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

Marcus Larsson marcus.larsson at oracle.com
Wed Sep 9 08:27:31 UTC 2015


On 2015-09-08 14:47, Kirk Pepperdine wrote:
>> On Sep 8, 2015, at 1:42 PM, Claes Redestad <claes.redestad at oracle.com> wrote:
>> On 2015-09-08 12:45, Kirk Pepperdine wrote:
>>> Hi,
>>> Indeed, not all messages are hierarchical. This is one of the reasons I originally questioned the use of log levels. For example, debugging and informational logs may come from the same source and I certainly wouldn’t want to be forced mix them to satisfy a forced structure. Is the intention you can use either or? I ask because I’m not sure that mixing them with tags will alleviate the problems I commonly see when tuning applications in the field.
>>>> 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.
>>> I would like to believe that this work is important enough to get right and that it’s never too late for fundamental changes up until it is integrated. I would like to believe that there are back out strategies in the cases where implementations show unintended consequences.
>> Can you give an example of what you believe will go wrong with allowing a mix of levels and tags and how removing levels from the JEP would improve things?
> I’m slowly working through the patch but I’ve not had time to get through all of it yet so some things mentioned here might be in the source already… but here are some comments from experiences.
> As you have discovered, you can’t define a tag system using levels. That said, if you have tags you can define levels but you don’t need to explicitly support them, it just come for free out of the box. So I would argue both the spec and implementation would be simplified by moving to a tag system. More over, it doesn’t preclude someone defining a hierarchical system if them so need to have one.

While implementing levels using tags might simplify the implementation, 
I think it would introduce more complexity for the log producers, since 
there would be a much wider choice between "levels" in contrast to the 
current six levels. It should be easy to specify how much logging you 
want, so I don't think it would be a good idea to omit levels completely 
either. For example, some consumers of the logging might only be 
interested in a certain level of log messages, but from any component or 
from a broad range of components. Others might want every piece of 
logging belonging to a (very) specific component.

> Next, I think there are two points of view that you have to consider. Usability for the producers and usability for the consumers. From a consumer POV what I would like to see is a logging system that promotes stability in the output. Using broad ill defined categories as they are currently defined in the JDK have historically lead to unstable output. This in turn breaks down stream tooling that maybe used to help triage system out in the wild. How does this happen? Well what gets put into a level is completely arbitrary. What goes into warning, fine, finest.. who knows? More to the point, the producers often have to make a quick decision and in their cubicles they often don’t see the entire downstream tooling chains that they are affecting when they do this even when the information being added it very valuable. I’m going to suggest a system, maybe not *the* system or even the *best* system. It’s just a simple system that people have been using in pub/sub messaging systems for years. I think the technique applies because logging is a form of journalling which is just another form of messaging (hence I suggested people looking at Peter Lawyer’s Chronicle which is designed to be used in HFT applications). We can also use ObjectName (jmx) to define tags.
> If you have a tag, let say, gc.agetable, it’s pretty clear what you should get. Also that output is very likely to remain stable over time. In fact, my experience with PrintTenuringDistrubtion is just that. The output has remained stable for years. I cannot say that about any overly broad category of GC logging such as PrintGCDetails (which I would equate to a level). If I have another tag say, gc.collection.scavenge I should get information about scavenges. Again purpose is very clear from both a producer/consumer POV. I can take age data or scavenge or both or gc.collection.* to get all sub categories for gc.collection. This looks like levels and indeed could be used to implement levels but it’s based on categories (tags). At any rate, the taxonomy is clearer and should offer guidelines that makes it easier for the developer to push the information out on the right channel while offering better stability needed for downstream tooling.

The problem of classifying log messages exists whether you use real 
levels or not; "Should this log message be tagged with verbose?" vs 
"Should this log message be on trace level?".

>> Having high-level tags which interact with the standard "levels" seem like a natural fit to me, while the framework proposed in the JEP make it possible for each VM team to support ever more detailed tags to each subsystem where that might be appropriate. For better or worse it adds a degree of freedom. Is your concern that we need more granular tags going forward to help zooming in on specific issues, and that levels for such niche tags might seem/feel superfluous?
> If the system has been built with tags on levels then I would suggest the system has been built upside down or inside out, however you want to describe it.. in the same way that JDK logging was build upside down (first problem, super class knows about subclasses and that is a primary source of classloader leaks and that is just the first issue).
> Kind regards,
> Kirk

I would say the system has been built with levels on tags. The tags 
define *what* you want to log, and the levels define *how much* 
information you want. The levels are sort of a filter on the tags you 


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