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From "Todd Lipcon (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-7714) Add support in native libs for OS buffer cache management
Date Thu, 06 Oct 2011 18:28:30 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-7714?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13122136#comment-13122136
] 

Todd Lipcon commented on HADOOP-7714:
-------------------------------------

That's a good point, Nathan. That was the original thinking behind the "offset - 1024" - but
probably better to actually do "offset - CACHE_DROP_LAG", setting CACHE_DROP_LAG to a couple
of MB.

The readahead certainly makes a big difference in the shuffle. I haven't run comparisons with
each change on/off yet, but will be interesting to see. I think the issue is that Linux's
native readahead is not very aggressive, and based on some heuristics which don't necessarily
kick in immediately. Another thing which might be kicking in here is that in mm/readahead.c:page_cache_async_readahead,
it checks {{bdi_read_congested}} on the backing device for the file before reading ahead.
if it detects that the block device is congested (as it will be during MR), readahead is disabled
at least in this code path (thus making the problem worse, not better!)
                
> Add support in native libs for OS buffer cache management
> ---------------------------------------------------------
>
>                 Key: HADOOP-7714
>                 URL: https://issues.apache.org/jira/browse/HADOOP-7714
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: native
>    Affects Versions: 0.24.0
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>         Attachments: hadoop-7714-20s-prelim.txt
>
>
> Especially in shared HBase/MR situations, management of the OS buffer cache is important.
Currently, running a big MR job will evict all of HBase's hot data from cache, causing HBase
performance to really suffer. However, caching of the MR input/output is rarely useful, since
the datasets tend to be larger than cache and not re-read often enough that the cache is used.
Having access to the native calls {{posix_fadvise}} and {{sync_data_range}} on platforms where
they are supported would allow us to do a better job of managing this cache.

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