Return-Path: X-Original-To: apmail-hadoop-mapreduce-user-archive@minotaur.apache.org Delivered-To: apmail-hadoop-mapreduce-user-archive@minotaur.apache.org Received: from mail.apache.org (hermes.apache.org [140.211.11.3]) by minotaur.apache.org (Postfix) with SMTP id 59AA3DFDE for ; Sun, 21 Oct 2012 06:46:17 +0000 (UTC) Received: (qmail 26287 invoked by uid 500); 21 Oct 2012 06:46:12 -0000 Delivered-To: apmail-hadoop-mapreduce-user-archive@hadoop.apache.org Received: (qmail 25741 invoked by uid 500); 21 Oct 2012 06:46:11 -0000 Mailing-List: contact user-help@hadoop.apache.org; run by ezmlm Precedence: bulk List-Help: List-Unsubscribe: List-Post: List-Id: Reply-To: user@hadoop.apache.org Delivered-To: mailing list user@hadoop.apache.org Received: (qmail 25706 invoked by uid 99); 21 Oct 2012 06:46:09 -0000 Received: from athena.apache.org (HELO athena.apache.org) (140.211.11.136) by apache.org (qpsmtpd/0.29) with ESMTP; Sun, 21 Oct 2012 06:46:09 +0000 X-ASF-Spam-Status: No, hits=1.5 required=5.0 tests=HTML_MESSAGE,RCVD_IN_DNSWL_LOW,SPF_PASS X-Spam-Check-By: apache.org Received-SPF: pass (athena.apache.org: domain of linlma@gmail.com designates 209.85.212.48 as permitted sender) Received: from [209.85.212.48] (HELO mail-vb0-f48.google.com) (209.85.212.48) by apache.org (qpsmtpd/0.29) with ESMTP; Sun, 21 Oct 2012 06:46:03 +0000 Received: by mail-vb0-f48.google.com with SMTP id e21so2202310vbm.35 for ; Sat, 20 Oct 2012 23:45:42 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :content-type; bh=DVp9iMXiFDKkabAn9r8oT2q2U3TiaIMwF8c2Lc0U9Ks=; b=FALPudN3rFvX6mzoZo3wzUHGxcC/GUGhbqUXyafWJg/xoLIb73uRNBbkb5jPNo5LNE SM9UpUhf0dfyMmaf7P/3S+2C5F68iOPQ+c2SVC+iFTKt7P0Psb8rb2mI2nYg79la/ZoH TQ7a6nPZ7Ku0QGMwbVDfSVl5r0E/UH6kPJx73lIeUrPdFXjlTIIt9HLTj1YumiLYf8Vy mBvd8bi6CIwYDfKcHH/LtEs30D61jInfaekXNlQB+u94d40ExY7MNo3MxCSnNfXmck2/ u7GYMQE0w40epxqTbpvrOkVrjOpWAzgj1nzJw+R7J8r0gCl8WjHw84IMJoQjr8CEnB+N fDjw== MIME-Version: 1.0 Received: by 10.52.17.19 with SMTP id k19mr7264168vdd.0.1350801942076; Sat, 20 Oct 2012 23:45:42 -0700 (PDT) Received: by 10.58.189.228 with HTTP; Sat, 20 Oct 2012 23:45:42 -0700 (PDT) In-Reply-To: References: Date: Sun, 21 Oct 2012 14:45:42 +0800 Message-ID: Subject: Re: Hadoop counter From: Lin Ma To: user@hadoop.apache.org, michael_segel@hotmail.com Content-Type: multipart/alternative; boundary=bcaec5040912eb5e2104cc8c151e X-Virus-Checked: Checked by ClamAV on apache.org --bcaec5040912eb5e2104cc8c151e Content-Type: text/plain; charset=ISO-8859-1 Thanks for the detailed reply, Mike. Yes, my most confusion is resolved by you. The last two questions (or comments) are used to confirm my understanding is correct, - is it normal use case or best practices for a job to consume/read the counters from previous completed job in an automatic way? I ask this because I am not sure whether the most use case of counter is human read and manual analysis, other then using another job to automatic consume the counters? - I want to confirm my understanding is correct, when each task completes, JT will aggregate/update the global counter values from the specific counter values updated by the complete task, but never expose global counters values until job completes? If it is correct, I am wondering why JT doing aggregation each time when a task completes, other than doing a one time aggregation when the job completes? Is there any design choice reasons? thanks. regards, Lin On Sat, Oct 20, 2012 at 3:12 PM, Michael Segel wrote: > > On Oct 19, 2012, at 10:27 PM, Lin Ma wrote: > > Thanks for the detailed reply Mike, I learned a lot from the discussion. > > - I just want to confirm with you that, supposing in the same job, when a > specific task completed (and counter is aggregated in JT after the task > completed from our discussion?), the other running task in the same job > cannot get the updated counter value from the previous completed task? I am > asking this because I am thinking whether I can use counter to share a > global value between tasks. > > > Yes that is correct. > While I haven't looked at YARN (M/R 2.0) , M/R 1.x doesn't have an easy > way for a task to query the job tracker. This might have changed in YARN > > - If so, what is the traditional use case of counter, only use counter > values after the whole job completes? > > Yes the counters are used to provide data at the end of the job... > > BTW: appreciate if you could share me a few use cases from your experience > about how counters are used. > > Well you have your typical job data like the number of records processed, > total number of bytes read, bytes written... > > But suppose you wanted to do some quality control on your input. > So you need to keep a track on the count of bad records. If this job is > part of a process, you may want to include business logic in your job to > halt the job flow if X% of the records contain bad data. > > Or your process takes input records and in processing them, they sort the > records based on some characteristic and you want to count those sorted > records as you processed them. > > For a more concrete example, the Illinois Tollway has these 'fast pass' > lanes where cars equipped with RFID tags can have the tolls automatically > deducted from their accounts rather than pay the toll manually each time. > > Suppose we wanted to determine how many cars in the 'Fast Pass' lanes are > cheaters where