Re: Very poor read performance, query independent

[Charles Nadeau <charles.nadeau@xxxxxxxxx>]

Mark,

First I must say that I changed my disks configuration from 4 disks in RAID 10 to 5 disks in RAID 0 because I almost ran out of disk space during the last ingest of data.

Here is the result test you asked. It was done with a cold cache:

flows=# \timing

Timing is on.

flows=# explain select count(*) from flows;

                                          QUERY PLAN                                           

-----------------------------------------------------------------------------------------------

 Finalize Aggregate  (cost=17214914.09..17214914.09 rows=1 width=8)

   ->  Gather  (cost=17214914.07..17214914.09 rows=1 width=8)

         Workers Planned: 1

         ->  Partial Aggregate  (cost=17213914.07..17213914.07 rows=1 width=8)

               ->  Parallel Seq Scan on flows  (cost=0.00..17019464.49 rows=388899162 width=0)

(5 rows)

Time: 171.835 ms

flows=# select pg_relation_size('flows');

 pg_relation_size 

------------------

     129865867264

(1 row)

Time: 57.157 ms

flows=# select count(*) from flows;

LOG:  duration: 625546.522 ms  statement: select count(*) from flows;

   count   

-----------

 589831190

(1 row)

Time: 625546.662 ms

The throughput reported by Postgresql is almost 198MB/s, and the throughput as mesured by dstat during the query execution was between 25 and 299MB/s. It is much better than what I had before! The i/o wait was about 12% all through the query. One thing I noticed is the discrepency between the read throughput reported by pg_activity and the one reported by dstat: pg_activity always report a value lower than dstat.

Besides the change of disks configuration, here is what contributed the most to the improvment of the performance so far:

Using Hugepage

Increasing effective_io_concurrency to 256

Reducing random_page_cost from 22 to 4

Reducing min_parallel_relation_size to 512kB to have more workers when doing sequential parallel scan of my biggest table

Thanks for recomending this test, I now know what the real throughput should be!

Charles

On Wed, Jul 12, 2017 at 4:11 AM, Mark Kirkwood <mark.kirkwood@xxxxxxxxxxxxxxx> wrote:

Hmm - how are you measuring that sequential scan speed of 4MB/s? I'd recommend doing a very simple test e.g, here's one on my workstation - 13 GB single table on 1 SATA drive - cold cache after reboot, sequential scan using Postgres 9.6.2:

bench=#  EXPLAIN SELECT count(*) FROM pgbench_accounts;
                                     QUERY PLAN
------------------------------------------------------------------------------------
 Aggregate  (cost=2889345.00..2889345.01 rows=1 width=8)
   ->  Seq Scan on pgbench_accounts (cost=0.00..2639345.00 rows=100000000 width=0)
(2 rows)


bench=#  SELECT pg_relation_size('pgbench_accounts');
 pg_relation_size
------------------
      13429514240
(1 row)

bench=# SELECT count(*) FROM pgbench_accounts;
   count
-----------
 100000000
(1 row)

Time: 118884.277 ms


So doing the math seq read speed is about 110MB/s (i.e 13 GB in 120 sec). Sure enough, while I was running the query iostat showed:

Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s wMB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda               0.00     0.00  926.00    0.00 114.89     0.00   254.10     1.90    2.03    2.03    0.00   1.08 100.00


So might be useful for us to see something like that from your system - note you need to check you really have flushed the cache, and that no other apps are using the db.

regards

Mark

On 12/07/17 00:46, Charles Nadeau wrote:

After reducing random_page_cost to 4 and testing more, I can report that the aggregate read throughput for parallel sequential scan is about 90MB/s. However the throughput for sequential scan is still around 4MB/s.

--

Charles Nadeau Ph.D.
http://charlesnadeau.blogspot.com/