RFR: 8252752: Clear card table for old regions during scan in G1 [v2]

[Kim Barrett]

On Fri, 25 Sep 2020 13:59:15 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Hi all,
>> 
>>   can I get reviews for this change that removes the need for explicit clearing of the card table for typically a large
>>   amount of regions in most cases?
>> 
>> Currently g1 unconditionally marks scanned dirty cards as "scanned" to remember that they had already been scanned in
>> earlier evacuation passes. Then in the "clear card table phase" g1 clears all these marks including the card table of
>> evacuated regions.  This change modifies g1 so that if possible, it clears dirty cards after scanning them instead of
>> setting them to "scanned" if there is only one evacuation pass. This saves a lot of work later (e.g. the amount of
>> regions to clear in the clear card table phase may be reduced by a magnitude or more, depending on the ration between
>> evacuated and scanned regions).  The main change here is which type of regions (ones in the collection set, ones that
>> are only scanned) are put into which list of regions g1 already manages: there is one "all" list containing all regions
>> that need clearing, and a "next" list containing the regions to be scanned in the current evacuation pass. Previously
>> g1 put regions into at least one list; now regions in the current collection set are always directly put into the "all"
>> list (as they independently of other reasons require card table cleaning), while to-be-scanned regions are always only
>> put into the "next" list.  Then, to achieve the desired effect that only regions that absolutely require clearing are
>> in the "all" list, g1 does not merge the "next" list into the "all" list when it detects that there will only be one
>> evacuation pass.  Performance impact: Decreases clear card table time significantly if the number of regions in the
>> collection set is much smaller than the number of scanned regions (as it decreases the number of regions to clear).
>> Some cursory perf testing showed slight overall pause time improvements.  Testing: hs-tier1-5 ("Initial import" change)
>> hs-tier1-2 (latest)
>
> Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Comments by ayang, kbarrett, iwalulya

Marked as reviewed by kbarrett (Reviewer).

src/hotspot/share/gc/g1/g1CollectedHeap.hpp line 786:

> 784:   // Actually do the work of evacuating the parts of the collection set.
> 785:   // The has_optional_evacuation_work flag for the initial collection set
> 786:   // evacuation indicates whether some or more optional evacuation steps may

s/some/one/

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PR: https://git.openjdk.java.net/jdk/pull/343