G1GC doesn't clean WeakReference at mixed gc in some situation?

[koji Lin]

My server is using 1.8.0_92 on CentOS 6.7, gc param is '-Xms16g -Xmx16g
-XX:+UseG1GC'. So the default InitiatingHeapOccupancyPercent is 45,
G1HeapWastePercent is 5 and G1MixedGCLiveThresholdPercent is 85. My
server's mixed gc starts from 7.2GB, but it clean less and less, finally
old gen keeps larger than 7.2GB, so it's always try to do concurrent mark.
Finally all heap are exhausted and full GC occurred,. After full gc, old
gen used is under 500MB.

https://i.stack.imgur.com/mzDlu.png

I dumped the heap from another machine with -Xmx4G.I used lettuce as my
redis client, and it has tracking featured using LatencyUtils. It make
LatencyStats(which contains some long[] with near 3000 elements) instances
weak referenced every 10 mins(Reset latencies after publish is true by
default, https://github.com/mp911de/lettuce/wiki/Command-Latency-Metrics).
So it will make lots of WeakReference of LatencyStats after long time.

Before Full GC.

https://i.stack.imgur.com/KRdOL.png
https://i.stack.imgur.com/wboak.png
https://i.stack.imgur.com/vGnmF.png

After Full GC.
https://i.stack.imgur.com/P0XJV.png

Currently I don't need tracking from lettuce, so just disable it and it
doesn't have full gc anymore. But want to know why mixed gc doesn't clear
them.

And I tried in local environment too, but can't reproduce at local
environment.

Thanks.
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