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https://issues.apache.org/jira/browse/CASSANDRA-6890?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13984805#comment-13984805
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Pavel Yaskevich commented on CASSANDRA-6890:
--------------------------------------------
Just mine 2 cents here.
To archive those results did you use stress in the mixed mode and/or were reads
happening simultaneously with writes. This regression could be caused by
contention on the vm_area_struct access inside mm_struct. Can you do 'perf
record' on the Cassandra process while stress is running and share the results?
bq. It's somewhat interesting to note that buffered I/O uses roughly 35% more
privileged cpu-time than memory-mapped, whereas user, interrupt, and all other
processor counters show comparable load between the two configurations.
This is expected because buffered i/o actually does syscall per read and has to
copy data over from page cache to the user buffer.
> Remove mmap'ed i/o from read path
> ---------------------------------
>
> Key: CASSANDRA-6890
> URL: https://issues.apache.org/jira/browse/CASSANDRA-6890
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Joshua McKenzie
> Assignee: Joshua McKenzie
> Priority: Minor
> Fix For: 3.0
>
> Attachments: mmap_gc.jpg, mmap_jstat.txt, mmap_perf.txt,
> nommap_gc.jpg, nommap_jstat.txt
>
>
> Since we actively unmap unreferenced SSTR's and also copy data out of those
> readers on the read path, the current memory mapped i/o is a lot of
> complexity for very little payoff. Clean out the mmapp'ed i/o on the read
> path.
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