RFR: JDK-8055845 - Add trace event for promoted objects

Staffan Friberg staffan.friberg at oracle.com
Thu Aug 28 21:42:25 UTC 2014 

Hi,

I uploaded a new webrev here, 
http://cr.openjdk.java.net/~sfriberg/8055845/webrev.02
Uploaded under my own account since my key got updated, but kept the 
revision number going from the earlier uploads from Bengt (Thank You!).

It contains suggested changes from Erik, as well as a bug fix in the G1 
path where the age could be higher than the tenuring threshold due to a 
forwarding pointer having overwritten the field.
This was not causing any issue in G1 as the object would never be copied 
in the end, but it made the tracing event look strange.

Cheers,
Staffan

On 08/28/2014 02:15 PM, Staffan Friberg wrote:
> Hi Erik,
>
> Thanks for the comments.
>> - Aren't the events for promotion to the tenured generation (SerialOld)
>>   and the CMS generation missing?
> The reason for leaving out these two, Serial completely and CMS 
> promotion, was due to that neither as far as I understand make use of 
> PLABs. So without a good sampling type of event it would end up being 
> an event for each aging and/or promotion that happened in the GC. As 
> this would be a very large number of events so I decided not to 
> support those cases due to performance concerns. If you have a good 
> idea on how to support it please let me know. I would see CMS as the 
> priority of the two, client workloads can usually be analyzed with a 
> different collector which might not be the case for a latency 
> sensitive application.
>
>> - We try to keep the trace dependency (the header file) in
>>   gcTraceSend.cpp, see the pattern for all the other events in
>>   gcTrace.cpp (they all have a "report" and a "send" function).
> Will update it to follow that pattern.
>
>> - Would it make sense to differentiate, either by separate events or by
>>   a field in a event, between promotions to to-space and to the old
>>   generation?
>> - The are two events for TLAB allocations,
>>   java/object_alloc_in_new_TLAB and java/object_alloc_outside_TLAB.
>>   What do you think about using two events for PLAB allocations as well:
>>   - java/object_alloc_in_new_PLAB
>>   - java/object_alloc_outside_PLAB
> I think this is a matter of taste and probably how similar we want the 
> event to be to the existing allocation event. I personally prefer a 
> single event but if the GC team and serviceability team feel it would 
> be better to have two I can certainly rewrite. The reason for me 
> preferring a single event is just ease of analysis, you can easily 
> filter a list of events on a field, it is harder to merge two 
> different events with different fields and work with them in an 
> straight forward fashion.
>
> Any general preference for having a single or multiple events?
>
>> - In PSPromotionManager, instead of utilizing the C++ friendship with
>>   PSScavenge, should we add a getter function for the gc_tracer? 
> Created a getter function.
>
> Will upload a new webrev shortly.
>
> //Staffan
>
> On 08/28/2014 02:27 AM, Erik Helin wrote:
>> Hi Staffan,
>>
>> thanks for the patch, looks like a useful event!
>>
>> A few initial comments:
>> - Aren't the events for promotion to the tenured generation (SerialOld)
>>   and the CMS generation missing?
>> - We try to keep the trace dependency (the header file) in
>>   gcTraceSend.cpp, see the pattern for all the other events in
>>   gcTrace.cpp (they all have a "report" and a "send" function).
>> - Would it make sense to differentiate, either by separate events or by
>>   a field in a event, between promotions to to-space and to the old
>>   generation?
>> - The are two events for TLAB allocations,
>>   java/object_alloc_in_new_TLAB and java/object_alloc_outside_TLAB.
>>   What do you think about using two events for PLAB allocations as well:
>>   - java/object_alloc_in_new_PLAB
>>   - java/object_alloc_outside_PLAB
>> - In PSPromotionManager, instead of utilizing the C++ friendship with
>>   PSScavenge, should we add a getter function for the gc_tracer?
>>
>> Thanks,
>> Erik
>>
>> On 2014-08-27 16:28, Bengt Rutisson wrote:
>>>
>>>
>>> Hi Staffan,
>>>
>>> Overall I think this change looks fine. I know that Erik Helin has some
>>> thoughts on the naming of the events. I'll let him communicate those
>>> thoughts.
>>>
