Re: how is number of mappers determined in mapside join?
Thanks Bejoy! That helps. On Tue, Mar 20, 2012 at 12:10 AM, Bejoy Ks bejoy...@yahoo.com wrote: Hi Bruce From my understanding, that formula is not for CombineFileInputFormat but for other basic Input Formats. I'd just brief you on CombineFileInputFormat to get things more clear. In the default TextInputFormat every hdfs block is processed by a mapper. But if the files are small say 5Mb, spawning that may mappers would be an overkill for the job. So here we use Combine file input format,where one mapper process more than one small file and the min data size a mapper should process is defined by the min split size and the maximum data that a mapper can process is defined by max split size. ie data processed by a mapper is guaranteed to be not less than the min split size and not more than max split size specified. As you asked, if you are looking at more mappers in CombinedFileInputFormat then reduce the value of Max split Size. Bump it down to 32 mb (your block size) and just try it out. Or If you are looking at num mappers = num blocks, just change the input format in hive. By the way 32 mb is too small for a hdfs block size, you may hit NN memory issues pretty soon. Consider increasing it at least to 64 mb, though all larger clusters use either 128 or 256 Mb blocks. Hope it helps!.. Regards Bejoy -- *From:* Bruce Bian weidong@gmail.com *To:* user@hive.apache.org; Bejoy Ks bejoy...@yahoo.com *Sent:* Monday, March 19, 2012 7:48 PM *Subject:* Re: how is number of mappers determined in mapside join? Hi Bejoy, Thanks for your reply. The function is from the book, Hadoop The Definitive Guide 2nd edition. On page 203 there is The split size is calculated by the formula (see the computeSplitSize() method in FileInputFormat): max(minimumSize, min(maximumSize, blockSize)) by default:minimumSize blockSize maximumSize so the split size is blockSize. And I've actually used the HDFS block size to control the number of mappers launched before. So as to your response, do you mean that any value of the data between 1B and 256MB is OK for the mappers to process? Then the only way I can think of to increase the #mappers is to reduce the max split size. Regards, Bruce On Mon, Mar 19, 2012 at 8:48 PM, Bejoy Ks bejoy...@yahoo.com wrote: Hi Bruce In map side join the smaller table is loader in memory and hence the number of mappers is dependent only on the data on larger table. Say If CombineHiveInputFormat is used and we have our hdfs block size as 32 mb, min split size as 1B and max split size as 256 mb. Which means one mapper would be processing data chunks not less than 1B and not more than 256 MB. So based on that mappers would be triggered, so a possibility in your case mapper 1 - 200 MB mapper 2 - 120 MB mapper 3 - 140 MB Every mapper is processing data whose size id between 1B and 256 MB. Totally of 460 MB, your table size. I'm not sure of the formula you posted here, Can you point me to the document from which you got this? Regards Bejoy -- *From:* Bruce Bian weidong@gmail.com *To:* user@hive.apache.org *Sent:* Monday, March 19, 2012 2:42 PM *Subject:* how is number of mappers determined in mapside join? Hi there, when I'm executing the following queries in hive set hive.auto.convert.join = true; CREATE TABLE IDAP_ROOT as SELECT a.*,b.acnt_no FROM idap_pi_root a LEFT OUTER JOIN idap_pi_root_acnt b ON a.acnt_id=b.acnt_id the number of mappers to run in the mapside join is 3, how is it determined? When launching a job in hadoop mapreduce, i know it's determined by the function max(Min split size, min(Max split size, HDFS blockSize)) which in my configuration is max(1B, min(256MB ,32MB)=32MB and the two tables are 460MB and 1.5MB respectively. Thus I thought the mappers to launch to be around 15, which is not the case. Thanks Bruce
how is number of mappers determined in mapside join?
Hi there, when I'm executing the following queries in hive set hive.auto.convert.join = true; CREATE TABLE IDAP_ROOT as SELECT a.*,b.acnt_no FROM idap_pi_root a LEFT OUTER JOIN idap_pi_root_acnt b ON a.acnt_id=b.acnt_id the number of mappers to run in the mapside join is 3, how is it determined? When launching a job in hadoop mapreduce, i know it's determined by the function max(Min split size, min(Max split size, HDFS blockSize)) which in my configuration is max(1B, min(256MB ,32MB)=32MB and the two tables are 460MB and 1.5MB respectively. Thus I thought the mappers to launch to be around 15, which is not the case. Thanks Bruce
Re: how is number of mappers determined in mapside join?
