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From "sankalp kohli (JIRA)" <>
Subject [jira] [Created] (CASSANDRA-7542) Reduce CAS contention
Date Mon, 14 Jul 2014 22:42:06 GMT
sankalp kohli created CASSANDRA-7542:

             Summary: Reduce CAS contention
                 Key: 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 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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