>>> Apart from Erik's comments and the missing test case (that I know you
>>> are working on) I think the change looks good.
>>>
>>> Thanks,
>>> Bengt
>>>
>>> On 8/26/14 10:50 AM, Bengt Rutisson wrote:
>>>>
>>>> Hi all,
>>>>
>>>> Staffan sent me an updated webrev based on Erik's comments. I uploaded
>>>> it here:
>>>> http://cr.openjdk.java.net/~brutisso/8055845/webrev.01/
>>>>
>>>> Thanks,
>>>> Bengt
>>>>
>>>>
>>>> On 2014-08-25 19:32, Staffan Friberg wrote:
>>>>> Hi Erik,
>>>>>
>>>>> No issue with naming the field class, since the event is similar to
>>>>> the Allocation event I used that as a starting point and it also uses
>>>>> class as name together with a couple of other events.
>>>>>
>>>>> Will go through the descriptions and remove periods.
>>>>>
>>>>> Object age is basically the how many times an object has been kept in
>>>>> survivor region, it is incremented each time a YC happens and the
>>>>> object is aged.
>>>>> Will see how I can update the description to make it clearer. Calling
>>>>> the field tenuringAge might help?
>>>>>
>>>>>> From the documentation,
>>>>>> http://docs.oracle.com/javase/8/docs/technotes/tools/unix/java.html
>>>>>>
>>>>>> -XX:MaxTenuringThreshold=/threshold/
>>>>>>
>>>>>>     Sets the maximum tenuring threshold for use in adaptive GC
>>>>>>     sizing. The largest value is 15. The default value is 15 for the
>>>>>>     parallel (throughput) collector, and 6 for the CMS collector.
>>>>>>
>>>>>>     The following example shows how to set the maximum tenuring
>>>>>>     threshold to 10:
>>>>>>
>>>>>>     -XX:MaxTenuringThreshold=10
>>>>>>
>>>>>>
>>>>>> -XX:+PrintTenuringDistribution
>>>>>>
>>>>>>     Enables printing of tenuring age information. The following is
>>>>>>     an example of the output:
>>>>>>
>>>>>>     Desired survivor size 48286924 bytes, new threshold 10 (max 10)
>>>>>>     - age 1: 28992024 bytes, 28992024 total
>>>>>>     - age 2: 1366864 bytes, 30358888 total
>>>>>>     - age 3: 1425912 bytes, 31784800 total
>>>>>>     ...
>>>>>>
>>>>>>     Age 1 objects are the youngest survivors (they were created
>>>>>>     after the previous scavenge, survived the latest scavenge, and
>>>>>>     moved from eden to survivor space). Age 2 objects have survived
>>>>>>     two scavenges (during the second scavenge they were copied from
>>>>>>     one survivor space to the next). And so on.
>>>>>>
>>>>>>     In the preceding example, 28 992 024 bytes survived one scavenge
>>>>>>     and were copied from eden to survivor space, 1 366 864 bytes are
>>>>>>     occupied by age 2 objects, etc. The third value in each row is
>>>>>>     the cumulative size of objects of age n or less.
>>>>>>
>>>>>>     By default, this option is disabled.
>>>>>>
>>>>>
>>>>> Thanks for the comments,
>>>>> Staffan
>>>>>
>>>>> On 08/25/2014 09:55 AM, Erik Gahlin wrote:
>>>>>> Did you manage to call the field "class"? It's a reserved word in
>>>>>> C++, so we had to use "klass" in the past
>>>>>>
>>>>>> Descriptions with only one sentence shouldn't end with "."
>>>>>>
>>>>>> How is "Object Age" measured?
>>>>>>
>>>>>> As a user, I would expect the number to be in seconds/minutes/hours,
>>>>>> but that is not the case. Perhaps an explanation in the description
>>>>>> and/or a field name that better reflects what we really mean with 
>>>>>> age.
>>>>>>
>>>>>> Otherwise trace.xml looks good!
>>>>>>
>>>>>> Erik
>>>>>>
>>>>>> Staffan Friberg skrev 25/08/14 18:28:
>>>>>>> Hi,
>>>>>>>
>>>>>>> Could I please get reviews for this RFE that adds a trace event for
>>>>>>> aging and promotion for young collections in G1, CMS and 
>>>>>>> Parallel GC.
>>>>>>> It works similarly to allocation events, and generates the event on
>>>>>>> the slow path when either a direct copy occurs or a new PLAB is
>>>>>>> request.
>>>>>>>
>>>>>>> Since I'm adding an event I cc:ed the serviceability list in case
>>>>>>> anyone has any comments on the event and changes to trace.xml.
>>>>>>>
>>>>>>> RFE: https://bugs.openjdk.java.net/browse/JDK-8055845
>>>>>>> Webrev: http://cr.openjdk.java.net/~brutisso/8055845/webrev.00
>>>>>>>
>>>>>>> Bengt has been kind and agreed to both sponsor and host the the
>>>>>>> webrev.
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Staffan
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>

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