Comments - Cluster Overall Memory Requirements (compared to multiple stand-alone nodes)
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galera doesn't use that much more memory. Its got an apply queue which in theory can be up to gcs.fc_limit ( wsrep_provider_options) entries of up to wsrep_max_ws_size size. If you're not doing large and multiple row updates (row based replication) updates hitting this limit will be hard.
wsrep_thread_count are full connection threads and in theory can consume about the same as real connection threads however as they are processing row based events some of the usual connection based buffers will never be used.
There are some other memory allocations but they are quite minor.
Unless you doing some massive updates where the hardware on the receiving node is significantly less than the update node, I wouldn't consider Galera usage to be any more than 1-2G (very conservative estimate with increased gcs.fc_limit at 1K).
bit more of a measure under the swap required on this page getting-started-with-mariadb-galera-cluster