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From Michael Segel <michael_se...@hotmail.com>
Subject Re: HBase parallel scanner performance
Date Thu, 19 Apr 2012 16:26:05 GMT

I think you are still missing the point. 
130 seconds to scan the table per iteration. 
Even if you have 10K rows 
130 * 10^4 or 1.3*10^6 seconds.  ~361 hours

Compare that to 10K rows where you then select a single row in your sub select that has a
list of all of the associated rows. 
You can then do  n number of get()s based on the data in the index. (If the data wasn't in
the index itself)

Assuming that the data was in the index, that's one get(). This is sub second. 
Just to keep things simple assume 1 second. 
That's 10K seconds vs 1.3 million seconds.  (2 hours vs 361hours) 
Actually its more like 10ms  so its 100 seconds to run your code.  (So its like 2 minutes
or so) 

Also since you're doing less work, you put less strain on the system.

Look, you're asking for help. You're fighting to maintain a bad design. 
Building the index table shouldn't take you more than a day to think, design and implement.

So you tell me, 2 minutes vs 361 hours. Which would you choose?



On Apr 19, 2012, at 10:04 AM, Narendra yadala wrote:

> Michael,
> Thanks for the response. This is a real problem and not a class project.
> Boxes itself costed 9k ;)
> I think there is some difference in understanding of the problem. The table
> has 2m rows but I am looking at the latest 10k rows only in the outer for
> loop. Only in the inner for loop i am trying to get all rows that contain
> the url that is given by the row in the outer for loop. So pseudo code is
> like this
> All scanners have a caching of 128.
> Scanner outerScanner =  tweetTable.getScanner(new Scan()); //This gets the
> entire row
> for (int index = 0; index < 10000; index++) {
> Result tweet =  outerScanner.next();
> NavigableMap<byte[],byte[]> linkFamilyMap =
> tweet.getFamilyMap(Bytes.toBytes("link"));
> String url = Bytes.toString( linkFamilyMap.firstKey());  //assuming only
> one link is there in the tweet.
> Scan linkScan = new Scan();
> linkScan.addColumn(Bytes.toBytes("link"), Bytes.toBytes(url)); //get only
> the link column family
> Scanner linkScanner = tweetTable.getScanner(linkScan); //ideally this for
> loop is taking 2 sec per sc
> for (Result linkResult = linkScanner.next(); linkResult != null;
> linkResult = linkScanner.next()) {
>    //do something with the link
> }
> linkScanner.close();
>        //do a similar for loop for hashtags
> }
> Each of my inner for loop is taking around 20 seconds (or more depending on
> number of rows returned by that particular scanner) for each of the 10k
> rows that I am processing and this is also triggering a lot of GC in turn.
> So it is 10000*40 seconds (4 days) for each thread. But the problem is that
> the batch process crashes before completion throwing IOException and
> SocketTimeoutException and sometimes GC OutOfMemory exceptions.
> I will definitely take the much elegant approach that you mentioned
> eventually. I just wanted to get to the core of the issue before choosing
> the solution.
> Thanks again.
> Narendra
> On Thu, Apr 19, 2012 at 7:42 PM, Michel Segel <michael_segel@hotmail.com>wrote:
>> Narendra,
>> Are you trying to solve a real problem, or is this a class project?
>> Your solution doesn't scale. It's a non starter. 130 seconds for each
>> iteration times 1 million seconds is how long? 130 million seconds, which
>> is ~36000 hours or over 4 years to complete.
>> (the numbers are rough but you get the idea...)
>> That's assuming that your table is static and doesn't change.
>> I didn't even ask if you were attempting any sort of server side filtering
>> which would reduce the amount of data you send back to the client because
>> it a moot point.
>> Finer tuning is also moot.
>> So you insert a row in one table. You then do n^2 operations to pull out
>> data.
>> The better solution is to insert data into 2 tables where you then have to
>> do 2n operations to get the same results. Thats per thread btw.  So if you
>> were running 10 threads, you would have 10n^2  operations versus 20n
>> operations to get the same result set.
>> A million row table... 1*10^13. Vs 2*10^6
>> I don't believe I mentioned anything about HBase's internals and this
>> solution works for any NoSQL database.
>> Sent from a remote device. Please excuse any typos...
>> Mike Segel
