Question re -XX:-FlightRecorder

[Andrey Petushkov]

Hi Erik,

On 06.10.2020 18:32, Erik Gahlin wrote:
> Hi Andrey,
>
> The flag is legacy from JDK 7 when we needed -XX:+FlightRecorder to allocate global buffers when the VM started. That restriction was removed in 8u40. it is probably a good idea to remove the flag completely. The method JfrRecorder::is_disabled() should be removed at the same time.
The flag is marked as deprecated so can I just remove it or should it be
promoted to the next stage (obsolete)?
> When it comes to earlier releases, I think we need to stick to the behaviour that exists. Changing it in an update release would cause even more confusion. 
Well, I assume you don't mean VM crash on assert is the behavior you'd
like to retain ;) So I read this as:
- FlightRecorder.isAvailable to return false
- FlightRecorder.getFlightRecorder, new Recording() etc are to work as
if no -XX:-FlightRecorder was specified
> Do you know why users want to use -XX:-FlightRecorder?
Not really. It might be that this were mostly related to JFR backport
into 8u and the customers were cautious about it's performance impact
>
> Thanks
> Erik
Thank you,
Andrey
>
>> On 6 Oct 2020, at 11:44, Andrey Petushkov <andrey.petushkov at gmail.com> wrote:
>>
>> Hi Erik, All,
>>
>> sometimes ([1], [2]) the message is given that the command line switch
>> -XX:-FlightRecorder disables the JFR functionality. The problem is
>> that this statement is not true and in fact causes buggy behavior of
>> the VM. Namely:
>> - on a release VM ran with -XX:-FlightRecorder the call to
>> FlightRecorder.getFlightRecorder() succeeds and the recording can be
>> successfully created (much like as if no command line flag was given)
>> - on a debug version of VM ran with same flag the call to
>> FlightRecorder.getFlightRecorder() crashes the VM on assert
>> #  Internal Error
>> (/home/andrey/ws/jdk/src/hotspot/share/jfr/recorder/jfrRecorder.cpp:236),
>> pid=331136, tid=331137
>> #  assert(!is_disabled()) failed: invariant
>>
>> It would be good to clarify the behavior and implement it in the
>> consistent way, that is, either:
>> - allow -XX:-FlightRecorder to actually disable JFR forever, and throw
>> IllegalStateException from FlightRecorder.getFlightRecorder() and
>> friends
>> - declare -XX:-FlightRecorder as no-op and remove the mentioned assert
>> Please note that the answer might be different for pre-13 and 13+
>> versions because of flag deprecation
>>
>> Any opinion on this?
>> This might not look like an important thing at all however there were
>> quite a number of users (Azul customers) stumbling on unclear behavior
>> of the said flag so I thought it's better to have this fixed
>>
>> Thanks a lot,
>> Andrey
>>
>> [1] https://bugs.openjdk.java.net/browse/JDK-8225312
>> [2] jdk/src/hotspot/share/jfr/recorder/jfrRecorder.cpp,
>> JfrRecorder::is_disabled()