RFR: 8223961: ZGC: Unexpected behaviour was observed caused by the incorrect update of _nticks in ZMetronome::wait_for_tick()

[Per Liden]

Hi,

There is code in ZStat that assumes ZMetronome::ntick() never skips over 
ticks. That would need to be addressed if we do something like this. 
Otherwise we risk not getting statistics print outs just because the 
metronome overslept. I'll take a look at it.

cheers,
Per

On 05/15/2019 04:33 PM, Jie Fu wrote:
> Hi all,
> 
> JBS: https://bugs.openjdk.java.net/browse/JDK-8223961
> 
> Please review the following patch which fixes the update of "_nticks" in 
> ZMetronome::wait_for_tick()
> -----------------------------------
> diff -r b0b20413d853 src/hotspot/share/gc/z/zMetronome.cpp
> --- a/src/hotspot/share/gc/z/zMetronome.cpp     Wed May 15 11:53:47 2019 
> +0200
> +++ b/src/hotspot/share/gc/z/zMetronome.cpp     Wed May 15 20:15:39 2019 
> +0800
> @@ -57,6 +57,8 @@
>       if (!_stopped && timeout_ms > 0) {
>         // Wait
>         ml.wait(timeout_ms);
> +    } else if (timeout_ms < -1 * (int64_t)_interval_ms) {
> +      _nticks += -timeout_ms / _interval_ms;
>       } else {
>         // Tick
>         return !_stopped;
> -----------------------------------
> For more info, please see the JBS.
> 
> Testing:
>    - make test TEST="hotspot_gc" CONF=release
> 
> Thanks a lot.
> Best regards,
> Jie
> 
>