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From "Eli Reisman (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (GIRAPH-301) InputSplit Reservations are clumping, leaving many workers asleep while other process too many splits and get overloaded.
Date Wed, 15 Aug 2012 17:55:38 GMT

    [ https://issues.apache.org/jira/browse/GIRAPH-301?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13435348#comment-13435348
] 

Eli Reisman commented on GIRAPH-301:
------------------------------------

one thing we can do to reduce ZK reads at the beginning is to check for RESERVED before FINISHED
splits, since we will encounter many more of those on the critical (busy) first pass to claim
nodes, and the loop can continue without doing the 2nd ZK read to check for FINSIHED (there
shouldn't be too many of those in the first pass). This will add reads for workers that awaken
after the first pass and look for another split to claim, since by then most will be finished.
But it avoids the mad rush. I might experiment with this change and see what effect is has.Given
the fact that we want to operate within pretty tight memory constraints, trying to tune for
1 worker per split when possible seems like the right move. Any other ideas? I do think one
was or another the idea of workers going back to sleep the first time they attempt to claim
a split is going to guarantee the clumping behavior if left as-is.
 
                
> InputSplit Reservations are clumping, leaving many workers asleep while other process
too many splits and get overloaded.
> -------------------------------------------------------------------------------------------------------------------------
>
>                 Key: GIRAPH-301
>                 URL: https://issues.apache.org/jira/browse/GIRAPH-301
>             Project: Giraph
>          Issue Type: Improvement
>          Components: bsp, graph, zookeeper
>    Affects Versions: 0.2.0
>            Reporter: Eli Reisman
>            Assignee: Eli Reisman
>              Labels: patch
>             Fix For: 0.2.0
>
>         Attachments: GIRAPH-301-1.patch
>
>
> With recent additions to the codebase, users here have noticed many workers are able
to load input splits extremely quickly, and this has altered the behavior of Giraph during
INPUT_SUPERSTEP when using the current algorithm for split reservations. A few workers process
multiple splits (often overwhelming Netty and getting GC errors as they attempt to offload
too much data too quick) while many (often most) of the others just sleep through the superstep,
never successfully participating at all.
> Essentially, the current algo is:
> 1. scan input split list, skipping nodes that are marked "Finsihed"
> 2. grab the first unfinished node in the list (reserved or not) and check its reserved
status.
> 3. if not reserved, attempt to reserve & return it if successful.
> 4. if the first one you check is already taken, sleep for way too long and only wake
up if another worker finishes a split, then contend with that worker for another split, while
the majority of the split list might sit idle, not actually checked or claimed by anyone yet.
> This does not work. By making a few simple changes (and acknowledging that ZK reads are
cheap, only writes are not) this patch is able to get every worker involved, and keep them
in the game, ensuring that the INPUT_SUPERSTEP passes quickly and painlessly, and without
overwhelming Netty by spreading the memory load the split readers bear more evenly. If the
giraph.splitmb and -w options are set correctly, behavior is now exactly as one would expect
it to be.
> This also results in INPUT_SUPERSTEP passing more quickly, and survive the INPUT_SUPERSTEP
for a given data load on less Hadoop memory slots.
>  

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