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From "zhangduo (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-13259) mmap() based BucketCache IOEngine
Date Wed, 18 Mar 2015 00:44:43 GMT

    [ https://issues.apache.org/jira/browse/HBASE-13259?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14366403#comment-14366403
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zhangduo commented on HBASE-13259:
----------------------------------

What about just use a different ByteArrayBuffer implementation? Then we do not need to check
'fileChannel != null' every time.

And one doubt, if the working set can fit in RAM, then why not just use offheap BucketCache?
We can not transfer data from HDFS using mmap...

And if we use mmap on a file much larger than available RAM as BucketCache, what will happen?
I think this is the common use case for FileIOEngine that your FileMmapEngine compared to.
Would be great if we can still be faster than FileIOEngine under this condition.

Thanks. 

> mmap() based BucketCache IOEngine
> ---------------------------------
>
>                 Key: HBASE-13259
>                 URL: https://issues.apache.org/jira/browse/HBASE-13259
>             Project: HBase
>          Issue Type: New Feature
>          Components: BlockCache
>    Affects Versions: 0.98.10
>            Reporter: Zee Chen
>             Fix For: 2.2.0
>
>         Attachments: HBASE-13259-v2.patch, HBASE-13259.patch, ioread-1.svg, mmap-0.98-v1.patch,
mmap-1.svg, mmap-trunk-v1.patch
>
>
> Of the existing BucketCache IOEngines, FileIOEngine uses pread() to copy data from kernel
space to user space. This is a good choice when the total working set size is much bigger
than the available RAM and the latency is dominated by IO access. However, when the entire
working set is small enough to fit in the RAM, using mmap() (and subsequent memcpy()) to move
data from kernel space to user space is faster. I have run some short keyval gets tests and
the results indicate a reduction of 2%-7% of kernel CPU on my system, depending on the load.
On the gets, the latency histograms from mmap() are identical to those from pread(), but peak
throughput is close to 40% higher.
> This patch modifies ByteByfferArray to allow it to specify a backing file.
> Example for using this feature: set  hbase.bucketcache.ioengine to mmap:/dev/shm/bucketcache.0
in hbase-site.xml.
> Attached perf measured CPU usage breakdown in flames graph.



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