RFR (L): 8046148: JEP 158 Unified JVM Logging
marcus.larsson at oracle.com
Tue Sep 8 09:08:54 UTC 2015
On 2015-09-07 17:53, Kirk Pepperdine wrote:
> Hi all,
> I thought that the idea behind tagging was to eliminate levels.
I believe the major concern with the first proposal was that not every
log message could be classified in a strictly hierarchical manner. Some
messages belong to multiple components, which is a use case the
hierarchy of loggers doesn't support. This is why the tag based solution
was proposed instead.
Log levels have always been part of the plan as far as I can tell. Both
the first and second proposal include the levels in their descriptions.
In any case, the JEP is due for integration and it is too late for such
a fundamental change to the feature now.
> Kind regards,
> Kirk Pepperdine
>> On Sep 7, 2015, at 3:33 PM, Marcus Larsson <marcus.larsson at oracle.com> wrote:
>> Please review the following patch adding the unified logging framework to hotspot.
>> See the JEP description for a general overview of the new feature. Below are some notes on the implementation.
>> The patch adds the new 'share/vm/logging' subdirectory containing the unified logging framework. The main entry point is log.hpp, which contains the necessary macros and definitions to use the framework.
>> Log tags are defined/listed in logTag.hpp, and are passed as template arguments to the Log class. Every combination of tags used in a log call has a corresponding LogTagSet instance, which keeps a track of all the outputs it should write the log message to (and their levels). Having tags as template arguments allows mapping directly from a set of tags to the LogTagSet instance, which means that the overhead for disabled logging should be low. Currently each log message can be tagged with up to 5 tags, but this can be increased if ever required (and with C++11's variadic templates the limit can be removed completely).
>> The LogConfiguration class keeps track of configured outputs (stdout, stderr, and possible file outputs). Configuration is done either by command line arguments (-Xlog) or by DCMD. Both methods will in turn use the LogConfiguration class to perform the parsing & configuration. This configuration includes iterating over all LogTagSet instances and updating them accordingly. The LogTagLevelExpression class is used to represent the selection of tags and levels for a given configuration request (the "what"-expression).
>> The LogDecorators class contains a selection of decorators. Instances of this class is kept in LogTagSet to track what decorators to use (this is the union of all decorators used by its outputs). Each log call will create a LogDecorations instance (note: different classes), which will contain the actual decoration strings for the log message. These decorations are used for each output the tagset is set to log on, and are then discarded.
>> The LogPrefix class allows messages of specific sets of tags to be prefixed. The prefix should supply a printf-style format with argument. (This allows GC logging to prefix messages of certain tagsets with GCId.) Prefixes are implemented using template specializations based on the specified tags, with the general/unspecialized case giving an empty prefix.
>> The LogOutput class defines the interface for all types of log outputs. LogFileStreamOutput corresponds to FILE* stream based log outputs. LogFileOutput builds on this and adds the file management and log rotation support.
>> A simple jtreg test is included in this patch. Additional tests will be added at a later stage.
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