RFR(s): 8073476: G1 logging ignores changes to PrintGC* flags via MXBeans
Bengt Rutisson
bengt.rutisson at oracle.com
Tue Apr 21 12:19:43 UTC 2015
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.
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?
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.
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.
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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