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From "Oliver Meyn (GBIF)" <om...@gbif.org>
Subject Re: strange PerformanceEvaluation behaviour
Date Wed, 15 Feb 2012 09:53:40 GMT
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 <stack@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?


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