On Mon, 8 Jul 2024 12:13:07 GMT, Axel Boldt-Christmas <abold...@openjdk.org> wrote:
>> When inflating a monitor the `ObjectMonitor*` is written directly over the >> `markWord` and any overwritten data is displaced into a displaced >> `markWord`. This is problematic for concurrent GCs which needs extra care or >> looser semantics to use this displaced data. In Lilliput this data also >> contains the klass forcing this to be something that the GC has to take into >> account everywhere. >> >> This patch introduces an alternative solution where locking only uses the >> lock bits of the `markWord` and inflation does not override and displace the >> `markWord`. This is done by keeping associations between objects and >> `ObjectMonitor*` in an external hash table. Different caching techniques are >> used to speedup lookups from compiled code. >> >> A diagnostic VM option is introduced called `UseObjectMonitorTable`. It is >> only supported in combination with the LM_LIGHTWEIGHT locking mode (the >> default). >> >> This patch has been evaluated to be performance neutral when >> `UseObjectMonitorTable` is turned off (the default). >> >> Below is a more detailed explanation of this change and how `LM_LIGHTWEIGHT` >> and `UseObjectMonitorTable` works. >> >> # Cleanups >> >> Cleaned up displaced header usage for: >> * BasicLock >> * Contains some Zero changes >> * Renames one exported JVMCI field >> * ObjectMonitor >> * Updates comments and tests consistencies >> >> # Refactoring >> >> `ObjectMonitor::enter` has been refactored an a >> `ObjectMonitorContentionMark` witness object has been introduced to the >> signatures. Which signals that the contentions reference counter is being >> held. More details are given below in the section about deflation. >> >> The initial purpose of this was to allow `UseObjectMonitorTable` to interact >> more seamlessly with the `ObjectMonitor::enter` code. >> >> _There is even more `ObjectMonitor` refactoring which can be done here to >> create a more understandable and enforceable API. There are a handful of >> invariants / assumptions which are not always explicitly asserted which >> could be trivially abstracted and verified by the type system by using >> similar witness objects._ >> >> # LightweightSynchronizer >> >> Working on adapting and incorporating the following section as a comment in >> the source code >> >> ## Fast Locking >> >> CAS on locking bits in markWord. >> 0b00 (Fast Locked) <--> 0b01 (Unlocked) >> >> When locking and 0b00 (Fast Locked) is observed, it may be beneficial to >> avoid inflating by spinning a bit. >> >> If 0b10 (Inflated) is observed or there is to... > > Axel Boldt-Christmas has updated the pull request incrementally with one > additional commit since the last revision: > > More graceful JVMCI VM option interaction Could you please revert 2814350 and export the following symbols to JVMCI? diff --git a/src/hotspot/share/jvmci/vmStructs_jvmci.cpp b/src/hotspot/share/jvmci/vmStructs_jvmci.cpp index faf2cb24616..7be31aa0f5f 100644 --- a/src/hotspot/share/jvmci/vmStructs_jvmci.cpp +++ b/src/hotspot/share/jvmci/vmStructs_jvmci.cpp @@ -241,6 +241,7 @@ nonstatic_field(JavaThread, _stack_overflow_state._reserved_stack_activation, address) \ nonstatic_field(JavaThread, _held_monitor_count, intx) \ nonstatic_field(JavaThread, _lock_stack, LockStack) \ + nonstatic_field(JavaThread, _om_cache, OMCache) \ JVMTI_ONLY(nonstatic_field(JavaThread, _is_in_VTMS_transition, bool)) \ JVMTI_ONLY(nonstatic_field(JavaThread, _is_in_tmp_VTMS_transition, bool)) \ JVMTI_ONLY(nonstatic_field(JavaThread, _is_disable_suspend, bool)) \ @@ -531,6 +532,8 @@ \ declare_constant_with_value("CardTable::dirty_card", CardTable::dirty_card_val()) \ declare_constant_with_value("LockStack::_end_offset", LockStack::end_offset()) \ + declare_constant_with_value("OMCache::oop_to_oop_difference", OMCache::oop_to_oop_difference()) \ + declare_constant_with_value("OMCache::oop_to_monitor_difference", OMCache::oop_to_monitor_difference()) \ \ declare_constant(CodeInstaller::VERIFIED_ENTRY) \ declare_constant(CodeInstaller::UNVERIFIED_ENTRY) \ ------------- PR Comment: https://git.openjdk.org/jdk/pull/20067#issuecomment-2214322632