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From "Sharad Agarwal (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-6502) DistributedFileSystem#listStatus is very slow when listing a directory with a size of 1300
Date Fri, 22 Jan 2010 08:42:21 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-6502?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12803636#action_12803636

Sharad Agarwal commented on HADOOP-6502:

We can do something like:
+    if (conf.getBoolean("mapred.jobconf.notpresent", false)) {
+      return;//classpath check already done and jobconf is not in classpath.
+    }
     try {
       Class<?> jobConfClass = 
@@ -86,9 +89,11 @@ public class ReflectionUtils {
         Method configureMethod = 
           jobConfigurableClass.getMethod("configure", jobConfClass);
         configureMethod.invoke(theObject, conf);
     } catch (ClassNotFoundException e) {
       //JobConf/JobConfigurable not in classpath. no need to configure
+      conf.setBoolean("mapred.jobconf.notpresent", true);
I think the performance issue only comes if jobconf is not in classpath resulting in ClassNotFoundException
on each check. Hairong, can you please verify if the above fix helps ?

> DistributedFileSystem#listStatus is very slow when listing a directory with a size of
> ------------------------------------------------------------------------------------------
>                 Key: HADOOP-6502
>                 URL: https://issues.apache.org/jira/browse/HADOOP-6502
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: util
>    Affects Versions: 0.20.0
>            Reporter: Hairong Kuang
>            Priority: Critical
>             Fix For: 0.20.2, 0.21.0, 0.22.0
> When listing a directory of around 1300 children, it takes hundreds of milliseconds.
It turns out the slowdowness is caused by the change made by HADOOP-4187. The return value
of listStatus is an array of FileStatus. When deserializing each element of the array, ReflectionUtils#newInstance(Class<T>,
Configuration) is called and then calls setConf, which calls setJobConf. SetJobConf checks
if JobConf is on the class path by calling Configuration#getClassByName. Even though Configuration#getClassByName
tries to optimize the lookup using a cached map, but since JobConf is not in the class path,
so it is not in the cache. Every checkup ends up calling Class.ForName which is very expensive.
Deserializing an array of 1300 entries requires calling of Class#ForName 1300 times!

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