jamal sasha
20121121, 16:38
Kartashov, Andy
20121121, 16:43
Bejoy KS
20121121, 16:50
Kartashov, Andy
20121121, 17:49
Manoj Babu
20121121, 17:58
Mohammad Tariq
20121121, 18:04
Bejoy KS
20121121, 18:21
jamal sasha
20121121, 18:27
Bejoy KS
20121121, 18:34
Manoj Babu
20121122, 04:45


guessing number of reducers.jamal sasha 20121121, 16:38
By default the number of reducers is set to 1..
Is there a good way to guess optimal number of reducers.... Or let's say i have tbs worth of data... mappers are of order 5000 or so... But ultimately i am calculating , let's say, some average of whole data... say average transaction occurring... Now the output will be just one line in one "part"... rest of them will be empty.So i am guessing i need loads of reducers but then most of them will be empty but at the same time one reducer won't suffice.. What's the best way to solve this.. How to guess optimal number of reducers.. Thanks 
RE: guessing number of reducers.Kartashov, Andy 20121121, 16:43
Jamal,
This is what I am using... After you start your job, visit jobtracker's WebUI <ipaddress>:50030 And look for Cluster summary. Reduce Task Capacity shall hint you what optimally set your number to. I could be wrong but it works for me. :) Cluster Summary (Heap Size is *** MB/966.69 MB) Running Map Tasks Running Reduce Tasks Total Submissions Nodes Occupied Map Slots Occupied Reduce Slots Reserved Map Slots Reserved Reduce Slots Map Task Capacity Reduce Task Capacity Avg. Tasks/Node Blacklisted Nodes Excluded Nodes Rgds, AK47 From: jamal sasha [mailto:[EMAIL PROTECTED]] Sent: Wednesday, November 21, 2012 11:39 AM To: [EMAIL PROTECTED] Subject: guessing number of reducers. By default the number of reducers is set to 1.. Is there a good way to guess optimal number of reducers.... Or let's say i have tbs worth of data... mappers are of order 5000 or so... But ultimately i am calculating , let's say, some average of whole data... say average transaction occurring... Now the output will be just one line in one "part"... rest of them will be empty.So i am guessing i need loads of reducers but then most of them will be empty but at the same time one reducer won't suffice.. What's the best way to solve this.. How to guess optimal number of reducers.. Thanks NOTICE: This email message and any attachments are confidential, subject to copyright and may be privileged. Any unauthorized use, copying or disclosure is prohibited. If you are not the intended recipient, please delete and contact the sender immediately. Please consider the environment before printing this email. AVIS : le pr?sent courriel et toute pi?ce jointe qui l'accompagne sont confidentiels, prot?g?s par le droit d'auteur et peuvent ?tre couverts par le secret professionnel. Toute utilisation, copie ou divulgation non autoris?e est interdite. Si vous n'?tes pas le destinataire pr?vu de ce courriel, supprimezle et contactez imm?diatement l'exp?diteur. Veuillez penser ? l'environnement avant d'imprimer le pr?sent courriel 
Re: guessing number of reducers.Bejoy KS 20121121, 16:50
Hi Sasha
In general the number of reduce tasks is chosen mainly based on the data volume to reduce phase. In tools like hive and pig by default for every 1GB of map output there will be a reducer. So if you have 100 gigs of map output then 100 reducers. If your tasks are more CPU intensive then you need lesser volume of data per reducer for better performance results. In general it is better to have the number of reduce tasks slightly less than the number of available reduce slots in the cluster. Regards Bejoy KS Sent from handheld, please excuse typos. Original Message From: jamal sasha <[EMAIL PROTECTED]> Date: Wed, 21 Nov 2012 11:38:38 To: [EMAIL PROTECTED]<[EMAIL PROTECTED]> ReplyTo: [EMAIL PROTECTED] Subject: guessing number of reducers. By default the number of reducers is set to 1.. Is there a good way to guess optimal number of reducers.... Or let's say i have tbs worth of data... mappers are of order 5000 or so... But ultimately i am calculating , let's say, some average of whole data... say average transaction occurring... Now the output will be just one line in one "part"... rest of them will be empty.So i am guessing i need loads of reducers but then most of them will be empty but at the same time one reducer won't suffice.. What's the best way to solve this.. How to guess optimal number of reducers.. Thanks 
