[jira] Created: (HADOOP-7016) Optimize locking codepath in LocalDirAllocator.getLocalPathToRead() and reduce creating many Path objects

["Rajesh Balamohan (JIRA)" <ji...@apache.org>]

Optimize locking codepath in LocalDirAllocator.getLocalPathToRead() and reduce creating many Path objects
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                 Key: HADOOP-7016
                 URL: https://issues.apache.org/jira/browse/HADOOP-7016
             Project: Hadoop Common
          Issue Type: Bug
          Components: fs
    Affects Versions: 0.20.2
            Reporter: Rajesh Balamohan
             Fix For: 0.20.2


LocalDirAllocator.getLocalPathToRead() is called from TaskTracker to retrieve temporary map output location. This method is synchronized and amount of time spent on this method directly affects the performance of TaskTracker's ability to serve faster.  This JIRA is created to reduce the additional fat in getLocalPathToRead() and reduce creating the number of Path obejcts which are expensive.

1. getLocalPathToRead() internally calls RawLocalFileSystem.exists() in order to check the existence of a local file. RawLocalFileSystem.exists() internally creates a FileStatus object which needs to be populated with lastModified, length, directory details etc. This might not be necessary for just checking  file existence.

2. Creating a Path object out of a string requires fair amount of processing. It might be worth to store "localDirsPath" in LocalDirAllocator instead of storing "localDirs". This would avoid the number of times Path() object is created in LocalDirAllocator.

Any other codepath using LocalDirAllocator would also benefit from the above 2 optimizations.

The attached patch addresses the above 2 issues.

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