On Wed, 23 Sep 2020 07:39:55 GMT, Stefan Karlsson <stef...@openjdk.org> wrote:
>> This PR the implementation of "JEP 376: ZGC: Concurrent Thread-Stack >> Processing" (cf. >> https://openjdk.java.net/jeps/376). >> Basically, this patch modifies the epilog safepoint when returning from a >> frame (supporting interpreter frames, c1, c2, >> and native wrapper frames), to compare the stack pointer against a >> thread-local value. This turns return polls into >> more of a swiss army knife that can be used to poll for safepoints, >> handshakes, but also returns into not yet safe to >> expose frames, denoted by a "stack watermark". ZGC will leave frames (and >> other thread oops) in a state of a mess in >> the GC checkpoint safepoints, rather than processing all threads and their >> stacks. Processing is initialized >> automagically when threads wake up for a safepoint, or get poked by a >> handshake or safepoint. Said initialization >> processes a few (3) frames and other thread oops. The rest - the bulk of the >> frame processing, is deferred until it is >> actually needed. It is needed when a frame is exposed to either 1) execution >> (returns or unwinding due to exception >> handling), or 2) stack walker APIs. A hook is then run to go and finish the >> lazy processing of frames. Mutator and GC >> threads can compete for processing. The processing is therefore performed >> under a per-thread lock. Note that disarming >> of the poll word (that the returns are comparing against) is only performed >> by the thread itself. So sliding the >> watermark up will require one runtime call for a thread to note that nothing >> needs to be done, and then update the poll >> word accordingly. Downgrading the poll word concurrently by other threads >> was simply not worth the complexity it >> brought (and is only possible on TSO machines). So left that one out. > > src/hotspot/share/gc/z/zCollectedHeap.cpp line 235: > >> 233: return true; >> 234: } >> 235: > > Weird placement between store barrier functions. But even weirder that we > still have those functions. I'll remove them > with JDK-8253516. Perfect, thanks. > src/hotspot/share/gc/z/zDriver.cpp line 108: > >> 106: return false; >> 107: } >> 108: > > Group needs_inactive_gc_locker, skip_thread_oop_barriers, and > allow_coalesced_vm_operations together? > > Add a comment about why we chose to skip coalescing here. Per explicitly wanted skip_thread_oop_barriers grouped with needs_inactive_gc_locker. But I should remove allow_coalesced_vm_operations; it is no longer used since I have removed VM operation coalescing completely already. ------------- PR: https://git.openjdk.java.net/jdk/pull/296
