RFR 8242263: Diagnose synchronization on primitive wrappers
Daniel D. Daugherty
daniel.daugherty at oracle.com
Mon Aug 10 20:39:41 UTC 2020
> Inc: http://cr.openjdk.java.net/~pchilanomate/8242263/v5/inc/webrev/
src/hotspot/share/jfr/metadata/metadata.xml
No comments.
src/hotspot/share/runtime/synchronizer.cpp
L577: char* newline = (char*)strchr(ss.base(), '\n');
L578: if (newline != NULL) {
L579: *newline = '\0';
L580: }
L581: fatal("Synchronizing on object " INTPTR_FORMAT " of klass
%s %s", p2i(obj()), obj->klass()->external_name(), base);
Does this work as expected on Win* where <CR><LF> is used? I don't
remember where we convert <LF> => <CR><LF> on Win*...
src/jdk.jfr/share/conf/jfr/default.jfc
No comments.
src/jdk.jfr/share/conf/jfr/profile.jfc
No comments.
test/hotspot/jtreg/runtime/Monitor/SyncOnPrimitiveWrapperTest.java
No comments.
Thumbs up. No need to see a new webrev if you end up making changes
for Win* above.
Dan
On 8/10/20 4:03 PM, Patricio Chilano wrote:
> Hi Erik,
>
> Thanks for looking into this and for the offline discussions.
>
> On 8/10/20 10:19 AM, Erik Gahlin wrote:
>>
>> Hi Patricio,
>>
>> I have tried to review the JFR changes, but I need more information
>> on how the feature is going to be used.
>>
>> Is this something temporary that will be used gain insight for the
>> Valhalla project, or do you think this will be needed longer term? If
>> it is longer term, the bar is higher on what can be accepted.
>>
>> If this is a temporary feature, we could mark the event as
>> experimental in metadata.xml, similar to what we did with GC events
>> during the development of ZGC. Events that are experimental do not
>> show up by default in visualization tools such as JMC and can be
>> removed when they are not needed, or we have a better solution.
>>
> Right, it's temporary so we can mark it as experimental. I also
> changed the category name to be "Java Virtual Machine, Diagnostics".
>
>> For events to be enabled in default.jfc, they should not cause more
>> than 1% overhead, not even in pathological cases. To me, it seems
>> this could happen if you make a loop where it is triggered all the time.
>>
>> For events in profile.jfc, the overhead should still be low (1-2%),
>> but the target is the typical application. For example, allocation
>> profiling is only enabled in profile.jfc. Some application that are
>> allocation intensive could cause a higher overhead than 1%, but that
>> is OK because that configuration is only to be used for a short
>> period of time. That said, there is still a budget on how much space
>> in a recording an event can take up, so it also depends on how
>> important the information is for the user. To me, it seems this event
>> will not be that important for the average user, which makes me think
>> it should be disabled by default.
>>
> After our off-list discussion I left it as enabled. I also removed the
> threshold setting and added the startTime=false in metadata.xml.
>
>> To complicate things, I noticed that aa command line flag is also
>> used to enable the event. This is something we have worked hard to
>> avoid when it comes to JFR events. All configuration should happen
>> using configuration files. We had problem with this in the past where
>> users gets confused why their events are not enabled. When we ported
>> JFR to Hotspot, we got rid of those of those flags/events and it is a
>> much better situation. Again, if this a temporary event to discover
>> usage pattern for a release or two, it might be OK, but if we believe
>> this event to stick around, I think we should look into alternatives,
>> or not use JFR at all for this.
>>
> Yes, I see your point. Since we want to diagnose synchronization on
> these classes but we don't want to affect performance the flag has to
> be specified at startup otherwise the diagnostic instrumentation is
> disabled, so I don't see a clear way to enable the events from JFR
> alone. In any case, this will be a temporary event.
>
> Here is v5. Let me know if you are okay with the changes.
>
> Inc: http://cr.openjdk.java.net/~pchilanomate/8242263/v5/inc/webrev/
> Full: http://cr.openjdk.java.net/~pchilanomate/8242263/v5/webrev/
>
> Thanks Erik!
>
> Patricio
>>
>> Thanks
>>
>> Erik
>>
>>> On 28 Jul 2020, at 21:16, Patricio Chilano
>>> <patricio.chilano.mateo at oracle.com
>>> <mailto:patricio.chilano.mateo at oracle.com>> wrote:
>>>
>>> Hi all,
>>>
>>> Please review the following change that adds diagnostic capabilities
>>> when synchronizing on primitive wrapper classes.
>>>
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8242263
>>> Webrev: http://cr.openjdk.java.net/~pchilanomate/8242263/v1/webrev/
>>>
>>> The new flag allows to identify synchronization on these classes and
>>> to take one of the following actions: exit the VM with fatal error,
>>> log a warning message, or issue a JFR event. The implementation uses
>>> a simple approach where a check is added at the beginning of the
>>> monitorenter generated code when the flag is enabled to check
>>> whether the object is of a primitive wrapper class. If it is, we
>>> jump to the slow path, otherwise we just continue as always. The
>>> extra instructions will be: load the klass of the object, load the
>>> _access_flags field for that klass, AND with a constant, and branch
>>> based on the result. The code will only be generated whenever the
>>> new opt-in diagnostic flag is enabled so performance won't be
>>> affected when off.
>>>
>>> In addition to the purpose described in the description of the bug,
>>> this flag will also be useful when trying to diagnose possible
>>> synchronization issues if these classes ever become inline types as
>>> part of the Valhalla project.
>>>
>>> I added test SyncOnPrimitiveWrapperTest.java that tests for the exit
>>> and logging cases. I added test TestSyncOnPrimitiveWrapperEvent.java
>>> to test for the JFR event case.
>>> I tested the patch running tiers1-6 in mach5 with the flag set to
>>> DiagnoseSyncOnPrimitiveWrappers=2.
>>> I checked it builds with arm32 and ppc but can't run any tests on
>>> those platforms, so it would be good if somebody can run the new
>>> test included in the patch.
>>>
>>> Let me know if you think I should run or add any more tests.
>>>
>>> Thanks!
>>>
>>> Patricio
>>
>
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