RFR (XXS): 8048085: Aborting marking just before remark results in useless additional clearing of the next mark bitmap
bengt.rutisson at oracle.com
Thu Jul 10 09:49:35 UTC 2014
On 2014-07-10 11:38, Thomas Schatzl wrote:
> Hi Bengt,
> thanks for the review.
> On Mon, 2014-07-07 at 15:44 +0200, Bengt Rutisson wrote:
>> Hi Thomas,
>> On 2014-07-07 14:46, Thomas Schatzl wrote:
>>> Hi all,
>>> can I have reviews for the following minor performance fix noticed
>>> during work on JDK-8038423 (G1: Decommit memory within the heap)?
>>> The situation is that when a full GC aborts concurrent marking, the
>>> concurrent mark thread needlessly clears the next mark bitmap again
>>> (concurrently this time) in ConcurrentMark::abort().
>>> The fix is to skip this phase in this case.
>>> An alternative, always keeping this phase but not doing this bitmap
>>> clear at Full GC abort (JDK-7098512) would be possible. However doing
>>> that would make JDK-8048084 (sending out reviews soon) much harder as we
>>> would need to keep track of regions that have become unavailable.
>>> This change also does not change anything about duration of Full GC. So
>>> I would prefer to postpone JDK-7098512.
>> Looks good.
>> A couple of minor comments:
>> The comment in ConcurrentMark::abort() for the bitmap clearing was
>> always misleading (as stated in JDK-7098512). Would maybe be good to
>> update it now to explain why it is really needed.
You added a call to:
I think that only checks that there are no bits set over TAMS. But you
can actually check that he whole bitmap is empty. I was thinking
something like (completely untested code):
== _nextMarkBitMap->endWord(), "Next bitmap has not been cleared");
Also, check_bitmaps() only hits if G1VerifyBitmaps. Maybe that is good.
In that case I guess the assert I suggest should be guarded by that too.
>> What do you think about asserting that the bitmap is clear in
>> ConcurrentMarkThread::run() if cm()->has_aborted() ?
> good thoughts.
> I updated the change and JDK-7098512, new webrev at
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