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From "Devaraj Das (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-1027) Fix the RAM FileSystem/Merge problems (reported in HADOOP-1014)
Date Fri, 23 Feb 2007 16:26:05 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-1027?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Devaraj Das updated HADOOP-1027:

    Attachment: 1027-new3.patch

Attached is a patch that applies cleanly to the current trunk. Also, set the merge threshold
count (for inmem merge) to 1000 files in hadoop-default.xml. 
I think it makes sense to have merge based also on the count of the number of files accumulated
in ramfs, by default. We will then have a better control of the memory usage. It will avoid
the various datastructures growing out of bounds (and thereby giving OutOfMemory errors) when
there are too many maps (generating small outputs) and the size of the ramfs is large (so
many many map outputs can be accomodated). 
Hopefully, since we can accomodate so many map outputs in the ramfs in the first place, the
sizes of the outputs are, quite likely, small and hence the on-disk spills are small too.
So although disk IO would be there, think it would not be the major bottleneck. 

> Fix the RAM FileSystem/Merge problems (reported in HADOOP-1014)
> ---------------------------------------------------------------
>                 Key: HADOOP-1027
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1027
>             Project: Hadoop
>          Issue Type: Bug
>          Components: mapred
>            Reporter: Devaraj Das
>         Assigned To: Devaraj Das
>         Attachments: 1027-new.patch, 1027-new2.patch, 1027-new3.patch
> 1) Merge algorithm implementation does not delete empty segments (sequence files with
no key/val data) in cases where single level merges don't happen on those segments (due to
the check "numberOfSegmentsRemaining <= factor" returning true). This affected the in-mem
merge in a subtle way :-
> For the in-mem merge, the merge-spill-file is given the same name as the name of the
0th entry file in the ramfs. If this file was an empty file, then it would not get deleted
from the ramfs, and if the subsequent merge on ramfs chose the same name for the merge-spill-file,
it would overwrite the previously created spill. This led to the inconsistent output sizes.
> 2) The InMemoryFileSystem has a "close" method which is not protected (only method where
pathToFileAttribs map is modified without first locking the InMemoryFileSystem instance) and
that quite likely leads to ConcurrentModificationException if some thread calls InMemoryFileSystem.close
(due to some exception) and some other thread is in the process of doing InMemoryFileSystem.getFiles().
However, this problem will not affect the correctness of the merge process (anyway the task
is going to fail) and the more important thing is that some other exception happened (like
insufficient disk space and so map outputs could not be written) which may not be related
to the merge process at all.
> 3) The number of outputs that is merged at once in RAM should be limited. This is to
prevent OutOfMemory errors. Consider a case where there are 10s of thousands of maps and all
maps generate empty outputs. Given the default size of the RAM FS as 75 MB, we can possibly
accomodate lots of map outputs in RAM without doing any merge but it also results in the various
other data structures exploding in size. We have to do a trade off here especially because
the inmem-merging is done in the TaskTracker process which already is under a good amount
of memory pressure.

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