CRR (XS): 7098085: G1: partially-young GCs not initiated under certain circumstances
y.s.ramakrishna at oracle.com
Thu Oct 6 00:44:36 UTC 2011
Hi Tony --
On 10/5/2011 5:27 PM, Tony Printezis wrote:
> 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:-
>> 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
> Yes, I do like this idea very much.
>>  you will have to invent a suitable other name, perhaps
>> 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
>> (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 :-)
>> Otherwise looks good.
>> -- ramki
>> On 10/5/2011 2:21 PM, Tony Printezis wrote:
>>> I'd like to get a couple of people to have a look at the following
>>> 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.
>>> PS Correct url? Check. Text actually applies to the CR in the
>>> subject? Check.
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