RFR(L): 8235795: replace monitor list mux{Acquire,Release}(&gListLock) with spin locks

Daniel D. Daugherty daniel.daugherty at oracle.com
Thu Jan 30 02:08:34 UTC 2020


Hi David,

Thanks for re-reviewing!


On 1/29/20 8:58 PM, David Holmes wrote:
> Hi Dan,
>
> On 28/01/2020 6:43 am, Daniel D. Daugherty wrote:
>> Greetings,
>>
>> I'm looking for a second reviewer on this thread. I've gone ahead and
>> made changes based on David H's comments on CR0.
>>
>>      JDK-8235795 replace monitor list 
>> mux{Acquire,Release}(&gListLock) with spin locks
>>      https://bugs.openjdk.java.net/browse/JDK-8235795
>>
>> Copyright years will be updated when the patches are rebased to JDK15.
>>
>> Here's the incremental webrev URL:
>>
>> http://cr.openjdk.java.net/~dcubed/8235795-webrev/1-for-jdk15.inc/
>>
>> Here's the full webrev URL:
>>
>> http://cr.openjdk.java.net/~dcubed/8235795-webrev/1-for-jdk15.full/
>>
>> Here's what changed between CR0 and CR1:
>>
>>    - refactor common code
>>    - refactor atomic load of LVars.population in 
>> monitors_used_above_threshold
>>    - simplify list walking in ObjectSynchronizer::om_release() so we 
>> lock fewer ObjectMonitors
>
> I'm a little confused by this. Here are the comments:
>
> 1402     // This list walk can only race with another list walker since
> 1403     // deflation can only happen at a safepoint so we don't have to
> 1404     // worry about an ObjectMonitor being removed from this list
> 1405     // while we are walking it.
> 1406
> 1407     // Lock the list head to avoid racing with another list walker.
>
> If we are at a safepoint how can we be racing with another list 
> walker? It would have to be a non-JavaThread, but then why would it be 
> walking the monitor lists?

om_release() is called by a JavaThread when we aren't at a safepoint.

If we have added a debugging audit_and_print_stats() call that is
also executed at a non-safepoint, then we could have a race. Or if
we have an exit_globals() call that doesn't happen at a safepoint
like the one we discovered with the ExitOnFullCodeCache option...

Just covering the possibilities...


>
>>    - remove unnecessary locking from 
>> ObjectSynchronizer::deflate_monitor_list()
>>    - add NoSafepointVerifier helpers to main list management functions
>>    - remove unnecessary storestore()
>>    - remove unnecessary comments
>>    - clarify/fix comments.
>
> All the actual changes seem fine.

Thanks again for the re-review!!

Dan


>
> Thanks,
> David
> -----
>
>> These changes have been tested in a Mach5 Tier[1-3] run with no
>> regressions. They have also been merged with 8235931 and 8236035 and
>> included in a Mach5 Tier[1-8] run with no known regressions (so far
>> since Tier8 is not quite finished).
>>
>> I did a SPECjbb2015 run on these bits with a jdk-14+32 baseline and 
>> 25 runs:
>>
>>      criticalJOPS                 0.25%  (Non-significant)
>>                     66754.32   66923.08
>>                    ± 1209.80  ± 1585.09
>>                               p = 0.674
>>
>>      maxJOPS                     -1.12%  (Non-significant)
>>                     90965.80   89948.80
>>                    ± 1788.39  ± 1989.22
>>                               p = 0.063
>>
>> I did a SPECjbb2015 run on the merge of 8235931, 8236035, and 8235795
>> with a jdk-14+32 baseline and 25 runs:
>>
>>      criticalJOPS                 0.37%  (Non-significant)
>>                     66754.32   67003.92
>>                    ± 1209.80  ± 1662.01
>>                               p = 0.547
>>
>>      maxJOPS                     -0.23%  (Non-significant)
>>                     90965.80   90754.00
>>                    ± 1788.39  ± 1851.64
>>                               p = 0.683
>>
>> All of these results were flagged as "Non-significant" by the perf
>> testing system. Looks like "p" values are still too high.
>>
>> Thanks, in advance, for comments, questions or suggestions.
>>
>> Dan
>>
>>
>> On 12/23/19 4:57 PM, Daniel D. Daugherty wrote:
>>> Greetings,
>>>
>>> I'm extracting another standalone fix from the Async Monitor Deflation
>>> project (JDK-8153224) and sending it out for review (and testing)
>>> separately.
>>>
>>>     JDK-8235795 replace monitor list 
>>> mux{Acquire,Release}(&gListLock) with spin locks
>>>     https://bugs.openjdk.java.net/browse/JDK-8235795
>>>
>>> Here's the webrev URL:
>>>
>>> http://cr.openjdk.java.net/~dcubed/8235795-webrev/0-for-jdk15/
>>>
>>> Folks that have reviewed JDK-8153224 will recognize these changes as
>>> a subset of the monitor list changes from the Async Monitor Deflation
>>> project. It's a subset because the Async Monitor Deflation project
>>> needs additional spin locking due to the async deflation work.
>>>
>>> The OpenJDK wiki for Async Monitor Deflation has several sections
>>> dedicated to the Spin-Lock Monitor List Management changes. This
>>> link will get you to the first section:
>>>
>>> Spin-Lock Monitor List Management In Theory
>>> https://wiki.openjdk.java.net/display/HotSpot/Async+Monitor+Deflation#AsyncMonitorDeflation-Spin-LockMonitorListManagementInTheory 
>>>
>>>
>>> The remaining monitor list sections are:
>>>
>>> Background: ObjectMonitor Movement Between the Lists
>>> https://wiki.openjdk.java.net/display/HotSpot/Async+Monitor+Deflation#AsyncMonitorDeflation-Background:ObjectMonitorMovementBetweentheLists 
>>>
>>>
>>> Spin-Lock Monitor List Management In Reality
>>> https://wiki.openjdk.java.net/display/HotSpot/Async+Monitor+Deflation#AsyncMonitorDeflation-Spin-LockMonitorListManagementInReality 
>>>
>>>
>>> Using The New Spin-Lock Monitor List Functions
>>> https://wiki.openjdk.java.net/display/HotSpot/Async+Monitor+Deflation#AsyncMonitorDeflation-UsingTheNewSpin-LockMonitorListFunctions 
>>>
>>>
>>> Of course, the OpenJDK wiki content is specific to the Async Monitor
>>> Deflation project, but this extract is a very close subset.
>>>
>>> These changes have been tested in various Mach5 Tier[1-7] runs.
>>> I'm also doing SPECjbb2015 runs.
>>>
>>> Thanks, in advance, for comments, questions or suggestions.
>>>
>>> Dan
>>>
>>



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