sankalp kohli created CASSANDRA-7542:
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Summary: Reduce CAS contention
Key: CASSANDRA-7542
URL: https://issues.apache.org/jira/browse/CASSANDRA-7542
Project: Cassandra
Issue Type: Improvement
Reporter: sankalp kohli
CAS updates on same CQL partition can lead to heavy contention inside C*. I am
looking for simple ways(no algorithmic changes) to reduce contention as the
penalty of it is high in terms of latency, specially for reads.
We can put some sort of synchronization on CQL partition at StorageProxy level.
This will reduce contention at least for all requests landing on one box for
same partition.
Here is an example of why it will help:
1) Say 1 write and 2 read CAS requests for the same partition key is send to C*
in parallel.
2) Since client is token-aware, it sends these 3 request to the same C*
instance A. (Lets assume that all 3 requests goto same instance A)
3) In this C* instance A, all 3 CAS requests will contend with each other in
Paxos. (This is bad)
To improve contention in 3), what I am proposing is to add a lock on partition
key similar to what we do in PaxosState.java to serialize these 3 requests.
This will remove the contention and improve performance as these 3 requests
will not collide with each other.
Another improvement we can do in client is to pick a deterministic live replica
for a given partition doing CAS.
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