RFR: JDK-8152952: Allow G1 phase logging to use individual number of threads

Bengt Rutisson bengt.rutisson at oracle.com
Wed Mar 30 17:36:27 UTC 2016


Hi again,

Another update to the webrev. I talked to Jon a bit and realized that it 
would be good to have at least a sanity test on the <double> version of 
WorkerDataArray. I refactored the tests a bit to be more readable and 
also fixed two issues with the previous versions. The original tests did 
not handle different locales and also had a too strict double comparison.

Here's an updated webrev:
http://cr.openjdk.java.net/~brutisso/8152952/webrev.03/

Only the internal VM tests have been changed. Here's a diff compared to 
the last version:
http://cr.openjdk.java.net/~brutisso/8152952/webrev.02-03.diff/

Thanks,
Bengt

On 2016-03-30 16:27, Bengt Rutisson wrote:
>
> One comment at the bottom of this email....
>
> On 2016-03-30 16:11, Bengt Rutisson wrote:
>>
>> Hi Thomas,
>>
>> Thanks for looking at this!
>>
>> On 2016-03-30 11:15, Thomas Schatzl wrote:
>>> Hi Bengt,
>>>
>>> On Tue, 2016-03-29 at 14:03 +0200, Bengt Rutisson wrote:
>>>> Hi everyone,
>>>>
>>>> Could I have a couple of reviews for this change?
>>>>
>>>> http://cr.openjdk.java.net/~brutisso/8152952/webrev.00/
>>>> https://bugs.openjdk.java.net/browse/JDK-8152952
>>>>
>>>> Currently if you run with UseDynamicNumberOfGCThreads you can
>>>> potentially get a different number of worker threads each GC. There
>>>> are improvements coming where we want to select a different number of
>>>> worker threads for individual phases. The G1GCPhaseTimes and
>>>> WorkerDataArray structures need to support this.
>>>>
>>>> The proposed patch sets all slots in the WorkerDataArray to an
>>>> uninitialized value and then only print any values that have actually
>>>> been set for that phase.
>>>>
>>>> The patch also extends the log message about the number for worker
>>>> threads to also say how many it could potentially have used. And it
>>>> also fixes a missing space in the level 3 and level 4 indentation.
>>>>
>>>> After applying this patch and running with -Xlog:gc*,phases*=trace
>>>> you get output like:
>>>> [0,581s][info][gc,task ] GC(0) GC Workers: using 2 out of 23
>>>> [0,588s][info][gc,phases] GC(0)   Evacuate Collection Set: 5,0ms
>>>> [0,588s][trace][gc,phases] GC(0)       GC Worker Start (ms):     Min:
>>>> 580,9, Avg: 580,9, Max: 580,9, Diff:  0,0
>>> It would be useful to have the information about the number of threads
>>> used for every top-level WorkerDataArray. That might differ for every
>>> phase in the future. Now it does not matter, because at the moment
>>> every thread at least sets the time spent to zero (i.e. is forced to),
>>> but that will not be the case later.
>>>
>>> Not working on something is different to taking "zero" time for it.
>>>
>>> My suggestion is to add a ", Workers: X" column to the summary 
>>> output, like
>>>
>>>> [0,588s][debug][gc,phases     ] GC(0)     Ext Root Scanning (ms):
>>>> Min:  1,7, Avg:  1,7, Max:  1,8, Diff:  0,0, Sum:  3,5
>>> Min:  1,7, Avg:  1,7, Max:  1,8, Diff:  0,0, Sum:  3,5, Workers: 2
>>
>> I added that. The log now looks like this:
>>
>> [0,613s][debug][gc,phases     ] GC(0)     Ext Root Scanning (ms):   
>> Min:  2,4, Avg:  2,5, Max:  2,7, Diff:  0,3, Sum:  5,0, Workers: 2
>>
>>>
>>>> [0,589s][trace][gc,phases,task] GC(0)
>>>> 1,8  1,7   -   -   -   - -   -   -   -   -   -   -   -   - -   -
>>>> -
>>>
>>>> [0,588s][trace][gc,phases,task] GC(0)
>>>> 580,9 580,9   -   -   - -   -   -   -   -   -   -   -   - -   -   -
>>> - g1GCPhaseTimes.cpp:109: typo in ASSERT_PHASE_UNINITILAIZED, should be
>>> ASSERT_PHASE_UNINITIALIZED.
>>
>> Fixed. Thanks for catching that! :)
>>
>>>
