Re: [PERFORM] Very poor read performance, query independent - Mailing list pgsql-performance
| From | Charles Nadeau |
|---|---|
| Subject | Re: [PERFORM] Very poor read performance, query independent |
| Date | |
| Msg-id | CADFyZw6_gpDoRtO_zqdD7bjsBy7twHM=FV3w_ukKRgcnJ79MSg@mail.gmail.com Whole thread Raw |
| In response to | Re: [PERFORM] Very poor read performance, query independent (Mark Kirkwood <mark.kirkwood@catalyst.net.nz>) |
| List | pgsql-performance |
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=# \timingTiming 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 msflows=# select pg_relation_size('flows');pg_relation_size------------------129865867264(1 row)Time: 57.157 msflows=# 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 HugepageIncreasing effective_io_concurrency to 256Reducing random_page_cost from 22 to 4Reducing 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@catalyst.net.nz> 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/
http://charlesnadeau.blogspot.com/
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