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From "Arpit Agarwal (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11535) Performance analysis of new DFSNetworkTopology#chooseRandom
Date Mon, 01 May 2017 17:16:04 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11535?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15991119#comment-15991119

Arpit Agarwal commented on HDFS-11535:

The tests look good, thanks for adding them [~vagarychen].

Minor comments:
# testPercentageStorageType and testPercentageStorageTypeWithMixedTopology seem to be testing
90% archival and 10% disk. Did you mean to test the opposite?
# You can move the @Ignore annotation to the class instead of each test case method.
# The following code can be replaced with {{assertNotNull(node)}}, in multiple places.
        if (node == null) {
          LOG.error("node not found, should not happen!");
          throw new IllegalArgumentException();
# Few code style fixes e.g. spaces around operators in {{i<OP_NUM}}, {{NODE_NUM*chance}}

> Performance analysis of new DFSNetworkTopology#chooseRandom
> -----------------------------------------------------------
>                 Key: HDFS-11535
>                 URL: https://issues.apache.org/jira/browse/HDFS-11535
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: namenode
>            Reporter: Chen Liang
>            Assignee: Chen Liang
>         Attachments: HDFS-11535.001.patch, HDFS-11535.002.patch, HDFS-11535.003.patch,
> This JIRA is created to post the results of some performance experiments we did.  For
those who are interested, please the attached .pdf file for more detail. The attached patch
file includes the experiment code we ran. 
> The key insights we got from these tests is that: although *the new method outperforms
the current one in most cases*. There is still *one case where the current one is better*.
Which is when there is only one storage type in the cluster, and we also always look for this
storage type. In this case, it is simply a waste of time to perform storage-type-based pruning,
blindly picking up a random node (current methods) would suffice.
> Therefore, based on the analysis, we propose to use a *combination of both the old and
the new methods*:
> say, we search for a node of type X, since now inner node all keep storage type info,
we can *just check root node to see if X is the only type it has*. If yes, blindly picking
a random leaf will work, so we simply call the old method, otherwise we call the new method.
> There is still at least one missing piece in this performance test, which is garbage
collection. The new method does a few more object creation when doing the search, which adds
overhead to GC. I'm still thinking of any potential optimization but this seems tricky, also
I'm not sure whether this optimization worth doing at all. Please feel free to leave any comments/suggestions.
> Thanks [~arpitagarwal] and [~szetszwo] for the offline discussion.

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