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From "Josh Elser (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-665) large values, complex iterator stacks, and RFile readers can consume a surprising amount of memory
Date Mon, 02 Jul 2012 13:19:21 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-665?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13405057#comment-13405057
] 

Josh Elser commented on ACCUMULO-665:
-------------------------------------

The big area that this can present itself is via the seekColumnFamilies on the SortedKeyValueIterator#seek()
method. In the IntersectingIterator inside the core module, you could run into "excessive"
memory usage.

Say you're intersecting over the two terms "foo" and "bar" and that you have some column "documents"
which is directly before the term "foo". Assume the keys in the "documents" column are in
their own locality group and have very large Values associated with them. The IntersectingIterator
only uses any seek-column-families that are passed in but does not set any itself. Meaning,
even though the "documents" column is in its own section of the RFile, by not specifically
setting the seek column families for each term to just the term itself, the underlying Accumulo
code will still open up all locality groups (the one for "documents" and the default locality
group).

The javadoc for SortedKeyValueIterator should also be updated to inform other users of the
implications that (not) setting the seekColumnFamilies has.
                
> large values, complex iterator stacks, and RFile readers can consume a surprising amount
of memory
> --------------------------------------------------------------------------------------------------
>
>                 Key: ACCUMULO-665
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-665
>             Project: Accumulo
>          Issue Type: Bug
>          Components: tserver
>    Affects Versions: 1.5.0, 1.4.0
>         Environment: large cluster
>            Reporter: Eric Newton
>            Assignee: Eric Newton
>            Priority: Minor
>
> On a production cluster, with a complex iterator tree, a large value (~350M) was causing
a 4G tserver to fail with out-of-memory.
> There were several factors contributing to the problem:
> # a bug: the query should not have been looking to the big data
> # complex iterator tree, causing many copies of the data to be held at the same time
> # RFile doubles the buffer it uses to load values, and continues to use that large buffer
for future values
> This ticket is for the last point.  If we know we're not even going to look at the value,
we can read past it without storing it in memory.  It is surprising that skipping past a large
value would cause the server to run out of memory, especially since it should fit into memory
enough times to be returned to the caller.

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