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