From core-dev-return-32324-apmail-hadoop-core-dev-archive=hadoop.apache.org@hadoop.apache.org Thu Mar 06 05:10:15 2008 Return-Path: Delivered-To: apmail-hadoop-core-dev-archive@www.apache.org Received: (qmail 11745 invoked from network); 6 Mar 2008 05:10:14 -0000 Received: from hermes.apache.org (HELO mail.apache.org) (140.211.11.2) by minotaur.apache.org with SMTP; 6 Mar 2008 05:10:14 -0000 Received: (qmail 70662 invoked by uid 500); 6 Mar 2008 05:10:10 -0000 Delivered-To: apmail-hadoop-core-dev-archive@hadoop.apache.org Received: (qmail 70653 invoked by uid 500); 6 Mar 2008 05:10:10 -0000 Mailing-List: contact core-dev-help@hadoop.apache.org; run by ezmlm Precedence: bulk List-Help: List-Unsubscribe: List-Post: List-Id: Reply-To: core-dev@hadoop.apache.org Delivered-To: mailing list core-dev@hadoop.apache.org Received: (qmail 70644 invoked by uid 99); 6 Mar 2008 05:10:10 -0000 Received: from nike.apache.org (HELO nike.apache.org) (192.87.106.230) by apache.org (qpsmtpd/0.29) with ESMTP; Wed, 05 Mar 2008 21:10:10 -0800 X-ASF-Spam-Status: No, hits=-2000.0 required=10.0 tests=ALL_TRUSTED X-Spam-Check-By: apache.org Received: from [140.211.11.140] (HELO brutus.apache.org) (140.211.11.140) by apache.org (qpsmtpd/0.29) with ESMTP; Thu, 06 Mar 2008 05:09:30 +0000 Received: from brutus (localhost [127.0.0.1]) by brutus.apache.org (Postfix) with ESMTP id 25E0E234C089 for ; Wed, 5 Mar 2008 21:08:58 -0800 (PST) Message-ID: <617883053.1204780138152.JavaMail.jira@brutus> Date: Wed, 5 Mar 2008 21:08:58 -0800 (PST) From: "Amar Kamat (JIRA)" To: core-dev@hadoop.apache.org Subject: [jira] Commented: (HADOOP-910) Reduces can do merges for the on-disk map output files in parallel with their copying MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Virus-Checked: Checked by ClamAV on apache.org [ https://issues.apache.org/jira/browse/HADOOP-910?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12575562#action_12575562 ] Amar Kamat commented on HADOOP-910: ----------------------------------- You will gain from on-disk merging if {noformat} 1) your map outputs are larger than the in-memory filesystem 2) if the reducers are idle (waiting for the maps) 2.1) too many lost trackers i.e re-execution 2.2) network is loaded 2.3) there are waves of maps. {noformat} In case of (1) and (2.3) there is a definite gain but (2.1) and (2.2) are opportunistic cases. > Reduces can do merges for the on-disk map output files in parallel with their copying > ------------------------------------------------------------------------------------- > > Key: HADOOP-910 > URL: https://issues.apache.org/jira/browse/HADOOP-910 > Project: Hadoop Core > Issue Type: Improvement > Components: mapred > Reporter: Devaraj Das > Assignee: Amar Kamat > Fix For: 0.17.0 > > Attachments: HADOOP-910-review.patch, HADOOP-910.patch, HADOOP-910.patch, HADOOP-910.patch > > > Proposal to extend the parallel in-memory-merge/copying, that is being done as part of HADOOP-830, to the on-disk files. > Today, the Reduces dump the map output files to disk and the final merge happens only after all the map outputs have been collected. It might make sense to parallelize this part. That is, whenever a Reduce has collected io.sort.factor number of segments on disk, it initiates a merge of those and creates one big segment. If the rate of copying is faster than the merge, we can probably have multiple threads doing parallel merges of independent sets of io.sort.factor number of segments. If the rate of copying is not as fast as merge, we stand to gain a lot - at the end of copying of all the map outputs, we will be left with a small number of segments for the final merge (which hopefully will feed the reduce directly (via the RawKeyValueIterator) without having to hit the disk for writing additional output segments). > If the disk bandwidth is higher than the network bandwidth, we have a good story, I guess, to do such a thing. -- This message is automatically generated by JIRA. - You can reply to this email to add a comment to the issue online.