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

Coleen Phillimore coleen.phillimore at oracle.com
Thu Sep 10 20:30:18 UTC 2015

Hi Marcus,

This code looks well written and documented in terms of appropriate 
comments in the header files.

I have a few comments:

1.  The NEW and strdup_check_oom() and other memory allocation functions 
use a combination of mtInternal and mtOther, neither of which is really 
going to be helpful for finding memory leaks in this.   Can you add an 
mtLogging tag and use that?  There is one strdup without an NMT tag 

2.  The header files end with #endif but no // BLAH to match the #ifdef 
in the header guard.   Can you add all of these?

3.  Nice job adding #includes in alphabetical order.  This is the only 
file that they are not:

4. Some classes don't have storage allocation classes.  It looks like 
they should be VALUE_OBJ_CLASS_SPEC.   Eg. LogOutputList, 
LogDecorations, LogTagSet, LogTag (AllStatic),

5. In 
there is unlikely to be a general 'rt' tag since runtime covers a broad 
set of different features in the vm.

Around line 51 in this file, can you explain that MaxTags is that you 
can only give so many tags in any given logging call, not that we can 
only define 5 tags total.   Also, giving a description of how to specify 
more than one tag and what it means (union or intersection of that 
logging tag) would be good to have here too.

On line 58, can Count be "TagCount" or something a bit more descriptive?

6. In log.hpp can this really long line be broken up to something 
reasonable around 100 chars?

template <LogTagType T0, LogTagType T1 = LogTag::__NO_TAG, LogTagType T2 
= LogTag::__NO_TAG, LogTagType T3 = LogTag::__NO_TAG, LogTagType T4 = 
LogTag::__NO_TAG, LogTagType GuardTag = LogTag::__NO_TAG>
class Log {

Thank you for the examples on how to use these.   The template macro 
combinations are pretty insane.   We used to not be allowed to use 
templates in Hotspot.

7. In general, could you add a blank line between the header guards in 
the file and the #include statements?

#include "logging/logLevel.hpp"

8. In logTag.hpp there is MaxTags = 5 but the log.hpp file definitions 
have only 5 tags.  It seems like MaxTags should be specified there 
instead or at the top level.

9. At the end of logTagSet.hpp, I can't guess what this line is:

template <LogTagType T0, LogTagType T1, LogTagType T2, LogTagType T3, 
LogTagType T4>
LogTagSet LogTagSetMapping<T0, T1, T2, T3, T4>::_tagset(T0, T1, T2, T3, T4);

it looks like a constructor call.  yes it is.  Can you add a comment or 
something for people who's decoding skills with templates isn't as 
strong?   And you have to define the static field _tagset in the .hpp 
file because it's a template.

Can you break up the long template line for LogTagSetMapping also?

This is all I have for now.  This is a much larger change than I 
appreciated in my pre-review, but I still don't see anything bad at all.


On 9/7/15 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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