This is 100% reproducible for me, so I doubt it is related to random number generation.
On Oct 9, 2011, at 2:53 PM, lars hofhansl wrote: > How frequently does this happen? > I did notice a while ago in the code that scanner ids are drawn just from a > Random number generator. > > So in theory it would be possible that multiple concurrent scans draw the > same scanner id. > > Since these are longs, this is astronomically unlikely, though (picking the > same number of 2^64, just does not happen :) ). > > > > ________________________________ > From: Bryan Keller <[email protected]> > To: [email protected] > Sent: Sunday, October 9, 2011 2:40 PM > Subject: Re: Using Scans in parallel > > This is just scanning (reads). I'll need to do more testing to find a cause, > hopefully it is something with my test. > > On Oct 9, 2011, at 1:13 PM, lars hofhansl wrote: > >> Which version of HBase? >> Are there concurrent inserts? If so, do you see splits in the log files >> happening while you do the scanning? >> >> I am pretty sure this has nothing to do with concurrent scans. >> >> From: Bryan Keller <[email protected]> >> To: Bryan Keller <[email protected]> >> Cc: [email protected] >> Sent: Sunday, October 9, 2011 11:03 AM >> Subject: Re: Using Scans in parallel >> >> On further thought, it seems this might be a serious issue, as two unrelated >> processes within an application may be scanning the same table at the same >> time. >> >> On Oct 9, 2011, at 10:59 AM, Bryan Keller wrote: >> >>> I was not able to get consistent results using multiple scanners in >>> parallel on a table. I implemented a counter test that used 8 scanners in >>> parallel on a table with 2m rows with 2k+ columns each, and the results >>> were not consistent. There were no errors thrown, but the count was off by >>> as much as 2%. Using a single thread gave the same (correct) result every >>> run. >>> >>> I tried various approaches, such as creating an HTable and opening a >>> connection per thread, but I was not able to get stable results. I would do >>> some testing before using parallel scanners as described here. >>> >>> >>> On Oct 5, 2011, at 10:11 PM, lars hofhansl wrote: >>> >>>> That's part of it, the other part is to get the region demarcations. >>>> You can also just get the smallest and largest key of the table and pick >>>> other demarcations for your scans. Then your individual scans will likely >>>> cover multiple regions and regionservers. >>>> >>>> >>>> Your threading model depends on your needs. If you interested in lowest >>>> latency you want to keep your regionservers busy for each query. >>>> What exactly that means depends on your setup. Maybe you split up the >>>> overall scan so that no more than N scans are active at any regionserver. >>>> >>>> If you're more interested in overall predictability, you might not want >>>> parallelize each scan too much. >>>> >>>> >>>> >>>> ----- Original Message ----- >>>> From: Sam Seigal <[email protected]> >>>> To: [email protected]; lars hofhansl <[email protected]> >>>> Cc: "[email protected]" <[email protected]> >>>> Sent: Wednesday, October 5, 2011 6:18 PM >>>> Subject: Re: Using Scans in parallel >>>> >>>> So the whole point of getting the region locations is to ensure that >>>> there is one thread per region server ? >>>> >>>> >>>> On Wed, Oct 5, 2011 at 4:42 PM, lars hofhansl <[email protected]> wrote: >>>>> Hi Sam, >>>>> >>>>> >>>>> There were some attempts to build this in. In the end I think the exact >>>>> patterns are different based on what one is trying to achieve. >>>>> Currently what you can do is getting all the region locations >>>>> (HTable.getRegionLocations). From the HRegionInfos you can >>>>> get the regions start and end keys. >>>>> Now you can issue parallel scan for as many regions as you want (by >>>>> create a Scan object with start and row set to the region's >>>>> start and end key). >>>>> You probably want to group the regions by regionserver and have one >>>>> thread per region server, or something. >>>>> >>>>> >>>>> -- Lars >>>>> ________________________________ >>>>> From: Sam Seigal <[email protected]> >>>>> To: [email protected] >>>>> Sent: Wednesday, October 5, 2011 4:29 PM >>>>> Subject: Using Scans in parallel >>>>> >>>>> Hi , >>>>> >>>>> Is there a known way to be able to do Scan's in parallel (in different >>>>> threads even) and then sort/combine the output ? >>>>> >>>>> For a row key like: >>>>> >>>>> prefix-event_type-event_id >>>>> prefix-event_type-event_id >>>>> >>>>> I want to declare two scan objects (for say event_id_type foo) >>>>> >>>>> Scan 1 => 0-foo >>>>> Scan 2 => 1-foo >>>>> >>>>> execute the scans in parallel (maybe even in different threads) and >>>>> then merge the results ? >>>>> >>>>> Thank you, >>>>> >>>>> Sam >>>>> >>>> >>> >> >>
