RFR: JDK-8134953: Make the GC ID available in a central place

Jon Masamitsu jon.masamitsu at oracle.com
Wed Sep 9 23:00:16 UTC 2015


When a CMS concurrent collection hands off to a STW foreground collection,
(done in acquire_control_and_collect()), I expected a new GCId would be
used.  Did I miss it?   That STW collection does not go through 


On 9/9/2015 7:40 AM, Bengt Rutisson wrote:
> Hi again,
> I found an issue with how G1 young collections kick off concurrent 
> marks. There is a short window where we might have two active GC IDs 
> at the same time. I've fixed this and updated the webrevs. In case 
> anyone had already started looking at the webrevs you need to do a 
> refresh now. The fix is in 
> G1CollectedHeap::do_collection_pause_at_safepoint().
> Thanks,
> Bengt
> On 09/09/15 13:38, Bengt Rutisson wrote:
>> Hi Per,
>> Thanks for looking at this!
>> On 2015-09-08 15:23, Per Liden wrote:
>>> Hi Bengt,
>>> On 2015-09-08 13:35, Bengt Rutisson wrote:
>>>> Hi everyone,
>>>> This is mostly a GC related patch, but it adds a field to the Thread
>>>> class, so I'm sending this out on the broader hotspot-dev list.
>>>> Could I have a couple of reviews for this patch?
>>>> http://cr.openjdk.java.net/~brutisso/8134953/webrev.01/
>>>> https://bugs.openjdk.java.net/browse/JDK-8134953
>>> Looks good. I think this is a nice simplification, especially for 
>>> G1's concurrent parts. Just two comments:
>>> genCollectedHeap.cpp:
>>> ---------------------
>>> - GenCollectedHeap::do_collection() has two GCIdMarks, in different 
>>> scopes. Do we really want that second mark?
>> We potentially do two GCs in GenCollectedHeap::do_collection(). First 
>> a young GC and then potentially a full GC. These two should have 
>> different GC IDs. So, yes, we need two GCIdMarks in this method. The 
>> scoping could be better, but I think that is a refactoring that 
>> should be done separately from my current patch. I'll talk to Jesper 
>> about it since he has been cleaning up this code lately.
>>> gcId.cpp:
>>> --------
>>> - I think you might have left currentNamedthread() in there. You 
>>> probably just want to use Thread::current() instead.
>> Yes, good catch. Thanks.
>> However, after thinking some more about the implications of moving 
>> the GC ID out from the GCTracer I figured it would be possible to 
>> just store the previous GC ID in the GCIdMark and then restore it in 
>> the destructor of that class. That way we don't have to store 
>> anything in the Thread class but can still have both a concurrent GC 
>> ID and a STW GC ID active at the same time. This also removes the 
>> need to copy the GC ID to the worker tasks.
>> Here's an updated webrev with a solution that does not add anything 
>> to the Thread class:
>> http://cr.openjdk.java.net/~brutisso/8134953/webrev.02/
>> And here's a diff compared to the last version:
>> http://cr.openjdk.java.net/~brutisso/8134953/webrev.01-02.diff/
>> Unfotunately the webrev tool had some bad luck when creating the diff 
>> webrev so at least the g1CollectedHeap.cpp seem to contain the 
>> complete changes rather than just the diff compared to the 01 
>> version. The rest of the diff looks correct as far as I can tell.
>> Thanks,
>> Bengt
>>> cheers,
>>> /Per
>>>> Currently the GC ID, that is used for event tracing and logging, is
>>>> stored in the GCTracer object. That has caused some minor problems 
>>>> since
>>>> there are multiple GCTracers active at the same time. The correct 
>>>> GC IDs
>>>> need to be passed around in many places.
>>>> For some upcoming GC logging changes I would like to have a more
>>>> consistent way of finding the currently active GC ID. I've played 
>>>> around
>>>> with a couple of different solutions, but eventually I found that the
>>>> simplest solution is to store the current GC ID in the thread. That 
>>>> way
>>>> there is a single way to look it up in all places. It is also fairly
>>>> straight forward to set it up.
>>>> I've reworked the GCId class a bit to support this and I've 
>>>> introduced a
>>>> GCIdMark class that is a scoped object that helps to set up and tear
>>>> down a current GC ID. By moving the GC ID out from the GCTracer 
>>>> class I
>>>> got rid of a few minor issues as well. One is that we no longer 
>>>> need to
>>>> keep track of the G1 "aborted concurrent GC ID". That was necessary
>>>> before since we did logging *after* we had told the corresponding
>>>> GCTracer that the concurrent cycle was aborted and it had thrown 
>>>> its GC
>>>> ID away. Now the GC ID can stay in scope until we have completed all
>>>> logging.
>>>> Also the HeapDumpBeforeFullGC and PrintClassHistogramBeforeFullGC used
>>>> to have to create a new GC ID since their logging was done before 
>>>> we had
>>>> the correct GCTracer set up. Now the GC ID can be available early 
>>>> enough
>>>> for this logging to be using the same (and correct) GC ID as the 
>>>> rest of
>>>> the GC. Same for HeapDumpAfterFullGC and 
>>>> PrintClassHistogramAfterFullGC.
>>>> I've added an uint to the Thread class to keep track of the currently
>>>> active GC ID. In the current code there are actually only NamedThreads
>>>> that need the GC ID. So, I'm not sure where the best place is for this
>>>> field is. But it seems like the Thread class contains most of the 
>>>> data,
>>>> even data that is only used by some subclasses, so I opted for putting
>>>> the field in Thread as opposed to in NamedThread. This opens up for
>>>> possible future extensions when Java threads may do part of the GC 
>>>> work.
>>>> However, I'm open to moving the field down to NamedThread if that is
>>>> seen as better.
>>>> In the GCTracer class there were many asserts that were checking that
>>>> the GC ID was properly set up. Mostly these assert verify that 
>>>> start/end
>>>> is called correctly and that the other methods are called inside of 
>>>> the
>>>> start/end scope. Now that the GC ID is moved out of the GCTracer class
>>>> these asserts had to be dealt with. I figured there were three ways to
>>>> handle it; just remove them, replace them with check that the GC ID 
>>>> from
>>>> the current thread is correct, or implement a new status field to keep
>>>> track of GC start/end state. Personally I'm not sure these asserts are
>>>> valuable enough to spend time on, so I went for the first approach:
>>>> removing them. I don't think making them use the thread GC ID will be
>>>> appropriate. But if anyone feels strongly about these asserts I can
>>>> implement a separate start/end state.
>>>> There are a few "Tasks" in the GC code that are executed by worker
>>>> threads. To make the worker threads use the correct GC ID I make sure
>>>> that the Task constructors copy the GC ID from the initiating thread
>>>> into a local variable. When the task is executed in its worker 
>>>> thread it
>>>> sets up the GC ID based on the local variable.
>>>> The proposed change does not alter any logging (except for the 
>>>> small bug
>>>> fix for
>>>> HeapDumpBefore(After)FullGC/PrintClassHistogramBefore(After)FullGC. 
>>>> This
>>>> means that no existing tests need to be updated. In particular the
>>>> test/gc/logging/TestGCId.java test passes even after these changes.
>>>> A big thanks to Per, who pre-reviewed these changes and came with some
>>>> really good feedback.
>>>> Thanks,
>>>> Bengt

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