RE: guessing number of reducers.Kartashov, Andy 20121121, 17:49
Bejoy,
I've read somethere about keeping number of mapred.reduce.tasks below the reduce task capcity. Here is what I just tested: Output 25Gb. 8DN cluster with 16 Map and Reduce Task Capacity: 1 Reducer  22mins 4 Reducers  11.5mins 8 Reducers  5mins 10 Reducers  7mins 12 Reducers  6:5mins 16 Reducers  5.5mins 8 Reducers have won the race. But Reducers at the max capacity was very clos. :) AK47 From: Bejoy KS [mailto:[EMAIL PROTECTED]] Sent: Wednesday, November 21, 2012 11:51 AM To: [EMAIL PROTECTED] Subject: Re: guessing number of reducers. Hi Sasha In general the number of reduce tasks is chosen mainly based on the data volume to reduce phase. In tools like hive and pig by default for every 1GB of map output there will be a reducer. So if you have 100 gigs of map output then 100 reducers. If your tasks are more CPU intensive then you need lesser volume of data per reducer for better performance results. In general it is better to have the number of reduce tasks slightly less than the number of available reduce slots in the cluster. Regards Bejoy KS Sent from handheld, please excuse typos. ________________________________ From: jamal sasha <[EMAIL PROTECTED]> Date: Wed, 21 Nov 2012 11:38:38 0500 To: [EMAIL PROTECTED]<[EMAIL PROTECTED]> ReplyTo: [EMAIL PROTECTED] Subject: guessing number of reducers. By default the number of reducers is set to 1.. Is there a good way to guess optimal number of reducers.... Or let's say i have tbs worth of data... mappers are of order 5000 or so... But ultimately i am calculating , let's say, some average of whole data... say average transaction occurring... Now the output will be just one line in one "part"... rest of them will be empty.So i am guessing i need loads of reducers but then most of them will be empty but at the same time one reducer won't suffice.. What's the best way to solve this.. How to guess optimal number of reducers.. Thanks NOTICE: This email message and any attachments are confidential, subject to copyright and may be privileged. Any unauthorized use, copying or disclosure is prohibited. If you are not the intended recipient, please delete and contact the sender immediately. Please consider the environment before printing this email. AVIS : le pr?sent courriel et toute pi?ce jointe qui l'accompagne sont confidentiels, prot?g?s par le droit d'auteur et peuvent ?tre couverts par le secret professionnel. Toute utilisation, copie ou divulgation non autoris?e est interdite. Si vous n'?tes pas le destinataire pr?vu de ce courriel, supprimezle et contactez imm?diatement l'exp?diteur. Veuillez penser ? l'environnement avant d'imprimer le pr?sent courriel 
Re: guessing number of reducers.Manoj Babu 20121121, 17:58
Hi,
How to set no of reducers in job conf dynamically? For example some days i am getting 500GB of data on heavy traffic and some days 100GB only. Thanks in advance! Cheers! Manoj. On Wed, Nov 21, 2012 at 11:19 PM, Kartashov, Andy <[EMAIL PROTECTED]>wrote: > Bejoy, > > > > I’ve read somethere about keeping number of mapred.reduce.tasks below the > reduce task capcity. Here is what I just tested: > > > > Output 25Gb. 8DN cluster with 16 Map and Reduce Task Capacity: > > > > 1 Reducer – 22mins > > 4 Reducers – 11.5mins > > 8 Reducers – 5mins > > 10 Reducers – 7mins > > 12 Reducers – 6:5mins > > 16 Reducers – 5.5mins > > > > 8 Reducers have