RFR: 8079315: UseCondCardMark broken in conjunction with CMS precleaning

Erik Österlund erik.osterlund at lnu.se
Tue Jun 2 10:22:52 UTC 2015


Hi Aleksey,

Den 02/06/15 11:25 skrev Aleksey Shipilev <aleksey.shipilev at oracle.com>:

>On 06/02/2015 10:44 AM, Andrew Haley wrote:
>> Have we come to a complete standstill?
>
>Appears to be so.
>
>I am still uneasy about global_store_fence/mprotect trick, since I don't
>quite believe it works without a hitch everywhere, even given Erik's
>extensive explanation it "appears" to work fine. Erik, can you follow up
>with Dave Dice on whether that is actually safe?

Sure, will do.

>The original StoreLoad patch by Andrew looks moderately okay to me,
>given it is conditionalized for CMS (I wonder if it should be
>conditionalized on CMSPrecleaningEnabled?). Google folks may chime in
>saying it breaks the performance for them...

I think the patch looks okay for now, and it¹s important to get a
correctness fix although potentially suboptimal in terms of performance
ASAP. I have a feeling the global_fence discussion could take a while
before reaching a conclusion, don¹t know how long. It would be good to
have the correctness fix while exploring better performant synchronization
solutions to elide the fences.

Yes, Google folks might say there are performance regressions then, but
others might complain the JVM unexpectedly crashes now as it is now. I
think this is worse.

Thanks,
/Erik

>Actually, GC guys should make this call.
>
>Thanks,
>-Aleksey
>
>



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