Re: Very poor read performance, query independent

From: pgsql-performance-owner(at)postgresql(dot)org [mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Charles Nadeau
Sent: Wednesday, July 12, 2017 6:05 AM
To: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: [PERFORM] Very poor read performance, query independent

flows=# explain (analyze, buffers) SELECT DISTINCT
flows-# srcaddr,
flows-# dstaddr,
flows-# dstport,
flows-# COUNT(*) AS conversation,
flows-# SUM(doctets) / 1024 / 1024 AS mbytes
flows-# FROM
flows-# flowscompact,
flows-# mynetworks
flows-# WHERE
flows-# mynetworks.ipaddr >>= flowscompact.srcaddr
flows-# AND dstaddr IN
flows-# (
flows(# SELECT
flows(# dstaddr
flows(# FROM
flows(# dstexterne
flows(# )
flows-# GROUP BY
flows-# srcaddr,
flows-# dstaddr,
flows-# dstport
flows-# ORDER BY
flows-# mbytes DESC LIMIT 50;
LOG: temporary file: path "pg_tblspc/36238/PG_9.6_201608131/pgsql_tmp/pgsql_tmp14573.6", size 1073741824
LOG: temporary file: path "pg_tblspc/36238/PG_9.6_201608131/pgsql_tmp/pgsql_tmp14573.7", size 1073741824
LOG: temporary file: path "pg_tblspc/36238/PG_9.6_201608131/pgsql_tmp/pgsql_tmp14573.8", size 639696896
LOG: duration: 2765020.327 ms statement: explain (analyze, buffers) SELECT DISTINCT
srcaddr,
dstaddr,
dstport,
COUNT(*) AS conversation,
SUM(doctets) / 1024 / 1024 AS mbytes
FROM
flowscompact,
mynetworks
WHERE
mynetworks.ipaddr >>= flowscompact.srcaddr
AND dstaddr IN
(
SELECT
dstaddr
FROM
dstexterne
)
GROUP BY
srcaddr,
dstaddr,
dstport
ORDER BY
mbytes DESC LIMIT 50;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=37762321.83..37762321.98 rows=50 width=52) (actual time=2764548.863..2764548.891 rows=50 loops=1)
Buffers: shared hit=1116590560 read=15851133, temp read=340244 written=340244
I/O Timings: read=5323746.860
-> Unique (cost=37762321.83..37769053.57 rows=2243913 width=52) (actual time=2764548.861..2764548.882 rows=50 loops=1)
Buffers: shared hit=1116590560 read=15851133, temp read=340244 written=340244
I/O Timings: read=5323746.860
-> Sort (cost=37762321.83..37763443.79 rows=2243913 width=52) (actual time=2764548.859..2764548.872 rows=50 loops=1)
Sort Key: (((sum(flows.doctets) / '1024'::numeric) / '1024'::numeric)) DESC, flows.srcaddr, flows.dstaddr, flows.dstport, (count(*))
Sort Method: quicksort Memory: 563150kB
Buffers: shared hit=1116590560 read=15851133, temp read=340244 written=340244
I/O Timings: read=5323746.860
-> GroupAggregate (cost=37698151.34..37714980.68 rows=2243913 width=52) (actual time=2696721.610..2752109.551 rows=4691734 loops=1)
Group Key: flows.srcaddr, flows.dstaddr, flows.dstport
Buffers: shared hit=1116590560 read=15851133, temp read=340244 written=340244
I/O Timings: read=5323746.860
-> Sort (cost=37698151.34..37699273.29 rows=2243913 width=20) (actual time=2696711.428..2732781.705 rows=81896988 loops=1)
Sort Key: flows.srcaddr, flows.dstaddr, flows.dstport
Sort Method: external merge Disk: 2721856kB
Buffers: shared hit=1116590560 read=15851133, temp read=340244 written=340244
I/O Timings: read=5323746.860
-> Gather (cost=19463936.00..37650810.19 rows=2243913 width=20) (actual time=1777219.713..2590530.887 rows=81896988 loops=1)
Workers Planned: 9
Workers Launched: 9
Buffers: shared hit=1116590559 read=15851133
I/O Timings: read=5323746.860
-> Hash Semi Join (cost=19462936.00..37622883.23 rows=249324 width=20) (actual time=1847579.360..2602039.780 rows=8189699 loops=10)
Hash Cond: (flows.dstaddr = flows_1.dstaddr)
Buffers: shared hit=1116588309 read=15851133
I/O Timings: read=5323746.860
-> Nested Loop (cost=0.03..18159012.30 rows=249324 width=20) (actual time=1.562..736556.583 rows=45499045 loops=10)
Buffers: shared hit=996551813 read=15851133
I/O Timings: read=5323746.860
-> Parallel Seq Scan on flows (cost=0.00..16039759.79 rows=62330930 width=20) (actual time=1.506..547485.066 rows=54155970 loops=10)
Buffers: shared hit=1634 read=15851133
I/O Timings: read=5323746.860
-> Index Only Scan using mynetworks_ipaddr_idx on mynetworks (cost=0.03..0.03 rows=1 width=8) (actual time=0.002..0.002 rows=1 loops=541559704)
Index Cond: (ipaddr >>= (flows.srcaddr)::ip4r)
Heap Fetches: 59971474
Buffers: shared hit=996550152
-> Hash (cost=19462896.74..19462896.74 rows=11210 width=4) (actual time=1847228.894..1847228.894 rows=3099798 loops=10)
Buckets: 4194304 (originally 16384) Batches: 1 (originally 1) Memory Usage: 141746kB
Buffers: shared hit=120036496
-> HashAggregate (cost=19462829.48..19462863.11 rows=11210 width=4) (actual time=1230049.015..1845955.764 rows=3099798 loops=10)
Group Key: flows_1.dstaddr
Buffers: shared hit=120036496
-> Nested Loop Anti Join (cost=0.12..19182620.78 rows=560417390 width=4) (actual time=0.084..831832.333 rows=113420172 loops=10)
Join Filter: (mynetworks_1.ipaddr >> (flows_1.dstaddr)::ip4r)
Rows Removed by Join Filter: 453681377
Buffers: shared hit=120036496
-> Index Only Scan using flows_srcaddr_dstaddr_idx on flows flows_1 (cost=0.12..9091067.70 rows=560978368 width=4) (actual time=0.027..113052.437 rows=541559704 loops=10)
Heap Fetches: 91
Buffers: shared hit=120036459
-> Materialize (cost=0.00..1.02 rows=4 width=8) (actual time=0.000..0.000 rows=2 loops=5415597040)
Buffers: shared hit=10
-> Seq Scan on mynetworks mynetworks_1 (cost=0.00..1.01 rows=4 width=8) (actual time=0.007..0.008 rows=4 loops=10)
Buffers: shared hit=10
Planning time: 6.689 ms
Execution time: 2764860.853 ms
(58 rows)

Regarding "Also using dstat I can see that iowait time is at about 25%", I don't think the server was doing anything else. If it is important, I can repeat the benchmarks.
Thanks!

Charles

Charles,

In your original posting I couldn’t find what value you set for temp_buffers.
Considering you have plenty of RAM, try setting temp_buffers=’6GB’ and then run ‘explain (analyze, buffers) select…’ in the same session. This should alleviate “disk sort’ problem.

Also, could you post the structure of flowscompact, mynetworks, and dstextern tables with all the indexes and number of rows. Actually, are they all – tables, or some of them – views?

Igor