RFR(S) 8021335: Missing synchronization when reading counters for live threads and peak thread count

[Mandy Chung]

On 10/17/18 2:13 PM, dean.long at oracle.com wrote:
> On 10/17/18 1:41 PM, Mandy Chung wrote:
>>
>>
>> On 10/16/18 7:33 PM, David Holmes wrote:
>>> Hi Dean,
>>>
>>> Thanks for tackling this.
>>>
>>> I'm still struggling to fully grasp why we need both the 
>>> PerfCounters and the regular counters. I get that we have to 
>>> decrement the live counts before ensure_join() has allowed 
>>> Thread.join() to return, to ensure that if we then check the number 
>>> of threads it has dropped by one. But I don't understand why that 
>>> means we need to manage the thread count in two parts. Particularly 
>>> as now you don't use the PerfCounter to return the live count, so it 
>>> makes me wonder what role the PerfCounter is playing as it is 
>>> temporarily inconsistent with the reported live count? 
>>
>> Perf counters were added long time back in JDK 1.4.2 for performance 
>> measurement before java.lang.management API.  One can use jstat tool 
>> to monitor VM perf counters of a running VM.   One could look into 
>> the possibility of deprecating these counters and remove them over time.
>>
>>> On 17/10/2018 9:43 AM, dean.long at oracle.com wrote:
>>> New webrev:
>>>
>>> http://cr.openjdk.java.net/~dlong/8021335/webrev.4/
>>
>> When the perf counters are updated when a thread is added/removed, 
>> it's holding Threads_lock.  Are the asserts in 
>> ThreadService::remove_thread necessary?
>>
>
> Not really.  They were intended to catch the case where the atomic 
> counters weren't decremented for some reason, not for the perf counters.
> Should I remove them?
>

Hmm... when remove_thread is called but decrement_thread_counts has not 
been called.   It's a bug in thread accounting.  It happens to have the 
perf counters that can be compared to assert.  It seems not obvious.  
Setting the perf counters same values as _atomic_threads_count and 
_atomic_daemon_threads_count makes sense to me.

I would opt for removing the asserts but I can't think of an alternative 
how to catch the issue you concern about.

>> For clarify, I think we could simply set _live_threads_count to the 
>> value of _atomic_threads_count and set _daemon_threads_count to the 
>> value of _atomic_daemon_threads_count.
>>
>
> I think that works, even inside decrement_thread_counts() without 
> holding the Threads_lock.  If you agree, I'll make that change.
>
+1

Mandy
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