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

Mandy Chung mandy.chung at oracle.com
Wed Oct 17 20:41:55 UTC 2018

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 

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.

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