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