Garbage Collection - timings and suspension

[Thomas Schatzl]

Hi Rohit,

  just some addition to Jenny's response:

On Mon, 2016-07-25 at 15:17 +0000, Rohit Mohta wrote:
> Hi All,
>  
>    Can someone please help me understand the below snippet of Garbage
> Collection logs?
> 
[...]
>  
> (b)    I can see “Stopping threads took: 0.0001230 seconds” statement
> in numerous occasions. Even when, GC was not taking place.

There are many other causes of the VM stopping all threads than garbage
collection, for many different purposes.

> ·        What does it mean by ‘stopping threads took’? Does it mean,
> garbage collector process was trying to ‘park’ all the threads,
> without interrupting ongoing instructions. So it had to wait for
> ongoing processing to complete, before it could put the thread in
> ‘park’ state?

This message means what you describe, i.e. this is the time from
signalling the threads in the VM to stop until all of them are stopped.

> ·        Why does JVM stop application threads even when we don’t see
> any Full GC events? In my system, full gc event is fired every 60
> mins – but I can see the above statement numerous times. Why does
> that happen?

The VM needs to do other kind of accounting next to garbage collection
that requires a stop-the-world pause.

> ·        When we talk about GC Suspension time, our definition is –
> “time for which application will not process anything. Application
> kinda hangs”. Should we consider these “Total time for which
> application threads were stopped” also as Suspension time?

Yes.

> ·        Any best practices to reduce the suspension time (apart from
> GC friendly code)?

This depends on the reason for the suspension that should be printed if
PrintSafepointStatistics is turned on. One potential knob to turn is
GuaranteedSafepointInterval. Note that it is not advised to make this
overly long or turn it off completely (default is 1s).

Thanks,
  Thomas