[jira] [Commented] (HDFS-8611) Improve the performance of retry cache eviction

["Xiaobing Zhou (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-8611?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15124494#comment-15124494 ] 

Xiaobing Zhou commented on HDFS-8611:
-------------------------------------

Thanks [~walter.k.su] for the proposal, however Circular FIFO queue is fixed size. This could be a problem since both PriorityQueue as member of LightWeightCache  and LightWeightGSet as base are of no upper bound regarding number of elements they stored.

> Improve the performance of retry cache eviction
> -----------------------------------------------
>
>                 Key: HDFS-8611
>                 URL: https://issues.apache.org/jira/browse/HDFS-8611
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: Kihwal Lee
>            Assignee: Xiaobing Zhou
>            Priority: Critical
>
> As discussed in HDFS-7609, removing expired entry from retry cache can be costly.  Following is the comment left by [~szetszwo] in HDFS-7609.
> {quote}
> PriorityQueue#remove is O\(n), so that definitely could be problematic. It's odd that there would be so many collisions that this would become noticeable though. Are any of you running a significant number of legacy applications linked to the RPC code before introduction of the retry cache support? If that were the case, then perhaps a huge number of calls are not supplying a call ID, and then the NN is getting a default call ID value from protobuf decoding, thus causing a lot of collisions.
> {quote}
> The priority queue can be improved using a balanced tree as stated in the java comment in LightWeightCache.  We should do it if it could fix the problem.
> {code}
> //LightWeightCache.java
>   /*
>    * The memory footprint for java.util.PriorityQueue is low but the
>    * remove(Object) method runs in linear time. We may improve it by using a
>    * balanced tree. However, we do not yet have a low memory footprint balanced
>    * tree implementation.
>    */
>   private final PriorityQueue<Entry> queue;
> {code}
> BTW, the priority queue is used to evict entries according the expiration time.  All the entries (with any key, i.e. any caller ID) are stored in it.



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