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https://issues.apache.org/jira/browse/MAPREDUCE-6197?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15333791#comment-15333791
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Junping Du commented on MAPREDUCE-6197:
---------------------------------------
Thanks [~jianhe] for review and comments.
bq. one question is how/why do you choose such policy for determining the
weight?
That's good question. To control cache size of a LoadingCache, we can either to
use maximumSize directly or maximumWeight. The reason to choose maximumWeight
instead of maximumSize is each cache item here is a flexible size which depends
on {{key size + value size}}. It means if we use a fixed maximumSize, we still
not sure how much memory it could end up with. The another reason is to keep
consistent with what we have in HIVE-9912. If we found any issue with current
settings/policy in large production deployment in future, we can change both
side together.
> Cache MapOutputLocations in ShuffleHandler
> ------------------------------------------
>
> Key: MAPREDUCE-6197
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-6197
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Affects Versions: 2.6.0
> Reporter: Siddharth Seth
> Assignee: Junping Du
> Attachments: MAPREDUCE-6197.patch
>
>
> ShuffleHandler currently seems to create a map of mapId - mapInfo (file.out /
> index information) when it receives a message.
> This should be caching map info across requests, so that the a scan of all
> directories is not required for each reducer fetching from the same map.
> Also, the scan for each map output / index file is performed twice per mapId
> within a request. In populateHeaders - once in the call to getMapOutputInfo,
> and then directly in the method.
> For an invocation where we do end up with more than 1000 (default) mapIds in
> a single call, and don't cache them in the map - the path constructed for
> such entries will be invalid. This is highly unlikely to be the case though,
> until there's proper caching.
> {code}
> MapOutputInfo info = mapOutputInfoMap.get(mapId);
> if (info == null) {
> info = getMapOutputInfo(outputBasePathStr, mapId, reduceId, user);
> }
> {code}
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