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https://issues.apache.org/jira/browse/CASSANDRA-7542?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14063405#comment-14063405
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Benedict commented on CASSANDRA-7542:
-------------------------------------
It seems to me the first and simplest improvement is to bound the _cost_ of
contention by tracking the, say, ~90th %ile of paxos round time, and to sleep
for anywhere between this time, and twice this time, once we detect contention.
Secondly, we could potentially have a lock within each C* process, where for
some period of time only one paxos request may be in flight for the given
partition+host (as it looks to me that a read against the same host as a write
can cause the write's paxos round to be interrupted, so the host fights with
itself... though I need to digest the code a little more to be sure about this
one).
> Reduce CAS contention
> ---------------------
>
> Key: CASSANDRA-7542
> URL: https://issues.apache.org/jira/browse/CASSANDRA-7542
> Project: Cassandra
> Issue Type: Improvement
> Reporter: sankalp kohli
> Assignee: Benedict
> Fix For: 2.0.10
>
>
> 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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