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

Daniel D. Daugherty daniel.daugherty at oracle.com
Mon Feb 3 21:32:45 UTC 2020 


On 2/3/20 2:56 PM, Daniel D. Daugherty wrote:
> On 2/3/20 2:30 PM, coleen.phillimore at oracle.com wrote:
>>
>> http://cr.openjdk.java.net/~dcubed/8235795-webrev/2-for-jdk15.inc/src/hotspot/share/runtime/objectMonitor.inline.hpp.udiff.html 
>>
>>
>> Can you add a little comment why you need Atomic::load() and store() 
>> above these functions?
>>
>> // The _om_next field can be concurrently read and modified, so 
>> disable optimizations that elide loading and storing this value.
>
> I'll add something like above the new functions.

Just for the record, this is the comment I'm adding:

+// The _next_om field can be concurrently read and modified so we
+// use Atomic operations to disable compiler optimizations that
+// might try to elide loading and/or storing this field.
+
  inline ObjectMonitor* ObjectMonitor::next_om() const {
    return Atomic::load(&_next_om);
  }

Dan


>
>
>> I don't need to see another webrev.
>
> Okay.
>
>
>> Looks good to me.
>
> Thanks for the fast re-review!
>
> Dan
>
>
>>
>> Coleen
>>
>> On 2/2/20 9:53 AM, Daniel D. Daugherty wrote:
>>> Greetings,
>>>
>>> I've made changes based on Coleen's comments on CR1. I'm looking for a
>>> re-review by both David H. and Coleen. Of course, anyone else is 
>>> welcome
>>> to chime in on this review thread.
>>>
>>>     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/2-for-jdk15.inc/
>>>
>>> Here's the full webrev URL:
>>>
>>> http://cr.openjdk.java.net/~dcubed/8235795-webrev/2-for-jdk15.full/
>>>
>>> Here's what changed between CR1 and CR2:
>>>
>>>   - add ObjectMonitor::next_om(), set_next_om(), and try_set_next_om()
>>>     and update direct uses of _next_om field to use them
>>>   - ObjectMonitor::_next_om field is now private
>>>   - rename ListGlobals -> ObjectMonitorListGlobals, rename LVars ->
>>>     om_list_globals, and prefix each ObjectMonitorListGlobals field 
>>> with '_'
>>>   - delete static set_next() function
>>>   - clarify comments
>>>   - delete stale comments about mux*()
>>>
>>> These changes have been tested in a Mach5 Tier[1-3] run with no
>>> failures. 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).
>>>
>>> Thanks, in advance, for comments, questions or suggestions.
>>>
>>> Dan
>>>
>>>
>>> On 1/27/20 3:43 PM, 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
>>>>   - 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.
>>>>
>>>> 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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