RFR(s): 8073476: G1 logging ignores changes to PrintGC* flags via MXBeans

Jesper Wilhelmsson jesper.wilhelmsson at oracle.com
Tue Apr 28 12:16:52 UTC 2015


Bengt Rutisson skrev den 28/4/15 10:31:
> On 27/04/15 18:24, Jesper Wilhelmsson wrote:
>> Bengt Rutisson skrev den 27/4/15 17:55:
>>>
>>> Hi Dima,
>>>
>>> On 2015-04-27 15:32, Dmitry Fazunenko wrote:
>>>> Hi Bengt,
>>>>
>>>> Updated variant of fix with all your comments addressed:
>>>>
>>>> http://cr.openjdk.java.net/~dfazunen/8073476/webrev.08/
>>>
>>> Thanks! Looks much better.
>>>
>>> One small detail. You have changed the behavior of this command line:
>>>
>>> -XX:+UnlockExperimentalVMOptions -XX:G1LogLevel="" -XX:+PrintGCDetails
>>
>> Why is this a valid command line? Shouldn't we require correct values for all
>> flags and exit with an error message if options get weird values?
>
> I agree, but as I wrote to Dima, I don't think this is worth fixing now.

No, I didn't mean to suggest that it should be fixed by this change. I just 
didn't think it was worth any extra effort to retain the behavior of a non-valid 
command line. In my view such command lines have undefined behavior.
/Jesper


>
> Bengt
>
>> /Jesper
>>
>>
>>>
>>> Before your change this would result in the log level being "finer" due to the
>>> PrintGCDetails flag. But after your change we will get log level None.
>>>
>>> It is not an interesting use case in my mind, but an easy fix would be to
>>> introduce a state variable in G1Log to track whether we should look at PrintGC*
>>> flags.
>>>
>>> So, in G1Log::init() you could do something like:
>>>
>>>    _log_level_flag_used = G1LogLevel != NULL && G1LogLevel[0] != '\0';
>>>    if (_log_level_flag_used) {
>>>
>>>    } else
>>>      update_level();
>>>    }
>>>
>>>
>>> And update_level() would also look at the stat flag instead of checking
>>> FLAG_IS_DEFAULT(G1LogLevel). That way init() and update_level() would be
>>> consistent and we would keep the same behavior as before your change.
>>>
>>> What do you think?
>>>
>>> Thanks,
>>> Bengt
>>>
>>>
>>>
>>>>
>>>> Thanks,
>>>> Dima
>>>>
>>>>
>>>> On 21.04.2015 16:18, Dmitry Fazunenko wrote:
>>>>> Bengt,
>>>>>
>>>>> Thanks for your time!
>>>>> A have a question inline:
>>>>>
>>>>> On 21.04.2015 15:19, Bengt Rutisson wrote:
>>>>>>
>>>>>> Hi Dima,
>>>>>>
>>>>>> On 2015-04-21 13:33, Dmitry Fazunenko wrote:
>>>>>>> Hi Bengt,
>>>>>>>
>>>>>>> This is modified fixed based on your comments:
>>>>>>> http://cr.openjdk.java.net/~dfazunen/8073476/webrev.07/
>>>>>>>
>>>>>>> Thanks for your assistance and good ideas!
>>>>>>
>>>>>> Thanks for making these change. Looks much better to me.
>>>>>>
>>>>>> A couple of comments:
>>>>>>
>>>>>> G1CollectedHeap::do_collection() is only used for full GCs. You probably
>>>>>> want to add a call to G1Log::update_level() in
>>>>>> G1CollectedHeap::do_collection_pause_at_safepoint() too to make sure that it
>>>>>> is called for young GCs as well.
>>>>>
>>>>> agree. good catch!
>>>>>
>>>>>>
>>>>>>
>>>>>> I think it looks odd that G1Log::init() and G1Log::update_level() use
>>>>>> different guards for when to take PrintGC and PrintGCDetails into account.
