. If an event is to be stored and the processed at a later time, these volatile values must be fixed bycalling FixVolatileData. There is a performance penalty incurred by calling FixVolatileData but is is essential to maintain data consistency
These "volatile" properties are represented by the FixFlags
enumeration containing flags such as Message, ThreadName, UserName, Identity ... so all volatile properties. It also contains the flag "None" (fix no properties), "All" (fix all properties) and "Partial" (fix only a certain predefine dset of properties).
Whem the BufferingAppenderSkeleton
is instanciated, by DEFAULT it sets the fixing to "All" meaning that all "volatile" properties should be fixed.
In that context, for each LoggingEvent appended into the BufferingAppenderSkeleton, ALL "volatile" properties will be fixed before the event is inserted in the buffer. This includes the properties Identity (username) and LocationInformation (stack trace) even if these properties are not included in the layout (but I guess it makes some kind of sense if the layout is changed to include these properties at a later time while a buffer has been already been filled with LoggingEvents).
However in my case this really HURTS performance. I am not including the Identity and LocationInformation in my layout and don\'t plan to (mainly for performance issues)
Now for the solution ...
There are two properties in BufferingAppenderSkeleton
which can be used to control the FixFlags
flag value of the BufferingAppenderSkeleton
(once again by default it is set to "ALL" which is not very nice !). These two properties are Fix
(FixFlags type) and OnlyFixPartialEventData
(bool type).
For a fine tune of the flag value or to disable all fix, the Fix
property should be used. For a specific partial predefined combination of flags (not including Identity or LocationInfo), the OnlyFixPartialEventData
can be used instead by setting it to "true".
If I reuse the configuration sample above (in my question), the only change made to the configuration to unleash performance is indicated below:
<appender name="BufferingForwardingAppender" type="log4net.Appender.BufferingForwardingAppender">
<bufferSize value="512" />
<appender-ref ref="RollingLogFileAppender" />
<Fix value="0"/> <!-- Set Fix flag to NONE -->
</appender>
Using this modified configuration, the benchmark code execution presented in my question above, is dropping from approx 14000ms to 230ms (60X faster) ! And if I use <OnlyFixPartialEventData value="true"/>
instead of disabling all fix it is taking approx 350ms.
Sadly, this flag is not very well documented (except in the SDK documentation, a little bit) .. so I had to dig deep into log4net sources to find the issue.
This is particularly problematic especially in the "reference" configuration samples, this flag appears nowhere (http://logging.apache.org/log4net/release/config-examples.html). So the samples provided for BufferingForwardingAppender, and AdoNetAppender (and other appenders inheriting from BufferingAppenderSkeleton) will give TERRIBLE performance to users, even if the layout they are using is pretty minimal.