CRR (XS): 7098085: G1: partially-young GCs not initiated under certain circumstances

Tony Printezis tony.printezis at oracle.com
Wed Oct 5 17:55:45 PDT 2011


Ramki,

We might want to include the fix in hs22 as the issue with 
partially-young GCs not starting is quite severe. So, we should probably 
keep the fix as simple as possible. So it'd be probably prudent to 
remove the bitmap clearing on a separate Cr. But thanks for the vote of 
confidence. :-)

Tony

On 10/5/2011 8:44 PM, Ramki Ramakrishna wrote:
>
> Hi Tony --
>
> On 10/5/2011 5:27 PM, Tony Printezis wrote:
>> Ramki,
>>
>> Thanks for taking a look at it! See inline.
>>
>> On 10/5/2011 7:48 PM, Ramki Ramakrishna wrote:
>>> Hi Tony --
>>>
>>> The changes look good.
>>>
>>> On a side note, was the removal of the bit map clearing a different 
>>> bug fix,
>>> or is it somehow related to the issue described in the CR?
>>> I can see that it is unnecessary and may be even wasteful,
>>> but it seemed to me to be unrelated to the issue described in the CR,
>>> unless I am missing something here.
>>
>> Agreed. I was looking at the abort() method to see what it did and I 
>> noticed that it was also clearing the bitmap. It's indeed wasteful 
>> and, in fact, it does not achieve what the comment says it should 
>> achieve. abort() is only called at the start of a Full GC but, in 
>> order to do the Full GC safepoint, the concurrent marking workers 
>> should have noticed that a safepoint was initiated and should have 
>> all yielded (otherwise the safepoint would have to wait for them). 
>> And, the concurrent marking thread will actually clear the next 
>> bitmap as the last thing it does before waiting for the next cycle to 
>> start. But, yeah, point taken: I'll back out of the 
>> concurrentMark.cpp changes and do them as a separate CR.
>
> No need (IMHO). The overhead of a separate CR is not necessary in my 
> opinion. (Just a comment in the Summary line
> of the changeset should suffice. Again IMHO :-)
>
>>
>>> As a related (but otherwise orthogonal) matter, it appears as though 
>>> the idiom:-
>>>
>>>    _sts.join();
>>>    do_something();
>>>    do_something_else();
>>>   _sts_leave();
>>>
>>> might gain from the usual constructor/destructor idiom used
>>> elsewhere for code that wants to synchronize with other
>>> parts of the system (in this case safepoints). For example:
>>>
>>>   {
>>>      SynchronizeWithSafepoints ss();     // or appropriate other 
>>> name [1]
>>>      do_something();
>>>      do_something_else();
>>>   }
>>
>> Yes, I do like this idea very much.
>>
>>> [1] you will have to invent a suitable other name, perhaps 
>>> DesynchronizeWithSafepoints
>>> for when you do the reverse, i.e. perform an action outside of the 
>>> _sts, as happens
>>> when you are doing those synch barriers amongst the concurrent marking
>>> threads in case of overflow and restart.
>>
>> I can't think of a good name off-hand but I'll think about it for a 
>> bit... (suggestions are welcome!)
>>
>>> The benefit is that the extra scopes help clearly demarcate
>>> the code that wants to be in the "critical section", if you
>>> will, wrt safepoints that will want to synchronize wrt these
>>> actions.
>>>
>>> (As far as I can tell all invocations of join/leave or stsJoin/stsLeave
>>> are paired and can become block-structured.)
>>
>> They should definitely be paired (like lock/unlock pairs). Otherwise, 
>> we're in trouble. :-)
>>
>>> The c'tor/d'tor idiom change is of course orthogonal to yr changes
>>> and would likely want to be a separate CR, if you agree that this
>>> may be another clean-up worth doing.
>>
>> Absolutely. So, I take an action item to open two hs23 CRs: one to 
>> avoid clearing the marking bitmap and one for the ctor/dtor idiom to 
>> join / leave the STS and vice versa.
>
> sounds good (but as i said the removal of the bit-map clearing can be 
> left in here, with
> a comment in the Summary line of the changeset, IMO, but whichever 
> route you prefer :-)
>
> reviewed!
> -- ramki
>
>>
>> Tony
>>
>>> Otherwise looks good.
>>> -- ramki
>>>
>>> On 10/5/2011 2:21 PM, Tony Printezis wrote:
>>>> Hi,
>>>>
>>>> I'd like to get a couple of people to have a look at the following 
>>>> webrev:
>>>>
>>>> http://cr.openjdk.java.net/~tonyp/7098085/webrev.0/
>>>>
>>>> The code changes are very minor, most of the added lines are in 
>>>> fact detailed comments on the changes.
>>>>
>>>> I'm going to do testing on two machines overnight, as this is quite 
>>>> tricky code (even though it doesn't look it). But, I thought I'd 
>>>> open the changes for code review early as the change is small and 
>>>> some early feedback, if possible, would be greatly appreciated.
>>>>
>>>> Tony
>>>>
>>>> PS Correct url? Check. Text actually applies to the CR in the 
>>>> subject? Check.
>>>>


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