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https://issues.apache.org/jira/browse/HDFS-10690?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15415563#comment-15415563
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Xiaoyu Yao commented on HDFS-10690:
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bq. I tried to replace TreeMap with linkedHashMap, but found LinkedHashMap
lacks of function "ceilingEntry" or similar alternative, which is key to
implement LRU-based replacement algorithm. LinkedHashMap also can't provide
getYoungest or getEldest or similar functions. That's to say, if we want to use
LinkedHashMap, we actually need to rewrite it. Any comments? Thanks.
[~fenghua_hu]/[~carp84], thanks again for working on this. I think we can use
[org.apache.commons.collections.map.LinkedMap|https://commons.apache.org/proper/commons-collections/jacoco/org.apache.commons.collections4.map/LinkedMap.java.html]
here which provides both firstKey(*eldest*) and lastKey(*youngest*). I
highlight eldest/youngest here because the firstKey/lastKey in the Javadoc is
wrong. Attach a patch for illustration (PS: I have not tested it myself), feel
free to modify/benchmark it and let us know the results. Hope that helps.
> 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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