>>> - ignore the following if wanted: in g1GCPhaseTimes.cpp:127 the if
>>> -clause is structured to have only the asserts in the if-part. I would
>>> prefer if the code that performs some useful work would be first, i.e.
>>> the condition reversed. As mentioned, ymmv.
>>
>> Done.
>>
>>>
>>> - I would kind of prefer if ADD_WORKER_KNOWN time were a method instead
>>> of a macro. In case of an uninitialized value that method could just
>>> return 0.0, which would be okay for this addition.
>>
>> Done.
>>
>>>
>>> - in WorkerDataArray::WDAPrinter::details, the "unknown value" imo does
>>> not need to be padded out to five spaces or so. I think the main
>>> motivation for this has been to show the values of the different phases
>>> of the evacuation phase in the same column.
>>>
>>> The details() method for the size_t values is not aligned at all.
>>>
>>> The main reason is waste of space. Not sure here, in doubt keep it.
>>
>> Removed the padding for both variants.
>>
>>>
>>> - I would prefer if the #include "memory/resourceArea.hpp" were put
>>> next to the other includes, of course guarded by the define.
>>
>> I prefer to keep it closer to the tests. If you have strong opinions 
>> I can move it up, but since it is only test related I prefer to have 
>> it there.
>>
>>>
>>> - in workerDataArray.hpp, at the definitions of sum() and average() it
>>> might be useful to mention what impact on sum/average uninitialized
>>> values have.
>>
>> Added a comment.
>>
>>>
>>> - I would prefer, if a phase has no data, that this would be detected
>>> automatically and the phase either not printed, or (preferably)
>>> indicated that it has not been executed. (I could also live with a
>>> solution where the programmer can choose what happens when there is no
>>> data. That would probably also remove the dependencies on a lot of
>>> other components, since as of right now there is quite a bit of
>>> checking for particular circumstances in G1GCPhaseTimes::print()).
>>>
>>> I do not really like the solution based on this change presented for
>>> JDK-8152428 that the programmer is responsible for explicitly
>>> specifying that a phase has not been executed. This will be forgotten,
>>> and only causes unnecessary failures potentially long after a change
>>> has been committed.
>>>
>>> The code to print has to iterate over all elements to sum/average them
>>> up anyway, so it already knows that there is no data for a particular
>>> phase.
>>>
>>> The reason why I prefer an indication that a phase has not been
>>> executed is that instead of a missing line that a missing line easy to
>>> overlook, while a "not executed" line is much more visible.
>>
>> I've changed to do mostly what you suggested. A skipped phase now logs:
>>
>> [6,339s][trace][gc,phases      ] GC(79)       Parallel Preserve CM 
>> Refs (ms):: skipped
>
> The double ":" should not be there. The log actually looks like:
>
> [6,458s][trace][gc,phases      ] GC(82)       Parallel Preserve CM 
> Refs (ms): skipped
>
> And I should also have mentioned that this makes my other fix for 
> JDK-8152428 void. I will withdraw that fix if this patch gets approved.
>
> Thanks,
> Bengt
>
>>
>> But you can't control that by a configuration on the WorkerDataArray. 
>> On the other hand you can control it just like you did before, but 
>> guarding the call in the print() menthod. Like for example 
>> StringDeduplication does.
>>
>> Updated webrev:
>> http://cr.openjdk.java.net/~brutisso/8152952/webrev.02/
>>
>> Diff compared to last version:
>> http://cr.openjdk.java.net/~brutisso/8152952/webrev.01-02.diff/
>>
>> Thanks,
>> Bengt
>>
>>>
>>> Thanks,
>>>    Thomas
>>>
>>
>



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