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HBase, mail # user - independent scans to same region processed serially


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James Taylor 2013-02-09, 01:49
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Marcos Ortiz 2013-02-09, 03:18
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lars hofhansl 2013-02-09, 02:51
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James Taylor 2013-02-09, 05:52
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lars hofhansl 2013-02-09, 17:02
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James Taylor 2013-02-09, 17:28
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lars hofhansl 2013-02-09, 19:04
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James Taylor 2013-02-10, 08:30
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ramkrishna vasudevan 2013-02-09, 10:48
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Re: independent scans to same region processed serially
lars hofhansl 2013-02-09, 16:49
HBASE-7336 only deal with parallel read on the same HFile, since each HFile only has a single reader.
For scans you want to do seek+read (as opposed to positional reads), the problem with seek+read is that is that can only be done with the single thread.
So HBASE-7336 just switches the read to a positional read if the reader is already locked. (somewhat of a hack)
-- Lars

________________________________
From: ramkrishna vasudevan <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Sent: Saturday, February 9, 2013 2:48 AM
Subject: Re: independent scans to same region processed serially

What do you see in the thread dump?  May be HBASE-7336 deals with scans
hitting the same block of data. But i see from your mail that the scans are
independent of each other and they scan different data but in the same
Region.

Regards
Ram

On Sat, Feb 9, 2013 at 11:22 AM, James Taylor <[EMAIL PROTECTED]>wrote:

> All data is the blockcache and there are plenty of handlers. To repro, you
> could:
> - create a table pre-split into, for example, three regions
> - execute serially a scan on the middle region
> - execute two parallel scans each on half of the middle region
> - you'd expect the parallel scan to execute near twice as fast, but we're
> seeing it execute slower than the serial scan.
> We're using the same HConnection with different HTable instances for each
> scan.
>
>     James
>
>
> On 02/08/2013 06:51 PM, lars hofhansl wrote:
>
>> Is your data all in the blockcache, otherwise you might have run into
>> HBASE-7336 (https://issues.apache.org/**jira/browse/HBASE-7336).Fixed<https://issues.apache.org/jira/browse/HBASE-7336).Fixed>0.94.4.
>> I assume you have enough handlers, etc. (i.e. does the same happen if
>> issue multiple scan request across different region of the same region
>> server?)
>>
>>
>> -- Lars
>>
>>
>>
>> ______________________________**__
>>   From: James Taylor <[EMAIL PROTECTED]>
>> To: HBase User <[EMAIL PROTECTED]>
>> Sent: Friday, February 8, 2013 5:49 PM
>> Subject: independent scans to same region processed serially
>>   Wanted to check with folks and see if they've seen an issue around this
>> before digging in deeper. I'm on 0.94.2. If I execute in parallel multiple
>> scans to different parts of the same region, they appear to be processed
>> serially. It's actually faster from the client side to execute a single
>> serial scan than it is to execute multiple parallel scans to different
>> segments of the region. I do have region observer coprocessors for the
>> table I'm scanning, but my code is not doing any synchronization.
>>
>> Is there a known limitation in this area? Anyone else see anything
>> similar?
>>
>>      James
>>
>
>