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