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From Jaesung Lee <>
Subject mmap I/O and shared memory
Date Thu, 17 Nov 2011 07:16:20 GMT
I am running 7 nodes cassandra(v1.0.2) cluster.
I am putting 20K rows per sec to the cluster.
This cluster has 1 KS, 3CFs.
Each CF has 4-5 secondary indices.

After I'v run for 1 week, nodes use swap memory.
I changed disk-access-mode to index_only or standard.
I got strange memory results.

> using mmap:
> VIRT: 566g  RES: 36g  SHR:12g
> standard disk access mode
> VIRT:24.7g  RES: 24g  SHR:68m

I allocated 24g memory for JVM heap.

I have some questions about mmap.
It is easy to analyze standard disk access mode's memory result.

I know cassandra use huge virtual memory for mmap I/O and each mmaped addresses are mapped
to indexed file not swap memory.

But, I don't understand why cassandra use shared memory, if using mmap I/O.

Are there some documents that explain this situation? 

Jaesung Lee
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