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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-7542) Reduce CAS contention
Date Wed, 16 Jul 2014 11:54:04 GMT


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:
>             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 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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