RFR (M): 8201492: Properly implement non-contiguous generations for reference discovery

Thomas Schatzl thomas.schatzl at oracle.com
Thu Apr 26 14:12:54 UTC 2018

Hi all,

  could someone please have a look at this?


On Wed, 2018-04-18 at 08:52 +0200, Thomas Schatzl wrote:
> Hi all,
>   can I have reviews for this change that adapts the reference
> discovery for the non-contiguous generations (and non-contiguous
> generations in general) of G1 to solve one day-one performance issue?
> So reference discovery does not properly support non-contiguous
> generations, resorting to an approximation of it. 
> This in turn causes G1 to require the "preserve cm referents" phase
> during GC during marking, which is very costly in some cases. 
> The reason for the preserve cm referents phase is that References
> (j.l.ref.References instances) that are discovered by concurrent
> discovery, will currently prevent discovery and evacuation of
> referents
> in the STW pauses, as G1 thinks that it already has discovered that
> Reference and skips it. Still their referents can be in young gen,
> while the Reference is in old gen (young gc may iterate over it
> during
> card scanning), and this may cause crashes later. 
> The preserve cm referents phase brute-force simply evacuates any
> "leftover" referents and its followers. 
> This is because the STW reference discovery currently does not treat
> References in old gen as roots (i.e. to be scanned and referents
> evacuated always), as the STW reference processor thinks that the g1
> heap is a single generation spanning the whole heap. 
> By giving the ref processor the correct idea of generations in G1,
> this
> automatically works, and obsoletes the "preserve cm referents" phase.
> To get an understanding how serious this issue may be, on
> theKitchensink reference stress test program, the "Preserve CM
> referents"
> phase may take 105ms out of 115ms.
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8201492
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8201492/webrev/
> Testing:
> hs-tier1-3, performance verification, lots of Kitchensink reference
> stress testing runs
> Thanks,
>   Thomas

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