[jira] [Created] (CASSANDRA-15810) Default StringTableSize parameter causes GC slowdown

["Tom van der Woerdt (Jira)" <ji...@apache.org>]

Tom van der Woerdt created CASSANDRA-15810:
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             Summary: Default StringTableSize parameter causes GC slowdown
                 Key: CASSANDRA-15810
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-15810
             Project: Cassandra
          Issue Type: Improvement
            Reporter: Tom van der Woerdt


While looking at tail latency on a Cassandra cluster, it came up that the default StringTableSize in Cassandra is set to a million:
{code:java}
# Larger interned string table, for gossip's benefit (CASSANDRA-6410)
-XX:StringTableSize=1000003{code}
This was done for CASSANDRA-6410 by [~jbellis] in '13, to optimize heap usage on a test case, running with 500 nodes and num_tokens=512.

Until Java 13, this string table is implemented as native code, and has to be traversed entirely during the GC initial marking phase, which is a STW event.

Some testing on my end shows that the pause time of a GC cycle can be reduced by approximately 10 milliseconds if we lower the string table size back to the Java 8 default of 60013 entries.

Thus, I would recommend this patch (3.11 branch, similar patch for 4.0):
{code:java}
diff --git a/conf/jvm.options b/conf/jvm.options
index 01bb1685b3..c184d18c5d 100644
--- a/conf/jvm.options
+++ b/conf/jvm.options
@@ -107,9 +107,6 @@
 # Per-thread stack size.
 -Xss256k

-# Larger interned string table, for gossip's benefit (CASSANDRA-6410)
--XX:StringTableSize=1000003
-
 # Make sure all memory is faulted and zeroed on startup.
 # This helps prevent soft faults in containers and makes
 # transparent hugepage allocation more effective.
 {code}
It does need some testing on more extreme clusters than I have access to, but I ran some Cassandra nodes with {{-XX:+PrintStringTableStatistics}} which suggested that the Java default will suffice here.



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