Andrew Wang updated HDFS-10690:
    Fix Version/s: 3.0.0-alpha2

> Optimize insertion/removal of replica in ShortCircuitCache
> ----------------------------------------------------------
>                 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
>             Fix For: 2.8.0, 3.0.0-alpha2
>         Attachments: HDFS-10690.001.patch, HDFS-10690.002.patch, 
> HDFS-10690.003.patch, HDFS-10690.004.patch, HDFS-10690.005.patch, 
> HDFS-10690.006.patch, HDFS-10690.007.patch, HDFS-10690.008.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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