won the race. But Reducers at the max capacity was very > clos. J > > > > AK47 > > > > > > *From:* Bejoy KS [mailto:[EMAIL PROTECTED]] > *Sent:* Wednesday, November 21, 2012 11:51 AM > *To:* [EMAIL PROTECTED] > *Subject:* Re: guessing number of reducers. > > > > Hi Sasha > > In general the number of reduce tasks is chosen mainly based on the data > volume to reduce phase. In tools like hive and pig by default for every 1GB > of map output there will be a reducer. So if you have 100 gigs of map > output then 100 reducers. > If your tasks are more CPU intensive then you need lesser volume of data > per reducer for better performance results. > > In general it is better to have the number of reduce tasks slightly less > than the number of available reduce slots in the cluster. > > Regards > Bejoy KS > > Sent from handheld, please excuse typos. >  > > *From: *jamal sasha <[EMAIL PROTECTED]> > > *Date: *Wed, 21 Nov 2012 11:38:38 0500 > > *To: *[EMAIL PROTECTED]<[EMAIL PROTECTED]> > > *ReplyTo: *[EMAIL PROTECTED] > > *Subject: *guessing number of reducers. > > > > By default the number of reducers is set to 1.. > Is there a good way to guess optimal number of reducers.... > Or let's say i have tbs worth of data... mappers are of order 5000 or so... > But ultimately i am calculating , let's say, some average of whole data... > say average transaction occurring... > Now the output will be just one line in one "part"... rest of them will be > empty.So i am guessing i need loads of reducers but then most of them will > be empty but at the same time one reducer won't suffice.. > What's the best way to solve this.. > How to guess optimal number of reducers.. > Thanks > NOTICE: This email message and any attachments are confidential, subject > to copyright and may be privileged. Any unauthorized use, copying or > disclosure is prohibited. If you are not the intended recipient, please > delete and contact the sender immediately. Please consider the environment > before printing this email. AVIS : le présent courriel et toute pièce > jointe qui l'accompagne sont confidentiels, protégés par le droit d'auteur > et peuvent être couverts par le secret professionnel. Toute utilisation, > copie ou divulgation non autorisée est interdite. Si vous n'êtes pas le > destinataire prévu de ce courriel, supprimezle et contactez immédiatement > l'expéditeur. Veuillez penser à l'environnement avant d'imprimer le présent > courriel > 
Re: guessing number of reducers.Mohammad Tariq 20121121, 18:04
Hello Jamal,
I use a different approach based on the no of cores. If you have, say a 4 cores machine then you can have (0.75*no cores)no. of MR slots. For example, if you have 4 physical cores OR 8 virtual cores then you can have 0.75*8=6 MR slots. You can then set 3M+3R or 4M+2R and so on as per your requirement. Regards, Mohammad Tariq On Wed, Nov 21, 2012 at 11:19 PM, Kartashov, Andy <[EMAIL PROTECTED]>wrote: > Bejoy, > > > > I’ve read somethere about keeping number of mapred.reduce.tasks below the > reduce task capcity. Here is what I just tested: > > > > Output 25Gb. 8DN cluster with 16 Map and Reduce Task Capacity: > > > > 1 Reducer – 22mins > > 4 Reducers – 11.5mins > > 8 Reducers – 5mins > > 10 Reducers – 7mins > > 12 Reducers – 6:5mins > > 16 Reducers – 5.5mins > > > > 8 Reducers have won the race. But Reducers at the max capacity was very > clos. J > > > > AK47 > > > > > > *From:* Bejoy KS [mailto:[EMAIL PROTECTED]] > *Sent:* Wednesday, November 21, 2012 11:51 AM > *To:* [EMAIL PROTECTED] > *Subject:* Re: guessing number of reducers. > > > > Hi Sasha > > In general the number of reduce tasks is chosen mainly based on the data > volume to reduce phase. In tools like hive and pig by default for every 1GB > of map output there will be a reducer. So if you have 100 gigs of map > output then 100 reducers. > If your tasks are more CPU intensive then you need lesser volume of