RFR (M): 8077144: Concurrent mark initialization takes too long (original) (raw)

Mikael Gerdin mikael.gerdin at oracle.com
Mon Mar 14 15:37:35 UTC 2016

Hi,

I had an IM discussion with Thomas around some issues with the design of the aggregation and verification code.

G1LiveDataClosureBase should become a utlility class instead of a base class. It's weird that the G1VerifyLiveDataHRClosure embeds another HRClosure and calls doHeapRegion on it, it would be better if the mark_marked_during_marking and mark_allocated_since_marking methods could be called right away.

Mark_marked_during_marking should return the marked bytes instead and let the caller take care of mutating the heap region and doing the yield check. There is currently a bug where the verification code calls add_to_marked_bytes on the HeapRegion. There is also an issue with how add_to_marked_bytes is called on HumongousContinues regions since marked_bytes is aggregated in each iteration and then the aggregate is added to the current hr.

It might be a good idea to hold off on reviews until the updated webrev is out.

/Mikael

On 2016-03-14 14:15, Thomas Schatzl wrote:

Hi all,

could I have reviews for this from-scratch solution for the problem that G1 startup takes too long? Current G1 uses per-mark thread liveness mark bitmaps that span the entire heap to be ultimately able to create information about areas in the heap where there are any live objects on a card basis. This information is needed for scrubbing remembered sets later. Basically, in addition to updating the previous bitmap required for SATB, the marking threads also, for every live object, mark all bits corresponding to the area the object covers on a per thread basis on these per-thread liveness mark bitmaps. During the remark pause, this information is aggregated into (two) global bitmaps ("Liveness Count Data"), then in the cleanup pause augmented with some more liveness information, and then used for scrubbing the remembered sets. The main problems with that solution: - the per-mark thread data structures take up a lot of space. E.g. with 64 mark threads, this data structure has the same size of the Java heap. Now, when you need to use 60 mark threads, the heap is big. And at those heap sizes, needing that much more memory hurts a lot. - management of these additional data structures is costly, it takes a long time to initialize, and regularly clear them. The increased startup time has actually been the cause for this issue. - it takes a significant amount of time to aggregate this data in the remark pause. - it slows down marking, the combined bitmap update (the prev bitmap and these per-thread bitmaps) is slower than doing these phases seperately. This proposed solution removes the per-thread additional mark bitmaps, and recreates this information from the (complete) prev bitmap in an extra concurrent phase after the Remark pause. This can be done since the Prev bitmap does not change after Remark any more. In total, this separation of the tasks is faster (lowers concurrent cycle time) than doing this work at once for the following reasons: - I did not observe any throughput regresssions with this change: actually, throughput of some large applications even increases with that change (not taking into account that you could increase heap size now since not so much is taken up by these additional bitmaps). - the concurrent phase to prepare for the next marking is much shorter now, since we do not need to clear lots of memory any more. - the remark pause can be much faster (I have measurements of a decrease in the order of a magnitude on large applications, where this aggregation phase dominates the remark pause). - startup and footprint naturally decreases significantly, particularly on large systems. As a nice side-effect, the change in effect removes a significant amount of LOC. There is a follow-up change to move (and later clean up) the still remaining data structures required for scrubbing into extra classes, since they will be used more cleverly in the future (JDK-8151386). There will be another follow-up change without CR yet to fix the use of an excessive amount of parallel gc threads for clearing the liveness count data. The change is based on JDK-8151614, JDK-8151126 (I do not think it conflicts with that actually), and JDK-8151534 (array allocator refactoring). CR: https://bugs.openjdk.java.net/browse/JDK-8077144 Webrev: http://cr.openjdk.java.net/~tschatzl/8077144/webrev.2/ Testing: jprt, vm.gc, kitchensink, some perf benchmarks Thanks, Thomas

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