>>>>>>
>>>>>>   51 void G1Log::init() {
>>>>>>   52   if (G1LogLevel != NULL && G1LogLevel[0] != '\0')
>>>>>>
>>>>>>   37 void G1Log::update_level() {
>>>>>>   38   if (FLAG_IS_DEFAULT(G1LogLevel)) {
>>>>>>
>>>>>> Is there a difference between (G1LogLevel != NULL && G1LogLevel[0] != '\0')
>>>>>> and (FLAG_IS_DEFAULT(G1LogLevel)) ? Can we use the same guard in both places?
>>>>>
>>>>> I though about using the same expression in both cases. The difference in
>>>>> handling the empty value:  -XX:G1LogLevel=
>>>>> So, the question,  how this case should be handled:
>>>>> - a warning to be printed out and level set to 'none'
>>>>> - treated in the same way as 'none'
>>>>>
>>>>>
>>>>>>
>>>>>>
>>>>>> g1Log.hpp:
>>>>>>
>>>>>> I don't think this comment helps much:
>>>>>>
>>>>>> +  // to be invoked on VM start
>>>>>>    static void init();
>>>>>>
>>>>>> The name init() makes it pretty clear to me what the method is expected to
>>>>>> do. I would just leave the comment out.
>>>>> I have a bit different vision, but I'm okay to remove the comment
>>>>>
>>>>>
>>>>>>
>>>>>> The comment for update_level() talks about where the method should be called
>>>>>> instead of what it does. That is normally not such a good thing for
>>>>>> comments. Better to explain what it does so it can be called in all places
>>>>>> where it is needed in the future.
>>>>>>
>>>>>> So instead of:
>>>>>>
>>>>>> +  // to be invoked on GC start or flag values change
>>>>>> +  static void update_level();
>>>>>>
>>>>>> I would prefer:
>>>>>>
>>>>>> //  Update the log level to reflect runtime changes to manageable flags.
>>>>> agree.
>>>>>
>>>>> Thanks,
>>>>> Dima
>>>>>
>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> Bengt
>>>>>>
>>>>>>>
>>>>>>> -- Dima
>>>>>>>
>>>>>>>
>>>>>>> On 20.04.2015 11:34, Bengt Rutisson wrote:
>>>>>>>>
>>>>>>>> Hi Dima,
>>>>>>>>
>>>>>>>> On 2015-04-17 15:51, Dmitry Fazunenko wrote:
>>>>>>>>> Bengt,
>>>>>>>>>
>>>>>>>>> Please find comments inline.
>>>>>>>>>
>>>>>>>>> On 17.04.2015 9:43, Bengt Rutisson wrote:
>>>>>>>>>>
>>>>>>>>>> Hi Dima,
>>>>>>>>>>
>>>>>>>>>> On 2015-04-16 15:36, Dmitry Fazunenko wrote:
>>>>>>>>>>> Hi Bengt,
>>>>>>>>>>>
>>>>>>>>>>> thanks for looking.
>>>>>>>>>>>
>>>>>>>>>>> On 16.04.2015 15:42, Bengt Rutisson wrote:
>>>>>>>>>>>>
>>>>>>>>>>>> Hello Dima,
>>>>>>>>>>>>
>>>>>>>>>>>> On 2015-04-16 13:40, Dmitry Fazunenko wrote:
>>>>>>>>>>>>> Hello,
>>>>>>>>>>>>>
>>>>>>>>>>>>> Would you review a simple fix in G1.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Short description:
>>>>>>>>>>>>> after introduction G1Log - dynamic changes of PrintGC and
>>>>>>>>>>>>> PrintGCDetails flag has no effect anymore, because G1Log looks for
>>>>>>>>>>>>> these flags during initialization only. The fix: sync the log level
>>>>>>>>>>>>> with the flags values.
>>>>>>>>>>>>>
>>>>>>>>>>>>> A huge thanks to Jesper who helped me a lot with my first product fix.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8073476
>>>>>>>>>>>>> Webrev: http://cr.openjdk.java.net/~dfazunen/8073476/webrev.06/
>>>>>>>>>>>>
>>>>>>>>>>>> Sorry, but I don't really like the way this is solved. With this
>>>>>>>>>>>> approach calling G1GCPhaseTimes::print() suddenly has the side effect
>>>>>>>>>>>> that it resets the log level. That's quite unexpected for me.