data > per reducer for better performance results. > > In general it is better to have the number of reduce tasks slightly less > than the number of available reduce slots in the cluster. > > Regards > Bejoy KS > > Sent from handheld, please excuse typos. >  > > *From: *jamal sasha <[EMAIL PROTECTED]> > > *Date: *Wed, 21 Nov 2012 11:38:38 0500 > > *To: *[EMAIL PROTECTED]<[EMAIL PROTECTED]> > > *ReplyTo: *[EMAIL PROTECTED] > > *Subject: *guessing number of reducers. > > > > By default the number of reducers is set to 1.. > Is there a good way to guess optimal number of reducers.... > Or let's say i have tbs worth of data... mappers are of order 5000 or so... > But ultimately i am calculating , let's say, some average of whole data... > say average transaction occurring... > Now the output will be just one line in one "part"... rest of them will be > empty.So i am guessing i need loads of reducers but then most of them will > be empty but at the same time one reducer won't suffice.. > What's the best way to solve this.. > How to guess optimal number of reducers.. > Thanks > NOTICE: This email message and any attachments are confidential, subject > to copyright and may be privileged. Any unauthorized use, copying or > disclosure is prohibited. If you are not the intended recipient, please > delete and contact the sender immediately. Please consider the environment > before printing this email. AVIS : le présent courriel et toute pièce > jointe qui l'accompagne sont confidentiels, protégés par le droit d'auteur > et peuvent être couverts par le secret professionnel. Toute utilisation, > copie ou divulgation non autorisée est interdite. Si vous n'êtes pas le > destinataire prévu de ce courriel, supprimezle et contactez immédiatement > l'expéditeur. Veuillez penser à l'environnement avant d'imprimer le présent > courriel > 
Re: guessing number of reducers.Bejoy KS 20121121, 18:21
Hi Andy
It is usually so because if you have more reduce tasks than the reduce slots in your cluster then a few of the reduce tasks will be in queue waiting for its turn. So it is better to keep the num of reduce tasks slightly less than the reduce task capacity so that all reduce tasks run at once in parallel. But in some cases each reducer can process only certain volume of data due to some constraints, like data beyond a certain limit may lead to OOMs. In such cases you may need to configure the number of reducers totally based on your data and not based on slots. Regards Bejoy KS Sent from handheld, please excuse typos. Original Message From: "Kartashov, Andy" <[EMAIL PROTECTED]> Date: Wed, 21 Nov 2012 17:49:50 To: [EMAIL PROTECTED]<[EMAIL PROTECTED]>; [EMAIL PROTECTED]<[EMAIL PROTECTED]> Subject: RE: guessing number of reducers. Bejoy, I've read somethere about keeping number of mapred.reduce.tasks below the reduce task capcity. Here is what I just tested: Output 25Gb. 8DN cluster with 16 Map and Reduce Task Capacity: 1 Reducer  22mins 4 Reducers  11.5mins 8 Reducers  5mins 10 Reducers  7mins 12 Reducers  6:5mins 16 Reducers  5.5mins 8 Reducers have won the race. But Reducers at the max capacity was very clos. :) AK47 From: Bejoy KS [mailto:[EMAIL PROTECTED]] Sent: Wednesday, November 21, 2012 11:51 AM To: [EMAIL PROTECTED] Subject: Re: guessing number of reducers. Hi Sasha In general the number of reduce tasks is chosen mainly based on the data volume to reduce phase. In tools like hive and pig by default for every 1GB of map output there will be a reducer. So if you have 100 gigs of map output then 100 reducers. If your tasks are more CPU intensive then you need lesser volume of data per reducer for better performance results. In general it is better to have the number of reduce tasks slightly less than the number of available reduce slots in the cluster. Regards Bejoy KS Sent from handheld, please excuse typos. ________________________________ From: jamal sasha <[EMAIL PROTECTED]> Date: Wed, 21 Nov 2012 11:38:38 0500 To: [EMAIL PROTECTED]<[EMAIL PROTECTED]> ReplyTo: [EMAIL PROTECTED] Subject: guessing number of reducers. By default the number of reducers is set to 1.. Is there a good way to guess optimal number of reducers.... Or let's say i have tbs worth of data... mappers are of order 5000 or so... But ultimately i am calculating , let's say, some average of whole data... say average transaction occurring... Now the output will be just one line in one "part"... rest of them will be empty.So i am guessing i need loads of reducers but then most of them will be empty but at the same time one reducer won't suffice.. What's the best way to solve this.. How to guess optimal number of reducers.. Thanks NOTICE: This email message and any attachments are confidential, subject to copyright and may be privileged. Any unauthorized use, copying or disclosure is prohibited. If you are not the intended recipient, please delete and contact the sender immediately. Please consider the environment before printing this email. AVIS : le pr?sent courriel et toute pi?ce jointe qui l'accompagne sont confidentiels, prot?g?s par le droit d'auteur et peuvent ?tre couverts par le secret professionnel. Toute utilisation, copie ou divulgation non autoris?e est interdite. Si vous n'?tes pas le destinataire pr?vu de ce courriel, supprimezle et contactez imm?diatement l'exp?diteur. Veuillez penser ? l'environnement avant d'imprimer le pr?sent courriel 
Re: guessing number of reducers.jamal sasha 20121121, 18:27
Thanks for the input guys. This helps alot
:) On Wednesday, November 21, 2012, Bejoy KS <[EMAIL PROTECTED]> wrote: > Hi Andy > > It is usually so because if you have more reduce tasks than the reduce slots in your cluster then a few of the reduce tasks will be in queue waiting for its turn. So it is better to keep the num of reduce tasks slightly less than the reduce task capacity so that all reduce tasks run at once in parallel. > > But in some cases each reducer can process only certain volume of data due to some constraints, like data beyond a certain limit may lead to OOMs. In such cases you may need to configure the number of reducers totally based on your data and not based on slots. > > Regards > Bejoy KS > > Sent from handheld, please excuse typos. > ________________________________ > From: "Kartashov, Andy" <[EMAIL PROTECTED]> > Date: Wed, 21 Nov 2012 17:49:50 +0000 > To: [EMAIL PROTECTED]<[EMAIL PROTECTED]>; [EMAIL PROTECTED] <[EMAIL PROTECTED]> > Subject: RE: guessing number of reducers. > > Bejoy, > > > > I’ve read somethere about keeping number of mapred.reduce.tasks below the reduce task capcity. Here is what I just tested: > > > > Output 25Gb. 8DN cluster with 16 Map and Reduce Task Capacity: > > > > 1 Reducer – 22mins > > 4 Reducers – 11.5mins > > 8 Reducers – 5mins > > 10 Reducers – 7mins > > 12 Reducers – 6:5mins > > 16 Reducers – 5.5mins > > > > 8 Reducers have won the race. But Reducers at the max capacity was very clos. J > > > > AK47 > > > > > > From: Bejoy KS [mailto:[EMAIL PROTECTED]] > Sent: Wednesday, November 21, 2012 11:51 AM > To: [EMAIL PROTECTED] > Subject: Re: guessing number of reducers. > > > > Hi Sasha > > In general the number of reduce tasks is chosen mainly based on the data volume to reduce phase. In tools like hive and pig by default for every 1GB of map output there will be a reducer. So if you have 100 gigs of map output then 100 reducers. > If your tasks are more CPU intensive then you need lesser volume of data per reducer for better performance results. > > In general it is better to have the number of reduce tasks slightly less than the number of available reduce slots in the cluster. > > Regards > Bejoy KS > > Sent from handheld, please excuse typos. > > ________________________________ > > From: jamal sasha <[EMAIL PROTECTED]> > > Date: Wed, 21 Nov 2012 11:38:38 0500 > > To: [EMAIL PROTECTED]<[EMAIL PROTECTED]> 
