RFR(S) 8021335: Missing synchronization when reading counters for live threads and peak thread count
David Holmes
david.holmes at oracle.com
Thu Oct 18 07:27:29 UTC 2018
Hi Dean,
On 18/10/2018 2:06 PM, dean.long at oracle.com wrote:
> On 10/17/18 7:07 PM, David Holmes wrote:
>> Hi Dean,
>>
>> This still seems racy to me. We increment all counts under the
>> Threads_lock but we still decrement without the Threads_lock. So we
>> can lose updates to the perfCounters.
>>
>> 117 _total_threads_count->inc();
>> 118 Atomic::inc(&_atomic_threads_count);
>> 119 int count = _atomic_threads_count;
>> <= context switch here
>> 120 _live_threads_count->set_value(count);
>>
>> If a second thread now exits while the above thread is descheduled, it
>> will decrement _atomic_threads_count and _live_threads_count, but when
>> the first thread resumes at line 120 above we will set
>> _live_threads_count to the wrong value!
>>
>> You can't maintain two counters in sync without all changes using
>> locking across both.
>>
>
> You're right, I missed that. I think the right thing to do is call
> current_thread_exiting while holding the Threads_lock.
> Then we can get rid of the parallel atomic counters. So, here's one
> more try:
>
> http://cr.openjdk.java.net/~dlong/8021335/webrev.7/
Okay that is the simple and obvious solution that doesn't require split
counts. So I have to ask Mandy if she recalls why this approach wasn't
taken 15 years ago when the exit counts were added as part of:
https://bugs.openjdk.java.net/browse/JDK-4530538 ?
Does taking the Threads_lock here cost too much and cause a thread
termination bottleneck?
Thanks,
David
-----
> dl
>
>> David
>>
>>
>>
>> On 18/10/2018 8:18 AM, dean.long at oracle.com wrote:
>>> On 10/17/18 2:38 PM, Mandy Chung wrote:
>>>>
>>>>
>>>> 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
>>>>
>>>
>>> New webrevs, full and incremental:
>>>
>>> http://cr.openjdk.java.net/~dlong/8021335/webrev.6/
>>> http://cr.openjdk.java.net/~dlong/8021335/webrev.6.diff/
>>>
>>> I like it better without all the asserts too.
>>>
>>> dl
>>>
>>>> Mandy
>>>
>
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