RFR (S): 8140597: Forcing an initial mark causes G1 to abort mixed collections

Thomas Schatzl thomas.schatzl at oracle.com
Thu Nov 5 09:06:45 UTC 2015


Hi all,

  here is the request for review for the updated and enhanced changes to
avoid that forcing an initial mark (e.g. by humongous allocation, gc
locker, metadata threshold reached, and other external requests) causes
G1 to abort any current reclamation activity.

Particularly in combination with adaptive IHOP (JDK-8136678) this will
cause guaranteed full gcs if these requests are timed just right.
Actually there is the possibility that G1 will never reclaim any space
if there is e.g. a lot of gclocker activity (or lots of humongous
objects).

Anecdotally, I remember some case some time ago on hotspot-gc-use where
we could not explain why G1 never did mixed gcs.

This change modifies the way G1 treats such initial mark requests: it
delays them until the mixed gc phase finishes - with JDK-8138740 G1 can
then even start the initial mark right after the mixed gc (as opposed to
need an intermediate young-only GC inbetween).

I think this change in behavior is justified due to the assumption that
when we already did the liveness analysis/marking we should definitely
use this information.

To hopefully speed up reviews some run-down of the changes:

G1CollectorPolicy::about_to_start_mixed_phase(): new method that defines
the marking cycle to span both the marking and the last young-only gc.
This prevents initial mark requests to skip the following mixed gc phase
if they occur between when marking actually finished, and the last
young-only gc.

This method is used in need_to_start_conc_mark() that schedules a
regular initial mark (since we already running marking, no need to do it
again), and decide_on_conc_mark_initiation(), which actually switches to
the initial mark.

G1CollectorPolicy::decide_on_conc_mark_initiation()'s purpose is to
switch to initial mark. Previously, it allowed that even during mixed gc
(and during the last young-only gc), simply aborting the mixed gcs.
Instead, G1 now disallows switching to initial-mark from initial-mark to
the last mixed gc. This obviates the need for the code that aborts mixed
gcs.

All other changes are comments, asserts and a minor cleanup in
whitebox.cpp.

CR:
https://bugs.openjdk.java.net/browse/JDK-8140597
Webrev:
http://cr.openjdk.java.net/~tschatzl/8140597/webrev
Testing:
jprt, manual verification of correct state transitions, vm.gc testlist

Thanks,
  Thomas




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