Re: guessing number of reducers.Bejoy KS 20121121, 18:34
Hi Manoj
If you intend to calculate the number of reducers based on the input size, then in your driver class you should get the size of the input dir in hdfs and say you intended to give n bytes to a reducer then the number of reducers can be computed as Total input size/ bytes per reducer. You can round this value and use it to set the number of reducers in conf programatically. Regards Bejoy KS Sent from handheld, please excuse typos. Original Message From: Manoj Babu <[EMAIL PROTECTED]> Date: Wed, 21 Nov 2012 23:28:00 To: <[EMAIL PROTECTED]> Cc: [EMAIL PROTECTED]<[EMAIL PROTECTED]> Subject: Re: guessing number of reducers. Hi, How to set no of reducers in job conf dynamically? For example some days i am getting 500GB of data on heavy traffic and some days 100GB only. Thanks in advance! Cheers! Manoj. On Wed, Nov 21, 2012 at 11:19 PM, Kartashov, Andy <[EMAIL PROTECTED]>wrote: > Bejoy, > > > > I’ve read somethere about keeping number of mapred.reduce.tasks below the > reduce task capcity. Here is what I just tested: > > > > Output 25Gb. 8DN cluster with 16 Map and Reduce Task Capacity: > > > > 1 Reducer – 22mins > > 4 Reducers – 11.5mins > > 8 Reducers – 5mins > > 10 Reducers – 7mins > > 12 Reducers – 6:5mins > > 16 Reducers – 5.5mins > > > > 8 Reducers have won the race. But Reducers at the max capacity was very > clos. J > > > > AK47 > > > > > > *From:* Bejoy KS [mailto:[EMAIL PROTECTED]] > *Sent:* Wednesday, November 21, 2012 11:51 AM > *To:* [EMAIL PROTECTED] > *Subject:* Re: guessing number of reducers. > > > > Hi Sasha > > In general the number of reduce tasks is chosen mainly based on the data > volume to reduce phase. In tools like hive and pig by default for every 1GB > of map output there will be a reducer. So if you have 100 gigs of map > output then 100 reducers. > If your tasks are more CPU intensive then you need lesser volume of data > per reducer for better performance results. > > In general it is better to have the number of reduce tasks slightly less > than the number of available reduce slots in the cluster. > > Regards > Bejoy KS > > Sent from handheld, please excuse typos. >  > > *From: *jamal sasha <[EMAIL PROTECTED]> > > *Date: *Wed, 21 Nov 2012 11:38:38 0500 > > *To: *[EMAIL PROTECTED]<[EMAIL PROTECTED]> > > *ReplyTo: *[EMAIL PROTECTED] > > *Subject: *guessing number of reducers. > > > > By default the number of reducers is set to 1.. > Is there a good way to guess optimal number of reducers.... > Or let's say i have tbs worth of data... mappers are of order 5000 or so... > But ultimately i am calculating , let's say, some average of whole data... > say average transaction occurring... > Now the output will be just one line in one "part"... rest of them will be > empty.So i am guessing i need loads of reducers but then most of them will > be empty but at the same time one reducer won't suffice.. > What's the best way to solve this.. > How to guess optimal number of reducers.. > Thanks > NOTICE: This email message and any attachments are confidential, subject > to copyright and may be privileged. Any unauthorized use, copying or > disclosure is prohibited. If you are not the intended recipient, please > delete and contact the sender immediately. Please consider the environment > before printing this email. AVIS : le présent courriel et toute pièce > jointe qui l'accompagne sont confidentiels, protégés par le droit d'auteur > et peuvent être couverts par le secret professionnel. Toute utilisation, > copie ou divulgation non autorisée est interdite. Si vous n'êtes pas le > destinataire prévu de ce courriel, supprimezle et contactez immédiatement > l'expéditeur. Veuillez penser à l'environnement avant d'imprimer le présent > courriel > 
Re: guessing number of reducers.Manoj Babu 20121122, 04:45
Thank you for the info Bejoy.
