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From "Xiaoyu Yao (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-10690) Optimize insertion/removal of replica in ShortCircuitCache.java
Date Tue, 23 Aug 2016 17:40:20 GMT

    [ https://issues.apache.org/jira/browse/HDFS-10690?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15433269#comment-15433269

Xiaoyu Yao commented on HDFS-10690:

[~fenghua_hu], I understand the tradeoff between memory footprint and the insert/delete performance
here.  Note the ShortCircuitCache#replicaInfoMap is a HashMap too, we are O( n) worst case
and O(1) on average anyway for ref()/unref() case. So I don't think that will change the time
complexity of ref()/unref() case. 

After taking another look at the get() usage issue you mentioned earlier, I don't think we
need the extra indexOf(). A LinkedMap is extended from AbstractLinkedMap, which extended from
AbstractHashedMap. Because of that, we are calling into the AbstractHashedMap#get(Object Key)
here according to https://commons.apache.org/proper/commonscollections/jacoco/org.apache.commons.collections4.map/AbstractHashedMap.java.html.
This will still be O(1) on average and O(n) worst case. 

I would appreciate if you could post the result of the simpler approach that I proposed under
the same test environment. If the difference is significant, we can look at alternatives as
you suggested. Let me know your thoughts. Thanks!

> 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, ShortCircuitCache_LinkedMap.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
> 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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