Request for review (S): 7110718 -XX:MarkSweepAlwaysCompactCount=0 crashes the JVM

Fri Nov 18 07:53:01 UTC 2011

Ramki,

Thanks for the extra testing!

Jon, John and Tony,

Thanks for the reviews! Are you OK with pushing this (handle values < 1 
as never force full compaction):
http://cr.openjdk.java.net/~brutisso/7110718/webrev.03/

Instead of this (forbid values < 1):
http://cr.openjdk.java.net/~brutisso/7110718/webrev.02/

I think your review comments were more related to webrev.02, so I would 
like to double check with you before I push my changes in webrev.03.

Thanks,
Bengt

On 2011-11-18 08:11, Srinivas Ramakrishna wrote:
> Hi Bengt -- this looks good to me. It also passed my testing...
> I also confirmed what Jon indicated -- that VerifyUpdateClosure is 
> dead code, as far as my cscope
> navigation showed and can be deleted (in a separate CR as Jon said).
>
> Rebiewed; thanks!
> -- ramki
>
> On Thu, Nov 17, 2011 at 6:13 AM, Bengt Rutisson 
> <bengt.rutisson at oracle.com <mailto:bengt.rutisson at oracle.com>> wrote:
>
>
>     Hi Ramki,
>
>     Is this what you were considering?
>     http://cr.openjdk.java.net/~brutisso/7110718/webrev.03/
>     <http://cr.openjdk.java.net/%7Ebrutisso/7110718/webrev.03/>
>
>     I think it should interpret MarkSweepAlwaysCompactCount <= 0 as
>     "never force full compaction". It would be great if you could do
>     some testing. There is one assert in
>     PSParallelCompact::VerifyUpdateClosure::do_addr() that worries me
>     a bit:
>
>       assert(HeapMaximumCompactionInterval > 1 ||
>     MarkSweepAlwaysCompactCount > 1 ||
>              forwarding_ptr == new_pointer, "new location is incorrect");
>
>     I think this should be safe since MarkSweepAlwaysCompactCount == 1
>     was an acceptable value even before my change. But I have to admit
>     that I don't really understand what the assert is trying to verify.
>
>     Bengt
>
>
>
>     On 2011-11-16 19:01, Srinivas Ramakrishna wrote:
>>     Hi Bengt, Not sure how much customers use this option. Its useful
>>     for "serviceability in production" kind of scenarios
>>     to have the code be more robust. I think it would be useful. I
>>     appreciate the need for more testing
>>     of course, and I am happy to do that testing for you -- just let
>>     me know and I'll grab yr patch and test
>>     here.
>>
>>     thanks!
>>     -- ramki
>>
>>     On Tue, Nov 15, 2011 at 6:50 AM, Bengt Rutisson
>>     <bengt.rutisson at oracle.com <mailto:bengt.rutisson at oracle.com>> wrote:
>>
>>
>>         Ramki,
>>
>>
>>         On 2011-11-14 20:32, Srinivas Ramakrishna wrote:
>>>
>>>         Thanks, Bengt, for the super-quick turnaround!! A comment
>>>         below on the choice of <= 0 for the option value....
>>
>>         Thanks for the review! See comments below.
>>
>>
>>>         On Mon, Nov 14, 2011 at 1:25 AM, Bengt Rutisson
>>>         <bengt.rutisson at oracle.com
>>>         <mailto:bengt.rutisson at oracle.com>> wrote:
>>>
>>>
>>>             Hi all,
>>>
>>>             Can I have a couple of reviews for this small change?
>>>             http://cr.openjdk.java.net/~brutisso/7110718/webrev.01/
>>>             <http://cr.openjdk.java.net/%7Ebrutisso/7110718/webrev.01/>
>>>
>>>             It is a fix for the issue that Ramki reported recently.
>>>             MarkSweepAlwaysCompactCount is used for division and
>>>             Hotspot crashes if it is set to 0.
>>>
>>>             I choose to log an error and exit the VM if someone
>>>             tries to start with -XX:MarkSweepAlwaysCompactCount=0.
>>>             An alternative is to just log a warning and set it to 1.
>>>
>>>             I prefer the error way since it is not really clear what
>>>             one wants to achieve with MarkSweepAlwaysCompactCount=0.
>>>             Always do full compactions or never do full compactions?
>>>             So I am not convinced that 1 is an appropriate value.
>>>
>>>             Also, since the VM, up until now, has crashed if someone
>>>             tried -XX:MarkSweepAlwaysCompactCount=0 I think we can
>>>             be sure that there are no customers that are running
>>>             with that setting. It should be safe to forbid it.
>>>
>>>
>>>         I agree with that statement. However, given that the value 0
>>>         was producing crashes, proving that no production code would
>>>         have been using that setting,
>>>         and based on yr comment above that the 0 value could as well
>>>         have been used to denote "never force full compaction", it
>>>         seems as though an alternative to exiting with
>>>         an error, is now to define all values <=0 to mean "never
>>>         _force_ full compaction"
>>>
>>>         Especially since tolerating allowed input values and mapping
>>>         them to specific non-exiting behaviours allows us to modify
>>>         production JVM's on the fly
>>>         without causing loss of availability. (Consider a future in
>>>         which this option becomes a "manageable"; you would then be
>>>         faced with the same
>>>         question, and it seems as though making this choice now
>>>         would help maintain consistency and robustness going forward
>>>         -- we could of course
>>>         always throw a "illegal value exception" or such at that
>>>         point, but allowing the specification of "never _force_ full
>>>         compaction" (unless the JVM
>>>         otherwise chooses to) would appear to be a choice to allow
>>>         users; mapping negative and 0 values to that setting would
>>>         avoid having to
>>>         throw an error.) However, I understand that this is somewhat
>>>         subjective, so I am willing to go with whatever the majority
>>>         consensus here
>>>         mght be. It just seemed more pleasant to:
>>>         (1) allow the specification of reasonable behaviour (i.e.
>>>         never _force_ ...)
>>>         (2) map the full domain of the option to a reasonable
>>>         behaviour (i.e. allow <= 0 to map to never _force_ ..)
>>>
>>>         Comments?
>>
>>         I see your point, and I think this should be fairly straight
>>         forward to fix. However it will require some more testing
>>         etc. I can do that, but I don't think I know enough to say
>>         whether or not the extra work is worth it. How important is
>>         this option? Is it something that customers use a lot?
>>
>>         Thanks,
>>         Bengt
>>
>>
>>>
>>>         -- ramki
>>>
>>>
>>>
>>>             CR:
>>>
>>>             7110718 -XX:MarkSweepAlwaysCompactCount=0 crashes the JVM
>>>             http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7110718
>>>
>>>             Thanks,
>>>             Bengt
>>>
>>>
>>
>>
>
>

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