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https://issues.apache.org/jira/browse/CASSANDRA-11383?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15202084#comment-15202084
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Pavel Yaskevich commented on CASSANDRA-11383:
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Ok, so it's going to be 1G * 9 per memtable and memtable itself is pretty big 
and I am assuming that all of the 9 columns there are only columns you have 
defined in the row, which means that a couple of flushed can potentially 
consume your while heap. You can try minimizing the memtable size to make it 
flush more frequently and let compaction deal with it, I'm attaching the patch 
to make memtable flush segment sizes configurable (and default of 128 MB) (you 
can specify max_memtable_flush_memory_in_mb in index options if you want to 
make it even smaller).

> SASI index build leads to massive OOM
> -------------------------------------
>
>                 Key: CASSANDRA-11383
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-11383
>             Project: Cassandra
>          Issue Type: Bug
>          Components: CQL
>         Environment: C* 3.4
>            Reporter: DOAN DuyHai
>         Attachments: new_system_log_CMS_8GB_OOM.log, system.log_sasi_build_oom
>
>
> 13 bare metal machines
> - 6 cores CPU (12 HT)
> - 64Gb RAM
> - 4 SSD in RAID0
>  JVM settings:
> - G1 GC
> - Xms32G, Xmx32G
> Data set:
>  - ≈ 100Gb/per node
>  - 1.3 Tb cluster-wide
>  - ≈ 20Gb for all SASI indices
> C* settings:
> - concurrent_compactors: 1
> - compaction_throughput_mb_per_sec: 256
> - memtable_heap_space_in_mb: 2048
> - memtable_offheap_space_in_mb: 2048
> I created 9 SASI indices
>  - 8 indices with text field, NonTokenizingAnalyser,  PREFIX mode, 
> case-insensitive
>  - 1 index with numeric field, SPARSE mode
>  After a while, the nodes just gone OOM.
>  I attach log files. You can see a lot of GC happening while index segments 
> are flush to disk. At some point the node OOM ...
> /cc [~xedin]



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