[11u] RFR: 8254185: Fix Code cache sweeper heuristics for JDK 11

[Nils Eliasson]

Thank you Vladimir!

// Nils

On 2020-10-12 20:36, Vladimir Kozlov wrote:
> +1
>
> Thanks,
> Vladimir K
>
> On 10/12/20 5:29 AM, Tobias Hartmann wrote:
>> Hi Nils,
>>
>> looks good to me.
>>
>> Best regards,
>> Tobias
>>
>> On 09.10.20 16:34, Nils Eliasson wrote:
>>> Hi,
>>>
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8254185
>>> Webrev: http://cr.openjdk.java.net/~neliasso/8254185/webrev.01/
>>>
>>> This fix is a replacement for JDK-8244660 that I consider to risky 
>>> to back port.
>>>
>>> This issue can't be seen unless JDK-8254185 is applied. That bug 
>>> causes the sweeper to trigger on every code cache allocation. When 
>>> JDK-8254185 is applied then no sweeps can be observed at all.
>>>
>>> The main issue is that the sweeper thread is never woken. And if it 
>>> isn't woken, it can't check the counters to see if it is time to sweep.
>>>
>>> This fix include two changes:
>>>
>>> 1) Notifying waiters on the codecache_lock when a threshold for 
>>> reclaimed nmethods has been reached. This fix is enough on its own.
>>>
>>> In JDK 15 I introduced a new lock for the sweeper because a adding a 
>>> notify in NMethodSweeper::possibly_enable_sweeper conflicted with 
>>> the compiledMethod_lock in add_osr_method. That lock was added when 
>>> deoptimization was converted to using handshakes. So in 11 it is 
>>> still ok to use the codecache_lock.
>>>
>>> 2) Moving the updates of _total_nof_code_cache_sweeps and 
>>> _last_sweep inside block that is only run when doing a sweep.
>>> This change just makes the counters update as expected. _last_sweep 
>>> is used to trigger sweeps after a number of safepoints. This is 
>>> counted by _time_counter. But since updates of _time_counter doesn't 
>>> wake the sweeper - it still doesn't work.
>>>
>>> In JDK 15 I removed all of this code since we can't even rely on 
>>> safepoints happening at all. I don't want to introduce any bugs in 
>>> JDK 11 by mistake - so I am leaving it as it is.
>>>
>>> Please review.
>>>
>>> Best regards,
>>> Nils Eliasson
>>>