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From "Fenghua Hu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-10690) Optimize insertion/removal of replica in ShortCircuitCache.java
Date Sat, 30 Jul 2016 01:08:20 GMT

    [ https://issues.apache.org/jira/browse/HDFS-10690?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15400336#comment-15400336
] 

Fenghua Hu commented on HDFS-10690:
-----------------------------------

Xiaoyu, thanks for the reply. Regarding the bulletin 2, look like i didn't explain the design
very well. Sorry for the misleading. I'd like to clarify here. This design won't need lookup
in link list, because there are two references in ShortCircuitReplica object. If we want to
remove a ShortCircuitReplica object from the list, just directly access its references and
unlink itself. That's why it can improve performance. 

> Optimize insertion/removal of replica in ShortCircuitCache.java
> ---------------------------------------------------------------
>
>                 Key: HDFS-10690
>                 URL: https://issues.apache.org/jira/browse/HDFS-10690
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: hdfs-client
>    Affects Versions: 3.0.0-alpha2
>            Reporter: Fenghua Hu
>            Assignee: Fenghua Hu
>         Attachments: HDFS-10690.001.patch, HDFS-10690.002.patch
>
>   Original Estimate: 336h
>  Remaining Estimate: 336h
>
> Currently in ShortCircuitCache, two TreeMap objects are used to track the cached replicas.
> private final TreeMap<Long, ShortCircuitReplica> evictable = new TreeMap<>();
> private final TreeMap<Long, ShortCircuitReplica> evictableMmapped = new TreeMap<>();
> TreeMap employs Red-Black tree for sorting. This isn't an issue when using traditional
HDD. But when using high-performance SSD/PCIe Flash, the cost inserting/removing an entry
 becomes considerable.
> To mitigate it, we designed a new list-based for replica tracking.
> The list is a double-linked FIFO. FIFO is time-based, thus insertion is a very low cost
operation. On the other hand, list is not lookup-friendly. To address this issue, we introduce
two references into ShortCircuitReplica object.
> ShortCircuitReplica next = null;
> ShortCircuitReplica prev = null;
> In this way, lookup is not needed when removing a replica from the list. We only need
to modify its predecessor's and successor's references in the lists.
> Our tests showed up to 15-50% performance improvement when using PCIe flash as storage
media.
> The original patch is against 2.6.4, now I am porting to Hadoop trunk, and patch will
be posted soon.



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