[CASSANDRA-7542] Reduce CAS contention

• Type: Improvement
• Status: Open
• Priority: Low
• Resolution: Unresolved
• Fix Version/s: 5.x
• Component/s: Feature/Lightweight Transactions, Legacy/Coordination
• Labels: LWT
• Assignee: Unassigned
• Reporter: Sankalp Kohli
• Created: 14/Jul/14 22:41
• Updated: 07/Mar/23 10:53

Description

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.

Issue Links

• contains: CASSANDRA-7343 CAS contention back off time should be configurable