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https://issues.apache.org/jira/browse/HDFS-3051?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13236868#comment-13236868
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Colin Patrick McCabe commented on HDFS-3051:
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Todd said:
bq. One interesting thing to take note of: in Linux prior to 2.6.37, the page
fault handler for file mappings actually held the mmap semaphore exclusively,
preventing other threads from modifying page mappings (or starting threads). So
doing mmapped IO may have some downsides as well, especially on older kernels.
Not sure if this issue is addressed in RHEL 6 or not. The Linux git hash is
d065bd810b6deb67d4897a14bfe21f8eb526ba99, see also
http://help.lockergnome.com/linux/PATCH-V2-Reduce-mmap_sem-hold-times-file-backed-page-faults--ftopict527005.html
Good point.
At least in theory, you can create threads on Linux without calling mmap. You
just can't create pthreads (note the "p"). I wonder what HotSpot does exactly
to create threads?
> A zero-copy ScatterGatherRead api from FSDataInputStream
> --------------------------------------------------------
>
> Key: HDFS-3051
> URL: https://issues.apache.org/jira/browse/HDFS-3051
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: hdfs client
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
>
> It will be nice if we can get a new API from FSDtaInputStream that allows for
> zero-copy read for hdfs readers.
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