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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 15:59:30 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-7714?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13122022#comment-13122022

Todd Lipcon commented on HADOOP-7714:

I'll try to prepare some graphs of node metrics with/without the patches. What I was seeing
last night was that, with the fadvises in, the CPU and network graphs were a lot "smoother"
- ie the nodes were kept pretty constant at 95-100% utilization, and the ratio of CPU to sys
to iowait was very smooth and constant within each phase of the terasort. Without fadvise,
the graph is very "jagged". I think we suffer from a lot of "hurry up and wait" - the threads
run for a little bit, then hit a read that isn't in cache, and stall for hundreds of milliseconds.
During that time, other disks might have excess IO capacity and end up sitting idle.

I have a talk on performance at Hadoop World, so I'll definitely be preparing more graphs
and explanations of these effects before then :)
> 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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