>> On Apr 19, 2012, at 7:03 AM, Narendra yadala <narendra.yadala@gmail.com>
>> wrote:
>>> Hi Michel
>>> Yes, that is exactly what I do in step 2. I am aware of the reason for
>> the
>>> scanner timeout exceptions. It is the time between two consecutive
>>> invocations of the next call on a specific scanner object. I increased
>> the
>>> scanner timeout to 10 min on the region server and still I keep seeing
>> the
>>> timeouts. So I reduced my scanner cache to 128.
>>> Full table scan takes 130 seconds and there are 2.2 million rows in the
>>> table as of now. Each row is around 2 KB in size. I measured time for the
>>> full table scan by issuing `count` command from the hbase shell.
>>> I kind of understood the fix that you are specifying, but do I need to
>>> change the table structure to fix this problem? All I do is a n^2
>> operation
>>> and even that fails with 10 different types of exceptions. It is mildly
>>> annoying that I need to know all the low level storage details of HBase
>> to
>>> do such a simple operation. And this is happening for just 14 parallel
>>> scanners. I am wondering what would happen when there are thousands of
>>> parallel scanners.
>>> Please let me know if there is any configuration param change which would
>>> fix this issue.
>>> Thanks a lot
>>> Narendra
>>> On Thu, Apr 19, 2012 at 4:40 PM, Michel Segel <michael_segel@hotmail.com
>>> wrote:
>>>> So in your step 2 you have the following:
>>>> FOREACH row IN TABLE alpha:
>>>>   SELECT something
>>>>   FROM TABLE alpha
>>>>   WHERE alpha.url = row.url
>>>> Right?
>>>> And you are wondering why you are getting timeouts?
>>>> ...
>>>> ...
>>>> And how long does it take to do a full table scan? ;-)
>>>> (there's more, but that's the first thing you should see...)
>>>> Try creating a second table where you invert the URL and key pair such
>>>> that for each URL, you have a set of your alpha table's keys?
>>>> Then you have the following...
>>>> FOREACH row IN TABLE alpha:
>>>> FETCH key-set FROM beta
>>>> WHERE beta.rowkey = alpha.url
>>>> Note I use FETCH to signify that you should get a single row in
>> response.
>>>> Does this make sense?
>>>> ( your second table is actually and index of the URL column in your
>> first
>>>> table)
>>>> HTH
>>>> Sent from a remote device. Please excuse any typos...
>>>> Mike Segel
>>>> On Apr 19, 2012, at 5:43 AM, Narendra yadala <narendra.yadala@gmail.com
>>>> wrote:
>>>>> I have an issue with my HBase cluster. We have a 4 node HBase/Hadoop
>>>> (4*32
>>>>> GB RAM and 4*6 TB disk space) cluster. We are using Cloudera
>> distribution
>>>>> for maintaining our cluster. I have a single tweets table in which we
>>>> store
>>>>> the tweets, one tweet per row (it has millions of rows currently).
>>>>> Now I try to run a Java batch (not a map reduce) which does the
>>>> following :
>>>>> 1. Open a scanner over the tweet table and read the tweets one after
>>>>> another. I set scanner caching to 128 rows as higher scanner caching
>> is
>>>>> leading to ScannerTimeoutExceptions. I scan over the first 10k rows
>>>> only.
>>>>> 2. For each tweet, extract URLs (linkcolfamily:urlvalue) that are
>> there
>>>>> in that tweet and open another scanner over the tweets table to see
>> who
>>>>> else shared that link. This involves getting rows having that URL from
>>>> the
>>>>> entire table (not first 10k rows).
>>>>> 3. Do similar stuff as in step 2 for hashtags
>>>>> (hashtagcolfamily:hashtagvalue).
>>>>> 4. Do steps 1-3 in parallel for approximately 7-8 threads. This number
>>>>> can be higher (thousands also) later.
>>>>> When I run this batch I got the GC issue which is specified here
>> http://www.cloudera.com/blog/2011/02/avoiding-full-gcs-in-hbase-with-memstore-local-allocation-buffers-part-1/
>>>>> Then I tried to turn on the MSLAB feature and changed the GC settings
>> by
>>>>> specifying  -XX:+UseParNewGC  and  -XX:+UseConcMarkSweepGC JVM flags.
>>>>> Even after doing this, I am running into all kinds of IOExceptions
>>>>> and SocketTimeoutExceptions.
>>>>> This Java batch opens approximately 7*2 (14) scanners open at a point
>> in
>>>>> time and still I am running into all kinds of troubles. I am wondering
>>>>> whether I can have thousands of parallel scanners with HBase when I
>> need
>>>> to
>>>>> scale.
>>>>> It would be great to know whether I can open thousands/millions of
>>>> scanners
>>>>> in parallel with HBase efficiently.
>>>>> Thanks
>>>>> Narendra

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