RFR (L): 8046148: JEP 158 Unified JVM Logging

Rachel Protacio rachel.protacio at oracle.com
Wed Sep 9 15:37:12 UTC 2015


For runtime logging, there are many locations where it is necessary to 
have a block led by a conditional checking whether the debug/info/trace 
logging is enabled with the appropriate tag. Would it be possible to add 
macros to logging/log.hpp such as

    #define log_debug_is_enabled(...) log_is_enabled(Debug, __VA_ARGS__)
    #define log_trace_is_enabled(...) log_is_enabled(Trace, __VA_ARGS__)

etc.? If they are added to your framework patch, that will help Max and 
me as we write our logging code.

Additionally, the print format for the logging was not immediately 
intuitive. For example, as I'm working on the safepoint logging, I might 
have an output line like

    [0.655s][debug  ][safepoint] Safepoint synchronization initiated. (20)

Are the spaces after debug intended to line up the tags with each other 
(since the longest one is warning, it seems to fit)? If so, is that 
meant for readability? And what happens if the number of seconds goes 
into double digits - won't the alignment be off?


On 9/7/2015 9:33 AM, Marcus Larsson wrote:
> Hi,
> Please review the following patch adding the unified logging framework 
> to hotspot.
> JEP:
> https://bugs.openjdk.java.net/browse/JDK-8046148
> Webrev:
> http://cr.openjdk.java.net/~mlarsson/8046148/webrev.00/
> 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.
> Thanks,
> Marcus

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