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From Mario Pastorelli <mario.pastore...@teralytics.ch>
Subject Re: Profile a (batch) scan
Date Wed, 31 Aug 2016 17:10:59 GMT
Below you can find a good example of what I'm struggling to understand
right now. It's a trace for a simple scan over some columns with a
BatchScanner using 75 threads. The scan takes 877 milliseconds and the main
contributor is the entry "tablet read ahead 1", which starts at 248 ms.
These are the questions that I cannot answer with this trace:

   1. why this heavy operation starts after 248ms? By summing up the delay
   before this operation you get a number which is not even close to 248ms.
   2. what does "tablet read ahead 1" means? In general, how to map the
   entries of a trace to their meaning? Is there a guide about this?
   3. why "tablet read ahead 1" takes 600ms? It's clearly not the sum of
   the entries under this one but that's the important part.
   4. I may be naive but...how much data have been read by this scan? How
   many entries? That's very important to understand what's going on.

Thanks for the help,

Mario

877+ 0 Dice@h01 counts
2+ 7 tserver@h12 startScan
6+ 10 tserver@h15 startScan
5+ 11 tserver@h15 metadata tablets read ahead 4
843+ 34 Dice@h01 batch scanner 74- 1
620+ 230 tserver@h09 startMultiScan
600+ 248 tserver@h09 tablet read ahead 1
22+ 299 tserver@h09 newDFSInputStream
22+ 299 tserver@h09 getBlockLocations
2+ 310 tserver@h09 ClientNamenodeProtocol#getBlockLocations
1+ 321 tserver@h09 getFileInfo
1+ 321 tserver@h09 ClientNamenodeProtocol#getFileInfo
2+ 322 tserver@h09 DFSInputStream#byteArrayRead
1+ 324 tserver@h09 DFSInputStream#byteArrayRead
2+ 831 tserver@h09 DFSInputStream#byteArrayRead
2+ 834 tserver@h09 DFSInputStream#byteArrayRead
1+ 835 tserver@h09 BlockReaderLocal#fillBuffer(1091850413)
1+ 874 tserver@h09 closeMultiScan

On Mon, Aug 29, 2016 at 5:06 PM, Josh Elser <josh.elser@gmail.com> wrote:

