I've tried to re-run the test for some specific values of effective_io_concurrency. The results were the same.

That's why I don't think the order of tests or variability in "hardware" performance affected the results.


On 31/01/2018 15:01, Rick Otten wrote:
We moved our stuff out of AWS a little over a year ago because the performance was crazy inconsistent and unpredictable.  I think they do a lot of oversubscribing so you get strange sawtooth performance patterns depending on who else is sharing your infrastructure and what they are doing at the time.

The same unit of work would take 20 minutes each for several hours, and then take 2 1/2 hours each for a day, and then back to 20 minutes, and sometimes anywhere in between for hours or days at a stretch.  I could never tell the business when the processing would be done, which made it hard for them to set expectations with customers, promise deliverables, or manage the business.  Smaller nodes seemed to be worse than larger nodes, I only have theories as to why.  I never got good support from AWS to help me figure out what was happening.

My first thought is to run the same test on different days of the week and different times of day to see if the numbers change radically.  Maybe spin up a node in another data center and availability zone and try the test there too.

My real suggestion is to move to Google Cloud or Rackspace or Digital Ocean or somewhere other than AWS.   (We moved to Google Cloud and have been very happy there.  The performance is much more consistent, the management UI is more intuitive, AND the cost for equivalent infrastructure is lower too.)

On Wed, Jan 31, 2018 at 7:03 AM, Vitaliy Garnashevich <vgarnashev...@gmail.com <mailto:vgarnashev...@gmail.com>> wrote:


    I've tried to run a benchmark, similar to this one:


    CREATE TABLESPACE test OWNER postgres LOCATION '/path/to/ebs';

    pgbench -i -s 1000 --tablespace=test pgbench

    echo "" >test.txt
    for i in 0 1 2 4 8 16 32 64 128 256 ; do
      sync; echo 3 > /proc/sys/vm/drop_caches; service postgresql restart
      echo "effective_io_concurrency=$i" >>test.txt
      psql pgbench -c "set effective_io_concurrency=$i; set
    enable_indexscan=off; explain (analyze, buffers)  select * from
    pgbench_accounts where aid between 1000 and 10000000 and abalance
    != 0;" >>test.txt

    I get the following results:

     Execution time: 40262.781 ms
     Execution time: 98125.987 ms
     Execution time: 55343.776 ms
     Execution time: 52505.638 ms
     Execution time: 54954.024 ms
     Execution time: 54346.455 ms
     Execution time: 55196.626 ms
     Execution time: 55057.956 ms
     Execution time: 54963.510 ms
     Execution time: 54339.258 ms

    The test was using 100 GB gp2 SSD EBS. More detailed query plans
    are attached.

    PostgreSQL 9.6.6 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu
    5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609, 64-bit

    The results look really confusing to me in two ways. The first one
    is that I've seen recommendations to set
    effective_io_concurrency=256 (or more) on EBS. The other one is
    that effective_io_concurrency=1 (the worst case) is actually the
    default for PostgreSQL on Linux.



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