RFR (7xS): 8071280, 8162928, 8177707, 8177044, 8178148, 8175554, 8178151: Remembered set update/scan improvements

[Thomas Schatzl]

Hi all,

  recent reviews have made changes necessary to parts of the changeset
chain.

Here is a list of links to updated webrevs. Since they have apparently
not been reviewed yet, I simply overwrote the old webrevs.

JDK-8177044: Remove _scan_top from HeapRegion
http://cr.openjdk.java.net/~tschatzl/8177044/webrev/

JDK-8178148: Log more detailed information about scan rs phase
http://cr.openjdk.java.net/~tschatzl/8178148/webrev/

JDK-8175554: Improve G1UpdateRSOrPushRefClosure
http://cr.openjdk.java.net/~tschatzl/8175554/webrev/

JDK-8178151: Clean up G1RemSet files
http://cr.openjdk.java.net/~tschatzl/8178151/webrev/

Please consider changing the subject of the email if you are reviewing
one specific webrev, as this seems helpful to quickly find the context
but keep the reviews together in the email thread.

Thanks,
  Thomas

On Tue, 2017-04-11 at 13:30 +0200, Thomas Schatzl wrote:
> Hi all,
> 
>   I would like you to ask for reviews of a set of related changes
> that optimize the way G1 updates/scans/refines cards of the
> remembered set.
> 
> As these CRs are highly dependent on each other I would like to have
> them reviewed and pushed together. Another reason is that all of them
> are self-contained, which means that they take one or the other
> detour I want to avoid questions about.
> 
> The basic premise is that refinement and Scan RS (and Update RS as it
> is mostly the same) phases are quite slow with G1. There are a lot of
> low-hanging fruits to be picked here, ranging from micro-
> optimizations, fixing "wrong" code sharing, re-using (slow) CMS code,
> doing stuff in
> non-straightforward ways etc.
> 
> These changes try to make it better (tm) ;)
> 
> Results are good, around 10%+ faster scan rs time/card, 10%+ faster
> update rs/card, refinement also faster (although it is too hard to
> measure), 8 LOC less (there is more to delete, but that one needs
> more care. Also these changes add some logging :))
> 
> Here we go:
> 
> 8071280: Specialize HeapRegion::oops_on_card_seq_iterate_careful()
> for
> use during concurrent refinement and updating the rset
> 
> CR: https://bugs.openjdk.java.net/browse/JDK-8071280
> Webrev: http://cr.openjdk.java.net/~tschatzl/8071280/webrev
> 
> As the title indicates, this change allows specializations of
> HeapRegion::oops_on_card_seq_iterate_careful() according to current
> GC
> phase (concurrent or during gc) to allow to remove some checks.
> This is mostly a preparatory change for the next ones.
> 
> 8162928: Micro-optimizations in scanning the remembered sets
> 
> CR: https://bugs.openjdk.java.net/browse/JDK-8162928
> Webrev: http://cr.openjdk.java.net/~tschatzl/8162928/webrev/
> 
> This change removes the use of HeapRegionDCTOC during scanning
> remembered sets, and uses the faster
> HeapRegion::oops_on_card_seq_iterate_careful() (measured).
> HeapRegionDCTOC also depends CMS code which is just bad.
> 
> 8177707: Specialize G1RemSet::refine_card for concurrent/during
> safepoint refinement
> 
> CR: https://bugs.openjdk.java.net/browse/JDK-8177707
> Webrev: http://cr.openjdk.java.net/~tschatzl/8177707/webrev/
> 
> This change temporarily duplicates the code in G1RemSet::refine_card
> for during refinement/gc to allow (initial) specialization of these
> methods for their purpose.
> 
> 8177044: Remove _scan_top from HeapRegion
> 
> CR: https://bugs.openjdk.java.net/browse/JDK-8177044
> Webrev: http://cr.openjdk.java.net/~tschatzl/8177044/
> 
> Removes _scan_top and all its awkward handling by explicitly creating
> a
> snapshot of the top values of the heap regions. This allows
> significant
> specialization of the refine_during_gc() method by subsuming all the
> various checks at the beginning into a single one.
> 
> This one is responsible for a large part of the improvement of the
> update rs phase.
> 
> 8178148: Log more detailed information about scan rs phase
> 
> CR: https://bugs.openjdk.java.net/browse/JDK-8178148
> Webrev: http://cr.openjdk.java.net/~tschatzl/8178148/webrev/
> 
> Adds some more statistics gathering and logging about what the scan
> rs
> phase is actually doing, allowing better performance analysis. This
> information will also be used further in the future.
> 
> 8175554: Improve G1UpdateRSOrPushRefClosure
> 
> CR: https://bugs.openjdk.java.net/browse/JDK-8175554
> Webrev: http://cr.openjdk.java.net/~tschatzl/8175554/webrev/
> 
> (This mostly deletes code, so it's still "S" ;))
> 
> Finally remove the G1ParPushHeapRSClosure and merge it with
> G1UpdateOrScanRSClosure. You may now notice that
> G1UpdateOrScanRSClosure is pretty similar to G1ParScanClosure - that
> is
> another issue.
> Also make sure that G1UpdateOrScanRSClosure properly does the
> humongous
> and "ext" region handling (which is benign so far).
> 
> 8178151: Clean up G1RemSet files
> 
> CR: https://bugs.openjdk.java.net/browse/JDK-8178151
> Webrev: http://cr.openjdk.java.net/~tschatzl/8178151/webrev/
> 
> Clean up after all these changes a little in the G1Remset* files,
> removing obsolete stuff, some renamings, etc. 
> 
> Thanks,
>   Thomas
>