they drive through the sensor and the sensor doesn't capture > the RFID tag. (Note its possible that you have a false positive where the > car has an RFID chip but doesn't trip the sensor.) Pushing the data in a > map/reduce job would require the use of counters. > > Does that help? > > -Mike > > regards, > Lin > > On Sat, Oct 20, 2012 at 5:05 AM, Michael Segel wrote: > >> Yeah, sorry... >> >> I meant that if you were dynamically creating a counter foo in the Mapper >> task, then each mapper would be creating their own counter foo. >> As the job runs, these counters will eventually be sent up to the JT. The >> job tracker would keep a separate counter for each task. >> >> At the end, the final count is aggregated from the list of counters for >> foo. >> >> >> I don't know how you can get a task to ask information from the Job >> Tracker on how things are going in other tasks. That is what I meant that >> you couldn't get information about the other counters or even the status of >> the other tasks running in the same job. >> >> I didn't see anything in the APIs that allowed for that type of flow... >> Of course having said that... someone pops up with a way to do just that. >> ;-) >> >> >> Does that clarify things? >> >> -Mike >> >> >> On Oct 19, 2012, at 11:56 AM, Lin Ma wrote: >> >> Hi Mike, >> >> Sorry I am a bit lost... As you are thinking faster than me. :-P >> >> From your this statement "It would make sense that the JT maintains a >> unique counter for each task until the tasks complete." -- it seems each >> task cannot see counters from each other, since JT maintains a unique >> counter for each tasks; >> >> From your this comment "I meant that if a Task created and updated a >> counter, a different Task has access to that counter. " -- it seems >> different tasks could share/access the same counter. >> >> Appreciate if you could help to clarify a bit. >> >> regards, >> Lin >> >> On Sat, Oct 20, 2012 at 12:42 AM, Michael Segel < >> michael_segel@hotmail.com> wrote: >> >>> >>> On Oct 19, 2012, at 11:27 AM, Lin Ma wrote: >>> >>> Hi Mike, >>> >>> Thanks for the detailed reply. Two quick questions/comments, >>> >>> 1. For "task", you mean a specific mapper instance, or a specific >>> reducer instance? >>> >>> >>> Either. >>> >>> 2. "However, I do not believe that a separate Task could connect with >>> the JT and see if the counter exists or if it could get a value or even an >>> accurate value since the updates are asynchronous." -- do you mean if a >>> mapper is updating custom counter ABC, and another mapper is updating the >>> same customer counter ABC, their counter values are updated independently >>> by different mappers, and will not published (aggregated) externally until >>> job completed successfully? >>> >>> I meant that if a Task created and updated a counter, a different Task >>> has access to that counter. >>> >>> To give you an example, if I want to count the number of quality errors >>> and then fail after X number of errors, I can't use Global counters to do >>> this. >>> >>> regards, >>> Lin >>> >>> On Fri, Oct 19, 2012 at 10:35 PM, Michael Segel < >>> michael_segel@hotmail.com> wrote: >>> >>>> As I understand it... each Task has its own counters and are >>>> independently updated. As they report back to the JT, they update the >>>> counter(s)' status. >>>> The JT then will aggregate them. >>>> >>>> In terms of performance, Counters take up some memory in the JT so >>>> while its OK to use them, if you abuse them, you can run in to issues. >>>> As to limits... I guess that will depend on the amount of memory on the >>>> JT machine, the size of the cluster (Number of TT) and the number of >>>> counters. >>>> >>>> In terms of global accessibility... Maybe. >>>> >>>> The reason I say maybe is that I'm not sure by what you mean by >>>> globally accessible. >>>> If a task creates and implements a dynamic counter... I know that it >>>> will eventually be reflected in the JT. However, I do not believe that a >>>> separate Task could connect with the JT and see if the counter exists or if >>>> it could get a value or even an accurate value since the updates are >>>> asynchronous. Not to mention that I don't believe that the counters are >>>> aggregated until the job ends. It would make sense that the JT maintains a >>>> unique counter for each task until the tasks complete. (If a task fails, it >>>> would have to delete the counters so that when the task is restarted the >>>> correct count is maintained. ) Note, I haven't looked at the source code >>>> so I am probably wrong. >>>> >>>> HTH >>>> Mike >>>> On Oct 19, 2012, at 5:50 AM, Lin Ma wrote: >>>> >>>> Hi guys, >>>> >>>> I have some quick questions regarding to Hadoop counter, >>>> >>>> >>>> - Hadoop counter (customer defined) is global accessible (for both >>>> read and write) for all Mappers and Reducers in a job? >>>> - What is the performance and best practices of using Hadoop >>>> counters? I am not sure if using Hadoop counters too heavy, there will be >>>> performance downgrade to the whole job? >>>> >>>> regards, >>>> Lin >>>> >>>> >>>> >>> >>> >> >> > > --bcaec5040912eb5e2104cc8c151e Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Thanks for the detailed reply, Mike. Yes, my most confusion is resolved by = you. The last two questions (or comments) are used to confirm my understand= ing is correct,