Cheers! Manoj. On Thu, Nov 22, 2012 at 12:04 AM, Bejoy KS <[EMAIL PROTECTED]> wrote: > ** > Hi Manoj > > If you intend to calculate the number of reducers based on the input size, > then in your driver class you should get the size of the input dir in hdfs > and say you intended to give n bytes to a reducer then the number of > reducers can be computed as > Total input size/ bytes per reducer. > > You can round this value and use it to set the number of reducers in conf > programatically. > > Regards > Bejoy KS > > Sent from handheld, please excuse typos. >  > *From: * Manoj Babu <[EMAIL PROTECTED]> > *Date: *Wed, 21 Nov 2012 23:28:00 +0530 > *To: *<[EMAIL PROTECTED]> > *Cc: *[EMAIL PROTECTED]<[EMAIL PROTECTED]> > *Subject: *Re: guessing number of reducers. > > Hi, > > How to set no of reducers in job conf dynamically? > For example some days i am getting 500GB of data on heavy traffic and some > days 100GB only. > > Thanks in advance! > > Cheers! > Manoj. > > > > On Wed, Nov 21, 2012 at 11:19 PM, Kartashov, Andy <[EMAIL PROTECTED]>wrote: > >> Bejoy, >> >> >> >> I’ve read somethere about keeping number of mapred.reduce.tasks below the >> reduce task capcity. Here is what I just tested: >> >> >> >> Output 25Gb. 8DN cluster with 16 Map and Reduce Task Capacity: >> >> >> >> 1 Reducer – 22mins >> >> 4 Reducers – 11.5mins >> >> 8 Reducers – 5mins >> >> 10 Reducers – 7mins >> >> 12 Reducers – 6:5mins >> >> 16 Reducers – 5.5mins >> >> >> >> 8 Reducers have won the race. But Reducers at the max capacity was very >> clos. J >> >> >> >> AK47 >> >> >> >> >> >> *From:* Bejoy KS [mailto:[EMAIL PROTECTED]] >> *Sent:* Wednesday, November 21, 2012 11:51 AM >> *To:* [EMAIL PROTECTED] >> *Subject:* Re: guessing number of reducers. >> >> >> >> Hi Sasha >> >> In general the number of reduce tasks is chosen mainly based on the data >> volume to reduce phase. In tools like hive and pig by default for every 1GB >> of map output there will be a reducer. So if you have 100 gigs of map >> output then 100 reducers. >> If your tasks are more CPU intensive then you need lesser volume of data >> per reducer for better performance results. >> >> In general it is better to have the number of reduce tasks slightly less >> than the number of available reduce slots in the cluster. >> >> Regards >> Bejoy KS >> >> Sent from handheld, please excuse typos. >>  >> >> *From: *jamal sasha <[EMAIL PROTECTED]> >> >> *Date: *Wed, 21 Nov 2012 11:38:38 0500 >> >> *To: *[EMAIL PROTECTED]<[EMAIL PROTECTED]> >> >> *ReplyTo: *[EMAIL PROTECTED] >> >> *Subject: *guessing number of reducers. >> >> >> >> By default the number of reducers is set to 1.. >> Is there a good way to guess optimal number of reducers.... >> Or let's say i have tbs worth of data... mappers are of order 5000 or >> so... >> But ultimately i am calculating , let's say, some average of whole >> data... say average transaction occurring... >> Now the output will be just one line in one "part"... rest of them will >> be empty.So i am guessing i need loads of reducers but then most of them >> will be empty but at the same time one reducer won't suffice.. >> What's the best way to solve this.. >> How to guess optimal number of reducers.. >> Thanks >> NOTICE: This email message and any attachments are confidential, >> subject to copyright and may be privileged. Any unauthorized use, copying >> or disclosure is prohibited. If you are not the intended recipient, please >> delete and contact the sender immediately. Please consider the environment >> before printing this email. AVIS : le présent courriel et toute pièce >> jointe qui l'accompagne sont confidentiels, protégés par le droit d'auteur >> et peuvent être couverts par le secret professionnel. Toute utilisation, >> copie ou divulgation non autorisée est interdite. Si vous n'êtes pas le >> destinataire prévu de ce courriel, supprimezle et contactez immédiatement 