>>>>>>>>>>>> Especially if you consider the code path in
>>>>>>>>>>>> G1CollectedHeap::log_gc_footer() where we do this:
>>>>>>>>>>>>
>>>>>>>>>>>> void G1CollectedHeap::log_gc_footer(double pause_time_sec) {
>>>>>>>>>>>>   if (!G1Log::fine()) {
>>>>>>>>>>>>     return;
>>>>>>>>>>>>   }
>>>>>>>>>>>>
>>>>>>>>>>>>   if (G1Log::finer()) {
>>>>>>>>>>>>     ...
>>>>>>>>>>>> g1_policy()->phase_times()->print(pause_time_sec);
>>>>>>>>>>>>     ...
>>>>>>>>>>>>   }
>>>>>>>>>>>>
>>>>>>>>>>>> If we don't have G1Log::fine() (which is PrintGC) enabled we will
>>>>>>>>>>>> never call the print() method and will thus not detect any changes
>>>>>>>>>>>> made by the MXBean. If we have G1Log::finer() enabled we enter the
>>>>>>>>>>>> logging code, print other things at the "finer" level (which is
>>>>>>>>>>>> PrintGCDetails) and then do the call to the print() method where we
>>>>>>>>>>>> can suddenly decide that PrintGCDetails no longer is enabled and not
>>>>>>>>>>>> do the rest of the logging. So for the same GC we will print some
>>>>>>>>>>>> stuff at PrintGCDetails level and some things at another level.
>>>>>>>>>>>> Strange.
>>>>>>>>>>>
>>>>>>>>>>> May it's not clear, but the change will have effect only if PrintGC or
>>>>>>>>>>> PrintGCDetails has been changed during execution.
>>>>>>>>>>> Otherwise, the level will not change.
>>>>>>>>>>
>>>>>>>>>> I understand that, but what I was saying with the example above is that
>>>>>>>>>> you will not get the desired effect if you change PrintGC or
>>>>>>>>>> PrintGCDetails at runtime. If they are disabled, and then turn them on
>>>>>>>>>> at runtime you will not enter the update code because of this check at
>>>>>>>>>> the start of the log_gc_footer() method:
>>>>>>>>>>
>>>>>>>>>>   if (!G1Log::fine()) {
>>>>>>>>>>     return;
>>>>>>>>>>   }
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Similarly turning them off at runtime will start to have an effect in
>>>>>>>>>> the middle of a GC. Which is probably not the effect you are looking for.
>>>>>>>>> fine() invokes level(), level() invokes update(), so G1::fine() will
>>>>>>>>> return the up to date value:
>>>>>>>>>
>>>>>>>>>     inline static bool fine() {
>>>>>>>>> -    return _level >= LevelFine;
>>>>>>>>> +    return level() >= LevelFine;
>>>>>>>>>     }
>>>>>>>>>
>>>>>>>>>     static LogLevel level() {
>>>>>>>>> +    // PringGC and PrintGCDetails are dynamic flags
>>>>>>>>> +    // _level should be in sync with the latest values
>>>>>>>>> +    update_level();
>>>>>>>>>       return _level;
>>>>>>>>>     }
>>>>>>>>>
>>>>>>>> Oh, I missed that you had changed fine() to call level(). This means that
>>>>>>>> all calls to G1Log interface has side effects. I don't like that at all.
>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> This is why making the level() getter having the side effect of changing
>>>>>>>>>> the log level is a bad idea. You have no control over when that happens.
>>>>>>>>>
>>>>>>>>> It must be up to those who change flags at runtime.
>>>>>>>>> But the following code should work:
>>>>>>>>>
>>>>>>>>> MXBean.set(PrintGC, false);
>>>>>>>>> System.gc();
>>>>>>>>> MXBean.set(PrintGC, true);
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> To me, it's strange, that when I disable PrintGC in G1 I still seeing
>>>>>>>>>>> gc log messages...
