Re: effective_io_concurrency on EBS/gp2 - Mailing list pgsql-performance

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 <vgarnashevich@gmail.com> wrote:

Hi,

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

https://www.postgresql.org/message-id/flat/CAHyXU0yiVvfQAnR9cyH%3DHWh1WbLRsioe%3DmzRJTHwtr%3D2azsTdQ%40mail.gmail.com#CAHyXU0yiVvfQAnR9cyH=HWh1WbLRsioe=mzRJTHwtr=2azsTdQ@mail.gmail.com

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
done

I get the following results:

effective_io_concurrency=0
 Execution time: 40262.781 ms
effective_io_concurrency=1
 Execution time: 98125.987 ms
effective_io_concurrency=2
 Execution time: 55343.776 ms
effective_io_concurrency=4
 Execution time: 52505.638 ms
effective_io_concurrency=8
 Execution time: 54954.024 ms
effective_io_concurrency=16
 Execution time: 54346.455 ms
effective_io_concurrency=32
 Execution time: 55196.626 ms
effective_io_concurrency=64
 Execution time: 55057.956 ms
effective_io_concurrency=128
 Execution time: 54963.510 ms
effective_io_concurrency=256
 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.

Thoughts?

Regards,
Vitaliy