Hi Bruce In map side join the smaller table is loader in memory and hence the number of mappers is dependent only on the data on larger table. Say If CombineHiveInputFormat is used and we have our hdfs block size as 32 mb, min split size as 1B and max split size as 256 mb. Which means one mapper would be processing data chunks not less than 1B and not more than 256 MB. So based on that mappers would be triggered, so a possibility in your case mapper 1 - 200 MB mapper 2 - 120 MB mapper 3 - 140 MB Every mapper is processing data whose size id between 1B and 256 MB. Totally of 460 MB, your table size. I'm not sure of the formula you posted here, Can you point me to the document from which you got this? Regards Bejoy From: Bruce Bian weidong@gmail.com To: user@hive.apache.org Sent: Monday, March 19, 2012 2:42 PM Subject: how is number of mappers determined in mapside join? Hi there, when I'm executing the following queries in hive set hive.auto.convert.join = true; CREATE TABLE IDAP_ROOT as SELECT a.*,b.acnt_no FROM idap_pi_root a LEFT OUTER JOIN idap_pi_root_acnt b ON a.acnt_id=b.acnt_id the number of mappers to run in the mapside join is 3, how is it determined? When launching a job in hadoop mapreduce, i know it's determined by the function max(Min split size, min(Max split size, HDFS blockSize)) which in my configuration is max(1B, min(256MB ,32MB)=32MB and the two tables are 460MB and 1.5MB respectively. Thus I thought the mappers to launch to be around 15, which is not the case. Thanks Bruce
Re: how is number of mappers determined in mapside join?
Hi Bejoy, Thanks for your reply. The function is from the book, Hadoop The Definitive Guide 2nd edition. On page 203 there is The split size is calculated by the formula (see the computeSplitSize() method in FileInputFormat): max(minimumSize, min(maximumSize, blockSize)) by default:minimumSize blockSize maximumSize so the split size is blockSize. And I've actually used the HDFS block size to control the number of mappers launched before. So as to your response, do you mean that any value of the data between 1B and 256MB is OK for the mappers to process? Then the only way I can think of to increase the #mappers is to reduce the max split size. Regards, Bruce On Mon, Mar 19, 2012 at 8:48 PM, Bejoy Ks bejoy...@yahoo.com wrote: Hi Bruce In map side join the smaller table is loader in memory and hence the number of mappers is dependent only on the data on larger table. Say If CombineHiveInputFormat is used and we have our hdfs block size as 32 mb, min split size as 1B and max split size as 256 mb. Which means one mapper would be processing data chunks not less than 1B and not more than 256 MB. So based on that mappers would be triggered, so a possibility in your case mapper 1 - 200 MB mapper 2 - 120 MB mapper 3 - 140 MB Every mapper is processing data whose size id between 1B and 256 MB. Totally of 460 MB, your table size. I'm not sure of the formula you posted here, Can you point me to the document from which you got this? Regards Bejoy -- *From:* Bruce Bian weidong@gmail.com *To:* user@hive.apache.org *Sent:* Monday, March 19, 2012 2:42 PM *Subject:* how is number of mappers determined in mapside join? Hi there, when I'm executing the following queries in hive set hive.auto.convert.join = true; CREATE TABLE IDAP_ROOT as SELECT a.*,b.acnt_no FROM idap_pi_root a LEFT OUTER JOIN idap_pi_root_acnt b ON a.acnt_id=b.acnt_id the number of mappers to run in the mapside join is 3, how is it determined? When launching a job in hadoop mapreduce, i know it's determined by the function max(Min split size, min(Max split size, HDFS blockSize)) which in my configuration is max(1B, min(256MB ,32MB)=32MB and the two tables are 460MB and 1.5MB respectively. Thus I thought the mappers to launch to be around 15, which is not the case. Thanks Bruce
