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

Coleen Phillimore coleen.phillimore at oracle.com
Wed Sep 16 19:59:07 UTC 2015

Hi Marcus,

I had to have someone tell me how the templates work so that the 
log_is_enabled(Debug, etc) statements are efficient.  My initial concern 
was that it would be expensive to test for logging in place of some 
runtime Trace flags which evaluate to constant 'false' in product mode.

If this still turns out to be a problem, we may ask for an RFE to add a 
flag_is_enabled(Develop, <tag>) or similar macro that evaluates to a 
constant false in product mode.  Also, I think it might be helpful to 
have more than one Develop level if we want certain logging flags to not 
be available in product mode, ie DevelopInfo, DevelopDebug and 
DevelopTrace.   But we can file RFEs for things we may need while doing 
the runtime logging.


Can you add to the comment at 103 that explains the magic that 
instantiates _tagset to be the static variable that holds the settings 
for each combination of tags found in the source code?  So that is_level 
is only a static test.

And here:


The constructor for LogTagSet is a static constructor that links all the 
static combinations of tags settings together so that they can be 
initialized with the values for the logging options on the command line.

I don't need to see the comments.  I think this change is ready for 


On 9/15/15 5:36 AM, Marcus Larsson wrote:
> Hi,
> Here is the updated version of the patch, including the feedback and 
> suggestions from Coleen, David and Ioi.
> Incremental webrev:
> http://cr.openjdk.java.net/~mlarsson/8046148/webrev.00-01/
> Full webrev:
> http://cr.openjdk.java.net/~mlarsson/8046148/webrev.01/
> Changes include:
> * New NMT tag 'mtLogging' used by UL
> * Added storage allocation classes for all new classes
> * Added Log::puts<Level>(const char*) for plain string messages of any 
> length
> * Fixed broken printf-attribute on Log::write functions
> * Moved VM startup timestamp to after os::init
> * Formatting and code style changes
> Thanks,
> Marcus
> On 2015-09-07 15:33, 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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