Request for sponsor - JDK-8031538

[Thomas Schatzl]

Hi again,

On Thu, 2014-01-23 at 10:28 +0100, Thomas Schatzl wrote:
> Hi,
> 
> On Wed, 2014-01-22 at 16:56 -0800, Staffan Friberg wrote:
[..]
> > [1] - https://bugs.openjdk.java.net/browse/JDK-8031538
> 
> I think there is a small problem after the change: when revising the
> young target list length concurrently, the code in lines 550/551 more or
> less make sure that the young gen will only ever increase because
> absolute_min_length is always young_list_length() + 1.
> 
> So this could mean that if this length is updated really frequently, we
> may exhaust the entire heap. E.g. recalculation of this length increases
> the amount of eden -> allocation can continue -> before OOM there is
> another recalculation -> goto start.
> 
>  542   // This is how many young regions we already have (survivors +
>        // eventual eden)
>  543   // The young list contain all survivors regions and any eden regions
> 
> Maybe add an explanation that during a GC pause the young list contains
> only survivor regions at this point.
> 
>  544   uint base_min_length = _g1->young_list()->length();
>  545   // This is the absolute minimum young length, which ensures that we
>  546   // can allocate one eden region in the worst-case.
>  547   uint absolute_min_length = base_min_length + 1;
> 
> I think the unconditional +1 is wrong. It should probably only add this single region if there are not yet eden regions added. I think you could get this information (whether the young list already contains an eden region) from YoungList, by subtracting length() with survivor_length(). If the result is > 0, there are eden regions in the list already. It's possibly best to add a method that returns that boolean value.

Another possible fix is

  uint absolute_min_length = _g1->recorded_survivor_regions() + 1; (or
_g1->young_list()->survivor_regions().

Did not think about what is better yet though (or whether they have
issues of their own). Both solutions would not require additional code.

Thomas