RFR(S): 8222518: Remove unnecessary caching of Parker object in java.lang.Thread
David Holmes
david.holmes at oracle.com
Fri Apr 26 05:32:17 UTC 2019
I pushed this today based on Dan and Robbin's reviews, but realized just
after the act that I should have waited for any feedback from core-libs
- apologies about that. If there are concerns I will roll it back.
Thanks,
David
-----
On 26/04/2019 2:57 pm, David Holmes wrote:
> Thanks Dan! Extraneous ; culled.
>
> David
>
> On 25/04/2019 1:16 am, Daniel D. Daugherty wrote:
>> On 4/24/19 3:12 AM, David Holmes wrote:
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8222518
>>> webrev: http://cr.openjdk.java.net/~dholmes/8222518/webrev/
>>
>> src/hotspot/share/classfile/javaClasses.cpp
>> L1629: macro(_park_blocker_offset, k, "parkBlocker",
>> object_signature, false);
>> Line ends with a ';' and the previous last line did not. When
>> the
>> THREAD_FIELDS_DO macro is called, it is already followed by a
>> ';':
>>
>> L1635: THREAD_FIELDS_DO(FIELD_COMPUTE_OFFSET);
>> L1640: THREAD_FIELDS_DO(FIELD_SERIALIZE_OFFSET);
>>
>> src/hotspot/share/classfile/javaClasses.hpp
>> No comments.
>>
>> src/hotspot/share/prims/unsafe.cpp
>> No comments.
>>
>> src/java.base/share/classes/java/lang/Thread.java
>> No comments.
>>
>> Thumbs up. I don't need to see another webrev if you choose to remove
>> the ';' on L1629.
>>
>> Dan
>>
>>>
>>> The original implementation of Unsafe.unpark simply extracted the
>>> JavaThread reference from the java.lang.Thread oop and if non-null
>>> extracted the Parker instance from it and invoked unpark. This was
>>> racy however as the native JavaThread could terminate at any time and
>>> deallocate the Parker.
>>>
>>> That logic was fixed by JDK-6271298 which used of combination of
>>> type-stable-memory "event" objects for the Parker, along with use of
>>> the Threads_lock to obtain the initial reference to the Parker (from
>>> a JavaThread guaranteed to be alive), together with caching the
>>> native Parker pointer in a field of java.lang.Thread. Even though the
>>> native thread may have terminated the Parker was still valid (even if
>>> associated with a different thread) and the unpark at worst was a
>>> spurious wakeup for that other thread.
>>>
>>> When JDK-8167108 introduced Thread Safe-Memory-Reclaimation (SMR) the
>>> logic was updated to always use the safe mechanism - we grab a
>>> ThreadsListHandle then check the cached field, else lookup the native
>>> thread to see if it is alive and locate the Parker instance that way.
>>> With SMR the caching of the Parker pointer no longer serves any
>>> purpose - we no longer have a lock-free use-the-cache path versus a
>>> lock-using populate-the-cache path. With SMR we've already"paid" for
>>> the ability to ensure the native thread can't terminate regardless of
>>> whether we lookup the field from the java.lang.Thread or the
>>> JavaThread. So we can simplify the code and save a little footprint
>>> by removing the cache from java.lang.Thread:
>>>
>>> /*
>>> * JVM-private state that persists after native thread termination.
>>> */
>>> private long nativeParkEventPointer;
>>>
>>> and the supporting code from unsafe.cpp and javaClass.*pp in the JVM.
>>>
>>> I considered restoring the fast-path use of the cache without
>>> recourse to Thread-SMR but performance measurements failed to show
>>> any benefit in doing. See bug report for details.
>>>
>>> Thanks,
>>> David
>>
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