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# general
s
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d
We use ZGC here and there, for example on some of our non-prod clusters. I am currently investigating if ZGC is the way to go for Historical because I have this theory that all of our 
DruidProcessingModule
threads are hanging because G1GC takes a very long time collecting garbage when our users are abusing Druid with their 
LIKE
queries.
ZGC is currently not-generational so some of the GC metrics are missing.
But I currently don’t know if swapping to ZGC will cause query performance degradation or not. I don’t know how to measure it.
g
do you have druid 
query/time
and 
jvm/gc/cpu
metrics being collected somewhere?
i usually look at those
d
jvm.gc.cpu is one of the metric that I weirdly no longer have. Even on G1 GC
I do have broker.query.time.{table}.groupBy.{95percentile|median|max|etc}.
g
you might need to add some additional jvm command line params to get it to work properly
d
ooo i remember, I got exception on boot due to Sigar library
I could never figure out how to fix it.
g
last one 
--add-opens=jdk.management/com.sun.management.internal=ALL-UNNAMED
i think is the one you need for full gc metrics
although it doesn't hurt to add all of them
d
this is what I have so far:
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--add-exports=jdk.management/com.sun.management.internal=ALL-UNNAMED
--add-exports=java.base/jdk.internal.perf=ALL-UNNAMED
--add-opens=java.base/java.lang=ALL-UNNAMED
--add-opens=java.base/jdk.internal.ref=ALL-UNNAMED
--add-opens=java.base/sun.nio.ch=ALL-UNNAMED
--add-opens=java.management/sun.management=ALL-UNNAMED
I see that they are quite different. This is list is helpful!
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