RFR: 8257774: G1: Trigger collect when free region count drops below threshold to prevent evacuation failures [v4]
Aditya Mandaleeka
adityam at openjdk.java.net
Thu Apr 22 21:40:39 UTC 2021
> _This PR picks up from [this previous PR](https://github.com/openjdk/jdk/pull/1650) by @charliegracie._
>
> I won't repeat the full description from the prior PR, but the general idea is to add the notion of a "proactive GC" to G1 which gets triggered in the slow allocation path if the number of free regions drops below the amount that would be required to complete a GC if it happened at that moment. The threshold is based on the survival rates from eden and survivor spaces along with the space required for tenured space evacuations.
>
> There are a couple of outstanding issues/questions known:
> - _Interaction with GCLocker_: In the case where we determine that a proactive GC is required and GC locker is active, we don't allow the young gen to expand (instead threads will stall). @tschatzl raised this and suggested that it should be discussed as part of the review.
> - _Disable proactive GC heuristic during initialization_: I ran into an issue in testing where code in the universe initialization codepath was tripping the proactive GC heuristic, leading to a GC being triggered before the VM has finished initialization. I need to find a good way to prevent this from happening. There may already be a mechanism for this, but I couldn't find one so I added a temporary placeholder (`zzinit_complete`) just to unblock testing.
Aditya Mandaleeka has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains eight commits:
- Remove temp init variable, update code after rebase.
- Refactor allocation funcs, removing attempt_lock_free_first.
- Fix format specifiers.
- Fix a few warnings as well as a Windows build error.
- Fix whitespace and comment.
- Further refactoring/changes.
- Address feedback and make other changes.
- Improve G1GC accounting for humongous objects
If there is a burst of humongous allocations they can
consume all of the free regions left in the heap. If
there are no free regions for to-space a collect will
fail to evacuate all objects. This makes the GC very
slow. Since, humongous regions can be collected in
G1GC young collects force a young collect before the
number of free regions becomes less than the number
the GC is likely to require.
Fix TestGCLogMessages.java so it still causes Evacuation
failures.
Signed-off-by: Charlie Gracie <charlie.gracie at microsoft.com>
-------------
Changes: https://git.openjdk.java.net/jdk/pull/3143/files
Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3143&range=03
Stats: 207 lines in 11 files changed: 167 ins; 13 del; 27 mod
Patch: https://git.openjdk.java.net/jdk/pull/3143.diff
Fetch: git fetch https://git.openjdk.java.net/jdk pull/3143/head:pull/3143
PR: https://git.openjdk.java.net/jdk/pull/3143
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