[jira] Commented: (HADOOP-3248) Improve Namenode startup performance

["girish vaitheeswaran (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-3248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12591121#action_12591121 ] 

girish vaitheeswaran commented on HADOOP-3248:
----------------------------------------------

As mentioned loadFSImage() is where most of the time is now spent since saveImage() is optmized well with the above changes. Within loadFSImage() most of the time is being spent in getExistingPathINode. 

Here is some data point on that (out of close to 7 minutes)

Time taken in getExistingPathINode : 4 minutes
Total number of calls to getExistingPathINode : 19941941

This was for an image file of size 2.5GB with close to 20 million files.

Since getExistingPathINode() does not have any significant object allocation, most of the optimization work going forward would entail parallelizing the operation in this method and use up all processors on the machine running the namenode. At this point only one processor is utilized and the remaining processors are idle. One other point to mention is that we are not I/O bound in this whole recovery operation and optimizations need to almost entirely be CPU focussed.




> Improve Namenode startup performance
> ------------------------------------
>
>                 Key: HADOOP-3248
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3248
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs
>            Reporter: girish vaitheeswaran
>         Attachments: FSImage.patch
>
>
> One of the things that would need to be addressed as part of Namenode scalability is the HDFS recovery performance especially in scenarios where the number of files is large. There are instances where the number of files are in the vicinity of 20 million and in such cases the time taken for namenode startup is prohibitive. Here are some benchmark numbers on the time taken for namenode startup. These times do not include the time to process block reports.
> Default scenario for 20 million files with the max  java heap size set to 14GB : 40 minutes
> Tuning various java options such as young size, parallel garbage collection, initial java heap size : 14 minutes
> As can be seen, 14 minutes is still a long time for the namenode to recover and code changes are required to bring this time down further. To this end some prototype optimizations were done to reduce this time. Based on some timing analysis saveImage and loadFSImage where the primary methods that were consuming most of the time. Most of the time was being spent on doing object allocations. The goal of the optimizations is to reduce the number of memory allocations as much as possible.
> Optimization 1: saveImage() 
> ======================
> Avoid allocation of the UTF8 object.
> Old code
> =======
> new UTF8(fullName).write(out);
> New Code
> ========
> out.writeUTF(fullName)
> Optimization 2: saveImage()
> ======================
> Avoid object allocation of the PermissionStatus Object and the FsPermission object. This is to be done for Directories and for files.
> Old code
> =======
> fileINode.getPermissionStatus().write(out)
> New Code
> =========
> out.writeBytes(fileINode.getUserName())
> out.writeBytes(fileINode.getGroupName())
> out.writeShort(fileINode.getFsPermission().toShort())
> Optimization 3
> ============
> loadImage() could use the same mechanism where we would avoid allocating the PermissionStatus object and the FsPermission object.
> Optimization 4
> ============
> A hack was tried out to avoid the cost of object allocation from saveImage() where the fullName was being constructed using string concatenation. This optimization also helped improve performance
> Overall these optimizations helped bring down the overall startup time down to slightly over 7 minutes. Most of all the remaining time is now spent in loadFSImage() since we allocate the INode and INodeDirectory objects. Any further optimizations will need to focus on loadFSImage()

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