> Right, no, I understand.
>
> I just meant that the metrics we do expose are lacking. It would be a huge
> benefit to everyone if we could find more things that we can expose, do
> that once, and then prevent the need for the next person to hand-roll some
> things like you are doing now :)
>
> Mario Pastorelli wrote:
>
>> I think suggestions in this mailing list are useful, Josh, that's why I
>> keep asking questions. I'm sorry that I'm asking so many questions but
>> I'm trying to improve my knowledge of Accumulo and documentation is
>> limited. Ideally, I would like to use only the metrics provided by
>> Accumulo, because that's less stuff that I have to maintain. The
>> StopWatch writing to the tracer could help.
>>
>>
>>
>> On Sun, Aug 28, 2016 at 10:14 PM, Josh Elser <josh.elser@gmail.com
>> <mailto:josh.elser@gmail.com>> wrote:
>>
>>     I know it's not a super-helpful response, but I would love to help
>>     you work through things we *can* expose and help you do that.
>>
>>     I imagine there is significantly more that we can add into the
>>     dist-tracing information for BatchScanners now which would give more
>>     insight into the tserver (amount of data read, number of ranges per
>>     scan RPC, amount of data returned). This would be ideal as it would
>>     prevent you from having to update your application code (although,
>>     the suggestion of writing some iterator for timing purposes is a
>>     simple way to move forward)
>>
>>     Mario Pastorelli wrote:
>>
>>         I would like to understand the performance of a batch scan and I
>>         would
>>         like to have some hints on how to proceed. I have enabled the
>>         distributed trace, and it tells me that some batch scanner
>>         threads take
>>         much more time than others to complete but this is not helpful
>>         enough
>>         because it's not telling me why some threads take more. My gut
>>         feeling
>>         is that one batch thread is scanning more data than the others,
>>         which
>>         means that the data is not well distributed for a query, but I
>> use a
>>         random shard byte as prefix of the keys which should guarantee
>>         that data
>>         of the same range is almost equally distributed among the
>>         tservers. I
>>         enabled JMX on the tservers and attached jvisualvm to get an
>>         idea of the
>>         state of each tserver but I couldn't find anything meaningful. I
>>         would
>>         like to know if there is a way to profile what's going on on a
>>         single
>>         tserver for a single scan thread and by this I mean:
>>
>>           1. where are the tablets required by a scan? Which tablet
>> server?
>>           2. how fast was the lookups on the index for that scan?
>>           3. how many bytes/records were read for that scan without the
>>         iterators
>>           4. how many seeks are done by the scan and possibly why
>>
>>         The main Accumulo UI is fine to get an overview of Accumulo but
>>         don't
>>         really give you any information about the performance of a
>>         single query
>>         and it seems to me that they are heavily affected by what
>>         iterators do.
>>         Profiling a single scan is much more interesting. Is there a way
>> to
>>         profile a single (batch) scan in Accumulo such that I have a
>>         complete
>>         overview of the entire process of reading and sending back
>>         records to
>>         the driver?
>>
>>         Thanks,
>>         Mario
>>
>>         --
>>         Mario Pastorelli| TERALYTICS
>>
>>         *software engineer*
>>
>>         Teralytics AG | Zollstrasse 62 | 8005 Zurich | Switzerland
>>         phone:+41794381682 <tel:%2B41794381682>
>>         email: mario.pastorelli@teralytics.ch
>>         <mailto:mario.pastorelli@teralytics.ch>
>>         <mailto:mario.pastorelli@teralytics.ch
>>
>>         <mailto:mario.pastorelli@teralytics.ch>>
>>         www.teralytics.net <http://www.teralytics.net>
>>         <http://www.teralytics.net/>
>>
>>         Company registration number: CH-020.3.037.709-7 | Trade register
>>         Canton
>>         Zurich
>>         Board of directors: Georg Polzer, Luciano Franceschina, Mark
>>         Schmitz,
>>         Yann de Vries
>>
>>         This e-mail message contains confidential information which is
>>         for the
>>         sole attention and use of the intended recipient. Please notify
>>         us at
>>         once if you think that it may not be intended for you and delete
>> it
>>         immediately.
>>
>>
>>
>>
>> --
>> Mario Pastorelli| TERALYTICS
>>
>> *software engineer*
>>
>> Teralytics AG | Zollstrasse 62 | 8005 Zurich | Switzerland
>> phone:+41794381682
>> email: mario.pastorelli@teralytics.ch
>> <mailto:mario.pastorelli@teralytics.ch>
>> www.teralytics.net <http://www.teralytics.net/>
>>
>> Company registration number: CH-020.3.037.709-7 | Trade register Canton
>> Zurich
>> Board of directors: Georg Polzer, Luciano Franceschina, Mark Schmitz,
>> Yann de Vries
>>
>> This e-mail message contains confidential information which is for the
>> sole attention and use of the intended recipient. Please notify us at
>> once if you think that it may not be intended for you and delete it
>> immediately.
>>
>>


-- 
Mario Pastorelli | TERALYTICS

*software engineer*

Teralytics AG | Zollstrasse 62 | 8005 Zurich | Switzerland
phone: +41794381682
email: mario.pastorelli@teralytics.ch
www.teralytics.net

Company registration number: CH-020.3.037.709-7 | Trade register Canton
Zurich
Board of directors: Georg Polzer, Luciano Franceschina, Mark Schmitz, Yann
de Vries

This e-mail message contains confidential information which is for the sole
attention and use of the intended recipient. Please notify us at once if
you think that it may not be intended for you and delete it immediately.

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