- is it normal use case or best practices for a job = to consume/read the counters from previous completed job in an automatic wa= y? I ask this because I am not sure whether the most use case of counter is= human read and manual analysis, other then using another job to automatic = consume the counters?
- I want to confirm my understanding is correct, when each task completes, = JT will aggregate/update the global counter values from the specific counte= r values updated by the complete task, but never expose global counters val= ues until job completes? If it is correct, I am wondering why JT doing aggr= egation each time when a task completes, other than doing a one time aggreg= ation when the job completes? Is there any design choice reasons? thanks.
regards,
Lin

On Sat, Oct 20, 2012 = at 3:12 PM, Michael Segel <michael_segel@hotmail.com> wrote:

On Oct 19, 2012, at 10:27 PM, Lin Ma <linlma@gmail.com> wrote:=

Thanks for the detailed reply Mike, I learned= a lot from the discussion.

- I just want to confirm with you that, = supposing in the same job, when a specific task completed (and counter is a= ggregated in JT after the task completed from our discussion?), the other r= unning task in the same job cannot get the updated counter value from the p= revious completed task? I am asking this because I am thinking whether I ca= n use counter to share a global value between tasks.

Yes that is correct.=A0
While I= haven't looked at YARN (M/R 2.0) , M/R 1.x doesn't have an easy wa= y for a task to query the job tracker. This might have changed in YARN

- If so, what is the traditional use case of counter, only use counter valu= es after the whole job completes?

Yes the counter= s are used to provide data at the end of the job...=A0
BTW: appreciate if you could share me a few use c= ases from your experience about how counters are used.

Well you have your typical job data like the number = of records processed, total number of bytes read, =A0bytes written...=A0

But suppose you wanted to do some quality control on= your input.=A0
So you need to keep a track on the count of bad records. =A0If this jo= b is part of a process, you may want to include business logic in your job = to halt the job flow if X% of the records contain bad data.=A0
Or your process takes input records and in processing them, they= sort the records based on some characteristic and you want to count those = sorted records as you processed them.=A0

For a mor= e concrete example, the Illinois Tollway has these 'fast pass' lane= s where cars equipped with RFID tags can have the tolls automatically deduc= ted from their accounts rather than pay the toll manually each time.=A0

Suppose we wanted to determine how many cars in the = 9;Fast Pass' lanes are cheaters where they drive through the sensor and= the sensor doesn't capture the RFID tag. (Note its possible that you h= ave a false positive where the car has an RFID chip but doesn't trip th= e sensor.) Pushing the data in a map/reduce job would require the use of co= unters.

