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From Xavier Stevens <xstev...@mozilla.com>
Subject Re: MR sharded Scans giving poor performance..
Date Mon, 26 Jul 2010 22:29:37 GMT
 We have something that might interest you.


We haven't fully tested everything yet, so don't blame us if something
goes wrong.  It's basically the exact same as TableInputFormat except it
takes an array of Scans to be used for row-key ranges.  It requires the
caller to setup the Scan array since they should have the best knowledge
about their row-key structure.

Preliminary results for us reduced a 15 minute job to under 2 minutes.



On 7/26/10 3:16 PM, Vidhyashankar Venkataraman wrote:
> I did not use a TableInputFormat: I ran my own scans on specific ranges (just for more
control from my side to tune the ranges and the ease with which I can run a hadoop streaming
> 1 MB for Hfile Block size.. Not the HDFS block size..
> I increased it since I didn't care too much for random read performance.. HDFS block
size is the default value... (I have a related question then: does the Hfile block size influence
only the size of the index and the efficiency of random reads?  I don't see an effect on scans
>   I had previously run 5 tasks per machine and at 20 rows, but that resulted in scanner
expiries (UnknownScannerexception) and DFS socket timeouts.. So that's why I reduced the number
of tasks.. Let me decrease the number of rows and see..
>   Just to make sure: the client uses zookeeper only for obtaining ROOT right whenever
it performs scans, isnt it? So scans shouldn't face any master/zk bottlenecks when we scale
up wrt number of nodes, am I right?
> Thank you
> Vidhya
> On 7/26/10 3:01 PM, "Ryan Rawson" <ryanobjc@gmail.com> wrote:
> Hey,
> A few questions:
> - sharded scan, are you not using TableInputFormat?
> - 1 MB block size - what block size?  You probably shouldnt set the
> HDFS block size to 1MB, it just causes more nn traffic.
> - Tests a year ago indicated that HFile block size really didnt
> improve speed when you went beyond 64k or so.
> - Run more maps/machine... one map task per disk probably?
> - Try setting the client cache to an in-between level, 2-6 perhaps.
> Let us know about those other questions and we can go from there.
> -ryan
> On Mon, Jul 26, 2010 at 2:43 PM, Vidhyashankar Venkataraman
> <vidhyash@yahoo-inc.com> wrote:
>> I am trying to assess the performance of Scans on a 100TB db on 180 nodes running
Hbase 0.20.5..
>> I run a sharded scan (each Map task runs a scan on a specific range: speculative
execution is turned false so that there is no duplication in tasks) on a fully compacted table...
>> 1 MB block size, Block cache enabled.. Max of 2 tasks per node..  Each row is 30
KB in size: 1 big column family with just one field..
>> Region lease timeout is set to an hour.. And I don't get any socket timeout exceptions
so I have not reassigned the write socket timeout...
>> I ran experiments on the following cases:
>>  1.  The client level cache is set to 1 (default: got he number using getCaching):
The MR tasks take around 13 hours to finish in the average.. Which gives around 13.17 MBps
per node. The worst case is 34 hours (to finish the entire job)...
>>  2.  Client cache set to 20 rows: this is much worse than the previous case: we get
around a super low 1MBps per node...
>>         Question: Should I set it to a value such that the block size is a multiple
of the above said cache size? Or the cache size to a much lower value?
>> I find that these numbers are much less than the ones I get when it's running with
just a few nodes..
>> Can you guys help me with this problem?
>> Thank you
>> Vidhya

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