RFR: 8240556: Abort concurrent mark after effective eager reclamation of humongous objects

[Albert Mingkun Yang]

On Tue, 15 Sep 2020 12:40:46 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   please review this change that implements concurrent mark abort as proposed by Liang Mao from Alibaba.
> 
> Basically, if at the end of the concurrent start pause we notice that we went below the IHOP threshold (and it has been
> a concurrent start pause caused by humongous allocation), instead of scheduling a mark operation, we schedule a
> "concurrent undo" operation that undoes the changes.  Regarding the removal of the
> test/hotspot/jtreg/gc/stress/jfr/TestStressBigAllocationGCEventsWithG1.java test: it only ever accidentally worked in
> G1. G1 never sent the AllocationRequiringGC events for GCs caused by going over the IHOP threshold for humongous
> allocations that the test actually expects.  That test previously only worked because G1 could not reclaim the
> humongous objects fast enough so crossing the IHOP threshold causes a full concurrent mark. Allocations during that
> concurrent mark do not cause a GC that can reclaim these objects, so ultimately some young GC that sends the
> AllocationRequiringGC event will be sent.  With concurrent undo this is not guaranteed any more, i.e. only in
> environments where concurrent undo is slow (and we'll improve it soon) this test works.  The test is too timing
> dependent, and checking for the wrong thing in this case, so removing it.  Testing: tbd.

src/hotspot/share/gc/g1/g1ConcurrentMarkThread.hpp line 112:

> 110:   bool idle() const;
> 111:   bool started() const;
> 112:   bool in_progress() const;

What's the motivation of moving them out of the header? Since they are quite small, placing them in the header makes
inlining possible and desirable.

src/hotspot/share/gc/g1/g1ConcurrentMarkThread.hpp line 43:

> 41:   G1ConcurrentMark* _cm;
> 42:
> 43:   enum ServiceState : uint {

Why `: uint`? The same question for `enum Command` below as well.

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PR: https://git.openjdk.java.net/jdk/pull/177