RFR: 8185525: [Event Request] Add Tracing event for DictionarySizes

[gerard ziemski]

On 4/10/19 1:12 PM, coleen.phillimore at oracle.com wrote:
>>>
>>> I noticed that events are only emitted if we are able to take the 
>>> resize lock. Can this be fixed? What prevents us from always getting 
>>> the data? That's how other periodic events work and losing data 
>>> sometimes may lead to subtle bugs that hard to understand and 
>>> replicate in systems that rely on the information. Could we retry on 
>>> a failure?
>> Good observation. If the resize lock is taken, then it's not likely 
>> that whoever owns it will be done soon, so retrying is most likely 
>> not going to succeed right away. Is it OK to tie up JFR periodic 
>> thread for some time? If so, how long?
>>
>> If the lock is taken, then it means that someone is scanning through 
>> the entire table, or the table is being resized. Either way, we're 
>> not loosing data, but are just temporarily blind - I don't see a 
>> problem here for a long running apps, they will start receiving 
>> events eventually (which happen every 10 sec by default)
>
> Robbin was talking about allowing scanning the table while resizing, 
> ie. not having the resize_lock, if we can accept that there might be 
> some entries double counted.

Yes, we could do that - are you suggesting that this is what we should 
do? Personally, I think I'd prefer not to emit the event at all, rather 
than emit one that might be wrong (that's exactly what we do currently 
for jcmd print statistics).

Erik, Robbin, do you have a preference here?


cheers