Detail
The rw_in_progress (also know has rw-hash) metric tracks the number of entries in the rw-hash queue.The entries represent parked transactions waiting on pending transactions processing on other nodes. This would include all write replicas and read duplicate resolutions.
Answer
Each rw-hash entry would use close to 5KB of Heap memory. This is usually non impacting when transaction to replicas are progressing at a reasonable rate and system is keeping up and timeout settings are not excessive.
This metric is worth tracking when using Index on Flash. In such cases, a system could OOM if the system was not sized to run a particular workload and the rw-hash is piling up with millions of entries waiting on progressing transactions.