RFR (L): 8046148: JEP 158 Unified JVM Logging
marcus.larsson at oracle.com
Wed Sep 9 07:33:06 UTC 2015
On 2015-09-08 22:53, Ioi Lam wrote:
> Also, is it possible to combine the output of two tags into a single
> file, like this?
> -Xlog:gc:file=out.txt -Xlog:comp:file=out.txt
> this way, I can have both GC and Compiler logs that's in an
> (approximate) temporal order. The alternatives are:
> - write both to stdout -- but I need to filter out junk that I don't want
> - write to different files -- but I lose the relative ordering between
> the two sets of logs
> From the current patch, it seems like if I assign the same file name
> to two tags, fopen() will be called twice and the result will be
> platform dependent.
> - Ioi
This is indeed possible! The syntax for configuring this would be
-Xlog:gc,comp:file=out.txt. Configuring logging for the same output
multiple times only overrides/replaces the previous configuration. The
second fopen will not happen, because the log configuration will notice
the output already exists and simply reconfigure the existing output.
> On 9/8/15 1:26 PM, Ioi Lam wrote:
>> Hi Marcus,
>> I am eagerly waiting for this to implement a new logging feature in
>> the VM. How much testing has been done on this patch? I can see only
>> two locations where any logs are bring generated:
>> + log_error(logging)("%s", errbuf);
>> + log_error(logging)("Could not open log file '%s' (%s).\n",
>> _file_name, strerror(errno));
>> Do you have a follow-on patch that's already in work to test the
>> various functionalists of the Unified JVM Logging framework? I am
>> asking so that I can know how much surprise to expect when I start
>> using it.
>> - Ioi
>> On 9/7/15 6:33 AM, Marcus Larsson 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
>>> 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
>>> 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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