[Jamvm-general] /hg/release/icedtea6-1.10: JamVM: Base default min and max heap ...

[Robert Lougher]

Hi Xerxes,

On 24 March 2011 21:55, Xerxes Ranby <xerxes at zafena.se> wrote:
> On 2011-03-24 17:22, Xerxes Rånby wrote:
>>
>> Hi the IcedTea buildbot found a multiplication overflow bug in the new
>> default min max heap size on physical memory calculation code when running
>> JamVM on a system with a lot of RAM.
>>
>>
>> http://git.berlios.de/cgi-bin/cgit.cgi/jamvm/commit/?id=cc6997003dd65ba5a32df471b7c0547fba9769f2
>>
>> http://builder.classpath.org/icedtea/buildbot/builders/icedtea6-jaunty-ia32-quick-jamvm/builds/35
>>
>> JamVM fails to launch on the icedtea buildbot with 3.6G of RAM
>
> Attached patch fix.
>
> The multiplication of num_pages*pages_size caused a long sign flip overflow.
> Then later when the sign flipped long got divided by 4 and converted back to
> unsigned form caused the calculated min-heap argument to end up in the 4Gb
> range.
>

>From the openjdk mailing list I see you've seen the push of the fix.
I didn't use your patch because I was already working on a fix.  After
making the embarrassing mistake of not taking into account long
overflow, I did some research and discovered that with PAE, a 32-bit
machine can have more than 4GB of physical memory (though each process
can't see more than 4GB).  Therefore, an unsigned long can still
overflow, and we need to use long long.  As min_heap and max_heap are
both unsigned long, I also added some extra precautions to ensure that
the conversion from long long will not overflow (check both limits
against DEFAULT_MAX_HEAP, which on 32-bit machines must be less than
4GB), even though in practice this can't occur (phys_mem/64 will only
overflow when physical memory is >= 256GB, and PAE is "limited" to
64GB, but this protects against potential changes to the min_heap
ratio).

Rob.

> Cheers and have a great day!
> Xerxes
>
>
>
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