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

[Srinivas Ramakrishna]

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>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> wrote:
>
>>
>> Hi all,
>>
>> Can I have a couple of reviews for this small change?
>> http://cr.openjdk.java.net/~brutisso/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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