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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 Mon, 26 Sep 2016 18:02:20 GMT

    [ https://issues.apache.org/jira/browse/HDFS-10690?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15523763#comment-15523763
] 

Xiaoyu Yao commented on HDFS-10690:
-----------------------------------

[~fenghua_hu], thanks for updating the patch. The patch v05 looks good to me overall. Nice
catch for the NoSuchElementException instead null returned from LinkedMap#firstKey(). Two
minor issues: 
1. Can we wrap this NoSuchElementException handling below into a helper function that returns
null when NoSuchElementException is thrown from firstKey() call? 

2. Do we need {{if (eldestKey == null)}} after the try/catch? I think it is needed only if
we wrap the try/catch with a helper function, which returns null when the list is empty.

{code}
        Object eldestKey;
897	        try {
898	          eldestKey = evictableMmapped.firstKey();
899	        } catch (NoSuchElementException e) {
900	          break;
901	        }
902	        if (eldestKey == null) {
903	          break;
904	        }
{code}

> 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, HDFS-10690.003.patch,
HDFS-10690.004.patch, HDFS-10690.005.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
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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