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https://issues.apache.org/jira/browse/HDFS-10690?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15397871#comment-15397871
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Xiaoyu Yao commented on HDFS-10690:
-----------------------------------
[~fenghua_hu], thanks for reporting the issue and posting the fix. If the goal
is to avoid the TreeMap overhead, can we use LinkedHashMap with the following
constructor ({{accessOrder=true}}) to simplify the code changes?
{code}
public LinkedHashMap(int initialCapacity,
float loadFactor,
boolean accessOrder)
Constructs an empty LinkedHashMap instance with the specified initial capacity,
load factor and ordering mode.
Parameters:
initialCapacity - the initial capacity
loadFactor - the load factor
accessOrder - the ordering mode - true for access-order, false for
insertion-order
{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
>
> 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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