Does that help?=A0

-Mike
=

regards,
Lin
On Sat, Oct 20, 2012 at 5:05 AM, Michael S= egel <michael_segel@hotmail.com> wrote:
Yeah, so= rry...=A0

I meant that if you were dynamically creating = a counter foo in the Mapper task, then each mapper would be creating their = own counter foo.=A0
As the job runs, these counters will eventually be sent up to the JT. = The job tracker would keep a separate counter for each task.=A0
<= br>
At the end, the final count is aggregated from the list of co= unters for foo.=A0


I don't know how you can get a task = to ask information from the Job Tracker on how things are going in other ta= sks. =A0That is what I meant that you couldn't get information about th= e other counters or even the status of the other tasks running in the same = job.=A0

I didn't see anything in the APIs that allowed for = that type of flow... Of course having said that... someone pops up with a w= ay to do just that. ;-)=A0


Does tha= t clarify things?=A0

-Mike


= On Oct 19, 2012, at 11:56 AM, Lin Ma <linlma@gmail.com> wrote:

Hi Mike,

Sorry I am a bit lost... As you are thinking faster than me= . :-P

From your this statement "It would make sense that the JT= maintains a unique counter for each task until the tasks complete." -= - it seems each task cannot see counters from each other, since JT maintain= s a unique counter for each tasks;

From your this comment "I meant that if a Task created and updated= a counter, a different Task has access to that counter. " -- it seems= different tasks could share/access the same counter.

Appreciate if = you could help to clarify a bit.

regards,
Lin

On Sat, Oct 20, 2012 = at 12:42 AM, Michael Segel <michael_segel@hotmail.com> wrote:

On Oct 19, 2012, at 11:27 AM, Lin Ma <linlma@gmail.com> wrote:

Hi Mike,

Thanks for the detailed reply= . Two quick questions/comments,

1. For "task", you mean a = specific mapper instance, or a specific reducer instance?

Either.=A0

2. "However, I do not believe that a separate Task could connect wit= h the JT and see if the counter exists or if it could get a value or even an=20 accurate value since the updates are asynchronous." -- do you mean if = a mapper is updating custom counter ABC, and another mapper is updating the= same customer counter ABC, their counter values are updated independently = by different mappers, and will not published (aggregated) externally until = job completed successfully?

I meant that if a Task created and updated a counter= , a different Task has access to that counter.=A0

= To give you an example, if I want to count the number of quality errors and= then fail after X number of errors, I can't use Global counters to do = this.

regards,
Lin

On Fri, Oct 19, 2012 at 10:35 PM, Michael Segel <michael_segel@hotmail.com> wrote:
As I und= erstand it... each Task has its own counters and are independently updated.= As they report back to the JT, they update the counter(s)' status. The JT then will aggregate them.=A0

In terms of pe= rformance, Counters take up some memory in the JT so while its OK to use th= em, if you abuse them, you can run in to issues.=A0
As to limits.= .. I guess that will depend on the amount of memory on the JT machine, the = size of the cluster (Number of TT) and the number of counters.=A0

In terms of global accessibility... Maybe.
The reason I say maybe is that I'm not sure by what you me= an by globally accessible.=A0
If a task creates and implements a = dynamic counter... I know that it will eventually be reflected in the JT. H= owever, I do not believe that a separate Task could connect with the JT and= see if the counter exists or if it could get a value or even an accurate v= alue since the updates are asynchronous. =A0Not to mention that I don't= believe that the counters are aggregated until the job ends. It would make= sense that the JT maintains a unique counter for each task until the tasks= complete. (If a task fails, it would have to delete the counters so that w= hen the task is restarted the correct count is maintained. ) =A0Note, I hav= en't looked at the source code so I am probably wrong.=A0

HTH
Mike
On Oct 19, 2012,= at 5:50 AM, Lin Ma <linlma@gmail.com> wrote:

Hi guys,

I have some quick questions regarding to Hadoop counter,
  • Hadoop counter (customer defined) is global accessible (for bo= th read and write) for all Mappers and Reducers in a job?
  • What is t= he performance and best practices of using Hadoop counters? I am not sure i= f using Hadoop counters too heavy, there will be performance downgrade to t= he whole job?
regards,
Lin








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