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

[Bengt Rutisson]

Hi Ramki,

Is this what you were considering?
http://cr.openjdk.java.net/~brutisso/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
>>
>>
>
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/hotspot-gc-dev/attachments/20111117/3f4e922f/attachment.htm>