sankalp kohli commented on CASSANDRA-12668:

By "always synchronous" I assume you mean always locking instead of using CAS? 

We did a test where you always write to a few CQL partition simultaneous to 
create contention. We have seen 2.0 has a higher throughput than 2.1 and 
looking at allocation points to this memtable issue.

Then we made the configuration changes added in  7546 to always lock and that 
reduced the throughput. 

Looking at the heap dumps did not point that memtable is smaller in 2.1 vs 2.0. 
So I dont think this is an issue. 

Apart from the testing, the only clusters this is an issue is where we have 
contention and hence this is change is an issue. 

> Memtable Contention in 2.1
> --------------------------
>                 Key: CASSANDRA-12668
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-12668
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: sankalp kohli
> We added a new Btree implementation in 2.1 which causes write performance to 
> go down in Cassandra if there is  lot of contention in the memtable for a CQL 
> partition. Upgrading a cluster from 2.0 to 2.1 with contention causes the 
> cluster to fall apart due to GC. We tried making the defaults added in 
> CASSANDRA-7546 configurable but that did not help. Is there anyway to fix 
> this issue?

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