>>>>>>>>>>> Or, I can't turn logging on during execution with G1.
>>>>>>>>>>> This works well for other collectors.
>>>>>>>>>>>
>>>>>>>>>>> Would it make sense to make PrintGC and PrintGCDetails unmanageable in
>>>>>>>>>>> case of g1?
>>>>>>>>>>
>>>>>>>>>> I think they can be manageble but their changed states need to be
>>>>>>>>>> handled more explicitly.
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> I would prefer to have a hook when the MXBean changes the value and
>>>>>>>>>>>> only update the level at that point.
>>>>>>>>>>>
>>>>>>>>>>> Yes, it would be ideal solution. But I haven't found such a mechanism
>>>>>>>>>>> supported. And it's unlikely worth adding just for this case.
>>>>>>>>>>> In my version, the sync will be done more frequently.
>>>>>>>>>>
>>>>>>>>>> The sync is done very frequently. Actually way too often in my view.
>>>>>>>>>>
>>>>>>>>>> I would prefer that you investigate the hook in the MXBean change so we
>>>>>>>>>> know how much work that would be.
>>>>>>>>>
>>>>>>>>> It was my first intention, but I haven't found a way how to implement it.
>>>>>>>>
>>>>>>>> I think what you need to do is to extend the Flag class with an update()
>>>>>>>> method and call the method in WriteableFlags::set_flag().
>>>>>>>>
>>>>>>>>>
>>>>>>>>>> If that is not possible I think a better solution is to call
>>>>>>>>>> G1Log::update_level() *once* at the start of a GC. That way the log
>>>>>>>>>> level is always consistent throughout the GC.
>>>>>>>>>
>>>>>>>>> Yes, this is a good idea!
>>>>>>>>
>>>>>>>> Yes, it is much cleaner.
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Having said that I am not sure that this bug is worth fixing right
>>>>>>>>>>>> now. I am currently working on the JEP to make the GC logging use the
>>>>>>>>>>>> new unified logging format. That will change all of this and most
>>>>>>>>>>>> likely remove the G1Log class all together. So, my suggestion would be
>>>>>>>>>>>> to leave this as is for now and instead add the MXBean requirement to
>>>>>>>>>>>> the unified logging work.
>>>>>>>>>>>
>>>>>>>>>>> Yes, I'm aware of unified logging and expect that G1Log class will go
>>>>>>>>>>> away someday.
>>>>>>>>>>> But I would like to be able to disable logging in JDK9, to not see GC
>>>>>>>>>>> events occurred at the certain period of time.
>>>>>>>>>>
>>>>>>>>>> Can you explain more why this is important to fix right now? Unified
>>>>>>>>>> logging is targeted for JDK 9.
>>>>>>>>>
>>>>>>>>> What I heard about unified logging is that there is no guarantee that it
>>>>>>>>> will happen in JDK9 time frame.
>>>>>>>>> I need the feature now to develop tests based on analysis of logs: I want
>>>>>>>>> to start logging not from begging, but after some steps.
>>>>>>>>>
>>>>>>>>> Will you be okay if G1Log::update_level() is invoked once per GC?
>>>>>>>>
>>>>>>>> I would be ok with it, but I'm pretty convinced unified logging will make
>>>>>>>> it in to JDK9 so if you can wait for it that might be better.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Bengt
>>>>>>>>
>>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>> Dima
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Thanks,
>>>>>>>>>> Bengt
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Thanks,
>>>>>>>>>>> Dima
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Thanks,
>>>>>>>>>>>> Bengt
>>>>>>>>>>>>>
>>>>>>>>>>>>> Testing:
>>>>>>>>>>>>> I ran manually the test from the bug report to make sure the change
>>>>>>>>>>>>> fixes the problem.
>>>>>>>>>>>>> A regression test will be delivered separately as a fix of
>>>>>>>>>>>>> JDK-8077056 <https://bugs.openjdk.java.net/browse/JDK-8077056>
>>>>>>>>>>>>>
>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>> Dima
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>


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