CMS: wrong max_eden_size for check_gc_overhead_limit

[Jon Masamitsu]

Ivan,

You're correct about this bug.

Is it correct that you're covered by the SAP contributor
agreement?

Have you filed a bug report on this?

Have you checked the other GC's for this problem?

I can sponsor this fix.

Thanks.

Jon



On 9/8/2015 2:21 AM, Galkin, Ivan wrote:
>
> Hello all,
>
> I believe the calculation of max_eden_size which is needed to check 
> the GC overhead in CMS is incorrect. Namely in 
> concurrentMarkSweepGeneration.cpp the following expression is used:
>
> size_t max_eden_size = _young_gen->max_capacity() - 
> _young_gen->to()->capacity() - _young_gen->from()->capacity();
>
> According to the implementation of DefNewGeneration::max_capacity() 
> the expression can be unfolded as:
>
>                 size_t max_eden_size = (“reserved size of young gen” – 
> “size of survivor space”) – “size of survivor space” – “size of 
> survivor space”;
>
> So the value becomes too small (survival spaces are accounted 3 
> times!), which can lead to the following problems:
>
> 1.max_eden_size is too small and GC overhead is wrongfully recognized 
> too early
>
> 2.max_eden_size == 0 (all young spaces have the same size; 
> -XX:SurvivorRatio=1) and GC overhead is never happens (see 
> implementation of AdaptiveSizePolicy::check_gc_overhead_limit:
>
> a.max_eden_size == 0 leads to mem_free_eden_limit == 0;
>
> b.free_in_eden < mem_free_eden_limit is always false, since both are 
> unsigned integers and mem_free_eden_limit is 0)
>
> I would therefore suggest the following fix (DefNewGeneration:: 
> max_eden_size() already contains the correctly calculated capacity of 
> eden):
>
> diff -r f74b3ce62e1f src/share/vm/gc/cms/concurrentMarkSweepGeneration.cpp
>
> --- a/src/share/vm/gc/cms/concurrentMarkSweepGeneration.cpp Fri Sep 04 
> 17:33:56 2015 -0700
>
> +++ b/src/share/vm/gc/cms/concurrentMarkSweepGeneration.cpp Mon Sep 07 
> 18:08:39 2015 +0200
>
> @@ -1563,9 +1563,7 @@
>
>    do_compaction_work(clear_all_soft_refs);
>
>    // Has the GC time limit been exceeded?
>
> -  size_t max_eden_size = _young_gen->max_capacity() -
>
> - _young_gen->to()->capacity() -
>
> - _young_gen->from()->capacity();
>
> +  size_t max_eden_size = _young_gen->max_eden_size();
>
>    GCCause::Cause gc_cause = gch->gc_cause();
>
> size_policy()->check_gc_overhead_limit(_young_gen->used(),
>
>        _young_gen->eden()->used(),
>
> Could anybody please review and sponsor the change?
>
> Thank you in advance,
>
> Ivan
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/hotspot-gc-dev/attachments/20150909/d0171699/attachment.htm>