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

Fri Nov 18 15:11:10 UTC 2011

Bengt,

I'm OK with the .03 version.

Jon

On 11/17/2011 11:53 PM, Bengt Rutisson wrote:
>
> 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
>>>>
>>>>
>>>
>>>
>>
>>
>
>