On 2012-02-15, at 9:09 AM, Oliver Meyn (GBIF) wrote: > On 2012-02-15, at 7:32 AM, Stack wrote: > >> On Tue, Feb 14, 2012 at 8:14 AM, Stack <st...@duboce.net> wrote: >>>> 2) With that same randomWrite command line above, I would expect a >>>> resulting table with 10 * (1024 * 1024) rows (so 10485700 = roughly 10M >>>> rows). Instead what I'm seeing is that the randomWrite job reports >>>> writing that many rows (exactly) but running rowcounter against the table >>>> reveals only 6549899 rows. A second attempt to build the table produces >>>> slightly different results (e.g. 6627689). I see a similar discrepancy >>>> when using 50 instead of 10 clients (~35% smaller than expected). Key >>>> collision could explain it, but it seems pretty unlikely (given I only >>>> need e.g. 10M keys from a potential 2B). >>>> >>> >> >> I just tried it here and got similar result. I wonder if its the >> randomWrite? What if you do sequentialWrite, do you get our 10M? > > Thanks for checking into this stack - when using sequentialWrite I get the > expected 10485700 rows. I'll hack around a bit on the PE to count the number > of collisions, and try to think of a reasonable solution.
So hacking around reveals that key collision is indeed the problem. I thought the modulo part of the getRandomRow method was suspect but while removing it improved the behaviour (I got ~8M rows instead of ~6.6M) it didn't fix it completely. Since that's really what UUIDs are for I gave that a shot (i.e UUID.randomUUID()) and sure enough now I get the full 10M rows. Those are 16-byte keys now though, instead of the 10-byte that the integers produced. But because we're testing scan performance I think using a sequentially written table would probably be cheating and so will stick with randomWrite with slightly bigger keys. That means it's a little harder to compare to the results that other people get, but at least I know my internal tests are apples to apples. Oh and I removed the outer 10x loop and that produced the desired number of mappers (ie what I passed in on the commandline) but made no difference in the key generation/collision story. Should I file bugs for these 2 issues? Thanks, Oliver
