RFR: 8272138: ZGC: Adopt relaxed ordering for self-healing [v5]

[Per Liden]

On Tue, 10 Aug 2021 14:33:56 GMT, Xiaowei Lu <github.com+39413832+weixlu at openjdk.org> wrote:

>> ZGC utilizes self-healing in load barrier to fix references to old objects. Currently, this fixing (ZBarrier::self_heal) adopts memory_order_conservative to guarantee that (1) the slow path (relocate, mark, etc. where we get healed address) always happens before self healing, and (2) the other thread that accesses the same reference is able to access the content at the healed address.
>> Let us consider changing the bounded releasing CAS in forwarding table installation to unbounded. In this way, the release (OrderAccess::release()) serves as a membar to block any subsequent store operations (forwarding table installation and self healing). More specifically, we can use release(), relaxed_cas() in the forwarding table and relaxed_cas() in the self healing. In the scenario when a thread reads healed address from forwarding table, relaxed_cas() is also fine due to the load-acquire in forwarding table.
>> Since release is done only in forwarding table, the majority of self heals, which only remap the pointer, is no longer restricted by the memory order. Performance is visibly improved, demonstrated by benchmark corretto/heapothesys on AArch64. The optimized version decreases both average concurrent mark/relocation time by 15%-20% and 10%-15%, respectively。
>> 
>> baseline
>> [1000.412s][info   ][gc,stats    ]       Phase: Concurrent Relocate                           0.000 / 0.000       161.031 / 232.546     147.641 / 289.855     147.641 / 289.855     ms
>> [1100.412s][info   ][gc,stats    ]       Phase: Concurrent Relocate                         220.739 / 220.739     162.373 / 232.546     149.909 / 289.855     149.909 / 289.855     ms
>> [1200.411s][info   ][gc,stats    ]       Phase: Concurrent Relocate                           0.000 / 0.000       157.900 / 232.546     148.912 / 289.855     148.912 / 289.855     ms
>> [1000.412s][info   ][gc,stats    ]       Phase: Concurrent Mark                               0.000 / 0.000       816.505 / 1508.676    759.533 / 1567.506    759.533 / 1567.506    ms
>> [1100.412s][info   ][gc,stats    ]       Phase: Concurrent Mark                            1262.680 / 1262.680    838.750 / 1508.676    772.495 / 1567.506    772.495 / 1567.506    ms
>> [1200.411s][info   ][gc,stats    ]       Phase: Concurrent Mark                               0.000 / 0.000       813.220 / 1422.825    769.346 / 1567.506    769.346 / 1567.506    ms
>> optimized
>> [1000.447s][info   ][gc,stats    ]       Phase: Concurrent Relocate                         248.035 / 248.035     162.719 / 253.498     124.061 / 273.227     124.061 / 273.227     ms
>> [1100.447s][info   ][gc,stats    ]       Phase: Concurrent Relocate                         217.434 / 217.434     161.209 / 253.498     126.767 / 273.227     126.767 / 273.227     ms
>> [1200.447s][info   ][gc,stats    ]       Phase: Concurrent Relocate                           0.000 / 0.000       164.023 / 253.498     129.314 / 273.227     129.314 / 273.227     ms
>> [1000.447s][info   ][gc,stats    ]       Phase: Concurrent Mark                            1224.059 / 1224.059    809.838 / 1518.214    582.867 / 1518.214    582.867 / 1518.214    ms
>> [1100.447s][info   ][gc,stats    ]       Phase: Concurrent Mark                            1302.235 / 1302.235    793.589 / 1518.214    600.215 / 1518.214    600.215 / 1518.214    ms
>> [1200.447s][info   ][gc,stats    ]       Phase: Concurrent Mark                               0.000 / 0.000       821.320 / 1518.214    615.187 / 1518.214    615.187 / 1518.214    ms
>
> Xiaowei Lu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   replace release() to a single place in insert()

Looks good.

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Marked as reviewed by pliden (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/5046