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https://issues.apache.org/jira/browse/HDFS-14617?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16908206#comment-16908206
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Stephen O'Donnell commented on HDFS-14617:
------------------------------------------
There has been some good discussion on the PR linked to this Jira with
[~jojochuang] and [~hexiaoqiao] and I think we are close to having this change
ready for the 3.3 branch, so if anyone else would like to review please check
out the latest PR.
If we agree to commit this change there will be a few followup Jiras we can
open:
# Implement parallel loading for the inode_reference and snapshot_diff
sections of the image. This change is large enough already and it only
addresses the INODE and INODE_DIR sections.
# OIV can handle images in the new format with no issues on the 3.3 branch,
but it would be good to improve OIV so it can create an image with the
additional sub-section entries in the image index.
# Nice to have would be OIV being able to load and process an image in
parallel with the sub-sections. I have thought about this in detail to see if
it is feasible, and it may depend on the output format requested.
> Improve fsimage load time by writing sub-sections to the fsimage index
> ----------------------------------------------------------------------
>
> Key: HDFS-14617
> URL: https://issues.apache.org/jira/browse/HDFS-14617
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: namenode
> Reporter: Stephen O'Donnell
> Assignee: Stephen O'Donnell
> Priority: Major
> Attachments: HDFS-14617.001.patch, ParallelLoading.svg,
> SerialLoading.svg, dirs-single.svg, flamegraph.parallel.svg,
> flamegraph.serial.svg, inodes.svg
>
>
> Loading an fsimage is basically a single threaded process. The current
> fsimage is written out in sections, eg iNode, iNode_Directory, Snapshots,
> Snapshot_Diff etc. Then at the end of the file, an index is written that
> contains the offset and length of each section. The image loader code uses
> this index to initialize an input stream to read and process each section. It
> is important that one section is fully loaded before another is started, as
> the next section depends on the results of the previous one.
> What I would like to propose is the following:
> 1. When writing the image, we can optionally output sub_sections to the
> index. That way, a given section would effectively be split into several
> sections, eg:
> {code:java}
> inode_section offset 10 length 1000
> inode_sub_section offset 10 length 500
> inode_sub_section offset 510 length 500
>
> inode_dir_section offset 1010 length 1000
> inode_dir_sub_section offset 1010 length 500
> inode_dir_sub_section offset 1010 length 500
> {code}
> Here you can see we still have the original section index, but then we also
> have sub-section entries that cover the entire section. Then a processor can
> either read the full section in serial, or read each sub-section in parallel.
> 2. In the Image Writer code, we should set a target number of sub-sections,
> and then based on the total inodes in memory, it will create that many
> sub-sections per major image section. I think the only sections worth doing
> this for are inode, inode_reference, inode_dir and snapshot_diff. All others
> tend to be fairly small in practice.
> 3. If there are under some threshold of inodes (eg 10M) then don't bother
> with the sub-sections as a serial load only takes a few seconds at that scale.
> 4. The image loading code can then have a switch to enable 'parallel loading'
> and a 'number of threads' where it uses the sub-sections, or if not enabled
> falls back to the existing logic to read the entire section in serial.
> Working with a large image of 316M inodes and 35GB on disk, I have a proof of
> concept of this change working, allowing just inode and inode_dir to be
> loaded in parallel, but I believe inode_reference and snapshot_diff can be
> make parallel with the same technique.
> Some benchmarks I have are as follows:
> {code:java}
> Threads 1 2 3 4
> --------------------------------
> inodes 448 290 226 189
> inode_dir 326 211 170 161
> Total 927 651 535 488 (MD5 calculation about 100 seconds)
> {code}
> The above table shows the time in seconds to load the inode section and the
> inode_directory section, and then the total load time of the image.
> With 4 threads using the above technique, we are able to better than half the
> load time of the two sections. With the patch in HDFS-13694 it would take a
> further 100 seconds off the run time, going from 927 seconds to 388, which is
> a significant improvement. Adding more threads beyond 4 has diminishing
> returns as there are some synchronized points in the loading code to protect
> the in memory structures.
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